Resonant tunnel diode element and electronic apparatus

Abstract

[Problem] The present disclosure provides: a resonant tunnel diode element capable of suppressing deterioration of negative conductance; and an electronic apparatus. [Solution] The present disclosure provides a resonant tunnel diode element comprising: a quantum well layer that is laminated on a nitride semiconductor and that has at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying an electric potential on the first barrier layer; a collector layer capable of applying an electric potential on the second barrier layer; and a first spacer layer laminated between the first barrier layer and the emitter layer. The average of the bandgap of the first spacer layer is set to be less than the average of the bandgap of the emitter layer.

Description

Resonant tunneling diode element and electronic device

[0001] The present disclosure relates to a resonant tunneling diode element and an electronic device.

[0002] In resonant tunneling diodes (RTDs) using nitride semiconductors, it is possible to form quantum well layers with quantum well structures with large barrier energies by taking advantage of the wide bandgap change from AlN to GaN to InN, which is expected to enable the fabrication of resonant tunneling diodes with large peak-to-valley current ratios.

[0003] JP 2009-152547 A

[0004] However, in typical nitride semiconductors (GaN-based materials) with a C-plane as the principal plane orientation, the potential near the well / barrier rises due to the effects of spontaneous polarization and piezoelectric polarization inherent in the crystal, and the effective barrier layer thickness increases, making it difficult for tunnel current to flow. For this reason, resonant tunneling diodes using GaN-based materials may end up with a small peak current value and a small negative conductance.

[0005] Therefore, the present disclosure provides a resonant tunneling diode element and an electronic device that can suppress a decrease in negative conductance.

[0006] In order to solve the above-mentioned problems, the present disclosure provides a resonant tunneling diode element comprising: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein the average value of the band gap of the first spacer layer is smaller than the average value of the band gap of the emitter layer.

[0007] The first spacer layer may be in contact with the first barrier layer.

[0008] A two-dimensional electron gas based on the band gap of the first spacer layer and the band gap of the first barrier layer may be generated at the interface between the first spacer layer and the first barrier layer.

[0009] The bandgap of the first spacer layer may be smallest in a region in contact with the first barrier layer.

[0010] The band gap of the first spacer layer may increase with increasing distance from the region in contact with the first barrier layer.

[0011] The first spacer layer may have an average impurity concentration lower than an average impurity concentration of the emitter layer.

[0012] At least a portion of the first spacer layer may be made of a nitride semiconductor containing indium (In).

[0013] The quantum well structure of the quantum well layer may be formed by a stacked structure of aluminum nitride (AlN), gallium nitride (GaN), and aluminum nitride (AlN).

[0014] The first barrier layer and the second barrier layer of the quantum well layer may be made of at least one of aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlInGaN), and the well layer may be made of at least one of gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlInGaN), and aluminum indium nitride (AlInN), which correspond to the first barrier layer and the second barrier layer.

[0015] The emitter layer may be formed of gallium nitride (GaN) doped with impurities, and the collector layer may be formed of gallium nitride (GaN) doped with impurities.

[0016] The first spacer layer may be made of indium gallium nitride (InGaN), and the indium (In) composition may increase toward the first barrier layer.

[0017] A second spacer layer of gallium nitride (GaN) may be formed between the collector layer and the second barrier layer.

[0018] The semiconductor device may further include: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, wherein the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

[0019] The intermediate layer may be doped with impurities.

[0020] The second spacer layer and the intermediate layer may be in contact with each other.

[0021] The intermediate layer may contain at least aluminum (Al) as a constituent element.

[0022] The intermediate layer may be made of aluminum gallium nitride (AlGaN).

[0023] In order to solve the above-mentioned problems, the present disclosure provides an electronic device comprising: a resonator having a resonant tunneling diode element; and a power supply circuit that applies a voltage to the resonant tunneling diode element, wherein the resonant tunneling diode element comprises: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer to which a potential can be applied to the first barrier layer; a collector layer to which a potential can be applied to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein the average value of the band gap of the first spacer layer is smaller than the average value of the band gap of the emitter layer.

[0024] The resonator may include an antenna, and the antenna may be connected to the resonant tunneling diode element.

[0025] The resonant tunneling diode element may further include: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, wherein the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

[0026] 12 is a cross-sectional view schematically showing an example of a resonant tunneling diode element. FIG. 12 is a diagram showing a conduction band structure of a resonant tunneling diode element when no bias is applied. FIG. 12 is a cross-sectional view schematically showing an example of a resonant tunneling diode element according to a comparative example. FIG. 12 is a diagram showing the relationship between the carrier density according to the present embodiment and the carrier density according to a comparative example. FIG. 12 is a diagram showing the relationship between the composition and width of a first spacer layer and the carrier density. FIG. 12 is a diagram showing the conduction band structure when a forward bias is applied. FIG. 12 is a diagram showing the voltage-current characteristics of a resonant tunneling diode element. FIG. 12 is a diagram showing the relationship between the composition and carrier density of a first spacer layer. FIG. 12 is a diagram showing the relationship between the doping concentration and carrier density of a first spacer layer. A cross-sectional view schematically showing an example of a resonant tunneling diode element according to a third embodiment. FIG. 12 is a diagram showing the conduction band structure when no bias is applied. A perspective view showing an example of a configuration of a terahertz oscillator. A cross-sectional view taken along the line AA' in FIG. 12. A circuit configuration diagram of a terahertz oscillator.

[0027] Hereinafter, embodiments of a resonant tunneling diode element and an oscillator element will be described with reference to the drawings. The following description will focus on the main components of the resonant tunneling diode element and the oscillator element, but the light-emitting element may include components and functions that are not shown or described. The following description does not exclude components and functions that are not shown or described.

[0028] 1 is a cross-sectional view schematically illustrating an example of a resonant tunneling diode element 10 according to a first embodiment of the present disclosure. As shown in FIG. 1, the resonant tunneling diode (RTD) element 10 is an element having a negative differential resistance characteristic, and includes a layered structure in which an emitter layer 100, a first spacer layer 102, a first barrier layer 104, a well layer 106, a second barrier layer 108, a second spacer layer 110, and a collector layer 112 are layered, in this order from the bottom, on a nitride semiconductor having a C-plane as its principal surface orientation.

[0029] As the substrate, in addition to a nitride semiconductor substrate (GaN substrate), SiC, sapphire, Si, etc. can be used. The substrate should have n-type or semi-insulating conductivity. Crystal growth can be achieved using MOCVD (metal organic chemical vapor deposition), a vapor phase growth method using organic metal precursors. Alternatively, MBE (molecular beam epitaxy), sputtering, or other growth methods can be used. The resonant tunneling diode element 10 according to this embodiment is made of a material such as aluminum nitride (AlN), gallium nitride (GaN), aluminum nitride (AlN), or indium gallium nitride (InGaN).

[0030] The quantum well structure consisting of the first barrier layer 104, the well layer 106, and the second barrier layer 108 is formed, for example, by the first barrier layer 104 of u (undoped, hereinafter referred to as "undoped")-AlN, the well layer 106 of u-GaN, and the second barrier layer 108 of u-AlN. That is, the quantum well structure is formed of aluminum nitride (AlN), gallium nitride (GaN), and aluminum nitride (AlN). Note that, according to this embodiment, the first barrier layer (first barrier layer) 104, the well layer (well layer) 106, and the second barrier layer (first barrier layer) 108 correspond to the quantum well layer. The first barrier layer 104 and the second barrier layer 108 can be formed of at least one of aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlInGaN). The well layer 106 can be made of at least one of gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlInGaN), and aluminum indium nitride (AlInN), which correspond to the first barrier layer 104 and the second barrier layer 108.

[0031] An emitter layer 100 is provided adjacent to the first barrier layer 104 on the emitter electrode side of one of the outer sides of the quantum well structure, with a first spacer layer 102 interposed therebetween. For example, an n-GaN emitter layer 100 can be used as the emitter layer 100. The n-GaN emitter layer 100 is doped with, for example, Si to a concentration of, for example, 1E+19 (cm-3) or more. Note that the doping concentration is an example and is not limited to this.

[0032] Furthermore, on the collector electrode side of the other outer side of the quantum well structure, a collector layer 112 is provided adjacent to the second barrier layer 108, with a second spacer layer 110 interposed therebetween. For example, a u-GaN spacer layer can be used as the second spacer layer 110, and an n-GaN collector layer 112 can be used as the collector layer 112. The n-GaN collector layer 112 is doped with, for example, Si, to a concentration of, for example, 1E+19 (cm-3) or more. Note that the doping concentration is an example and is not limited to this.

[0033] The first spacer layer 102 is configured so that the average band gap of the first spacer layer 102 is smaller than the average band gap of the emitter layer 100. This makes it possible to increase the concentration of two-dimensional electron gas (2DEG) generated at the interface between the first spacer layer 102 and the first barrier layer 104, as will be described later.

[0034] That is, the first spacer layer 102 includes a first spacer layer 102a and a first spacer layer 102b. The first spacer layer 102a is adjacent to the emitter layer 100 and is, for example, a spacer layer made of u-GaN. The first spacer layer 102b is adjacent to the first barrier layer 104 and is, for example, a spacer layer made of u-InGaN. That is, the first spacer layer 102b is made of indium gallium nitride (InGaN). In this way, the first spacer layer 102 is made of a nitride semiconductor. In addition, ohmic electrodes are formed on the emitter layer 100 and the collector layer 112, respectively, so that a bias voltage can be applied between the emitter layer 100 and the collector layer 112.

[0035] 2 is a diagram showing the conduction band structure of the resonant tunneling diode element 10 when no bias is applied. The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents energy. The conduction band structure Ec10 is shown as a reference example when there is no influence of spontaneous polarization or piezoelectric polarization. The dotted line represents the Fermi level E Ｆ Shows.

[0036] On the other hand, the conduction band structure Ec20 of the GaN-based resonant tunneling diode device 10 according to this embodiment is deformed from the conduction band structure Ec10 due to the influence of spontaneous polarization and piezoelectric polarization. In the conduction band structure Ec20, the potential of the quantum well structure consisting of the first barrier layer 104, the well layer 106, and the second barrier layer 108 increases. Furthermore, in the conduction band structure Ec20, a positive triangular potential gradient is generated on the second spacer layer 110 side of the second barrier layer 108. This increases the effective barrier layer thickness, making it more difficult for a tunneling current to flow than in the conduction band structure Ec10 without spontaneous polarization and piezoelectric polarization. In this embodiment, a potential change in the conduction band structure in which the band gap increases as the distance to the quantum well structure decreases may be referred to as a positive potential change, and a potential change in which the band gap decreases as the distance to the quantum well structure decreases may be referred to as a negative potential change.

[0037] Furthermore, a triangular potential gradient is generated in the interface region of the first barrier layer 104 on the emitter layer 100 side. In the resonant tunneling diode element 10 according to this embodiment, this negative triangular potential gradient makes it possible to generate two-dimensional electron gas (2DEG) at the interface between the first barrier layer 104 and the first spacer layer 102.

[0038] The resonant tunneling current passing through the quantum well structure is determined by the tunneling probability of electrons passing through the quantum well structure and the number of electrons passing through. When this tunneling probability is constant, it is effective to increase the carrier concentration on the emitter side of the first barrier layer 104.

[0039] Therefore, the resonant tunneling diode element 10 according to this embodiment can generate a larger negative triangular potential by adjusting the band gap of the first spacer layer 102. This makes it possible to increase the concentration of two-dimensional electron gas (2DEG) generated at the interface between the first spacer layer 102 and the first barrier layer 104. For example, in the resonant tunneling diode element 10 according to this embodiment, as described above, the first spacer layer 102 is configured so that the average band gap of the first spacer layer 102 is smaller than the average band gap of the emitter layer 100.

[0040] More specifically, a first spacer layer 102b made of u-InGaN, which has a bandgap smaller than that of u-GaN, is configured adjacent to the first barrier layer 104. This makes it possible to increase the negative triangular potential, thereby generating more two-dimensional electron gas (2DEG). In this way, the negative triangular potential in the first spacer layer 102b is made smaller. In other words, by reducing the bandgap on the first spacer layer 102b side at the interface between the first barrier layer 104 and the first spacer layer 102b, it is possible to increase the carrier concentration on the emitter side of the first barrier layer 104.

[0041] FIG. 3 is a cross-sectional view schematically illustrating an example of a resonant tunneling diode element 10a according to a comparative example. As shown in FIG. 3, the resonant tunneling diode element 10a according to the comparative example is an element having negative differential resistance characteristics and includes a layered structure in which, from bottom to top, an emitter layer 100, a first spacer layer 102a, a first barrier layer 104, a well layer 106, a second barrier layer 108, a second spacer layer 110, and a collector layer 112 are stacked on a nitride semiconductor having a C-plane as its principal surface orientation. The first spacer layer 102a is a u-GaN spacer layer. In other words, the resonant tunneling diode element 10a according to the comparative example differs from the resonant tunneling diode element 10 according to the first embodiment in that it does not include a u-InGaN first spacer layer 102b.

[0042] FIG. 4 is a diagram showing the relationship between the carrier density of the resonant tunneling diode element 10 according to this embodiment and the carrier density of the resonant tunneling diode element 10a according to a comparative example. For example, the carrier density is a current density. FIG. 4(a) shows the band gap of the resonant tunneling diode element 10a when no bias is applied. The horizontal axis shows the distance from the end of the upper surface of the collector layer 112, and the vertical axis shows the energy. The thickness of the first spacer layer 102a is set to 15 nm. FIG. 4(b) shows the carrier density in the band gap of FIG. 4(a). The horizontal axis shows the distance from the end of the upper surface of the collector layer 112, and the vertical axis shows the carrier density.

[0043] FIG. 4(c) shows the bandgap of the resonant tunneling diode element 10 when no bias is applied. The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the energy. In0.05GaN is used as the first spacer layer 102b on the first barrier layer 104 side of the resonant tunneling diode element 10. Its thickness is set to 5 nm, and the thickness of the first spacer layer 102a is set to 10 nm. FIG. 4(d) shows the carrier density in the bandgap of FIG. 4(c). The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the carrier density.

[0044] 4( a) and 4(c), by using In0.05GaN as the first spacer layer 102b, the band gap at the interface between the first spacer layer 102b and the first barrier layer 104 becomes smaller than when In0.05GaN is not used. As a result, as shown in FIGS. 4(b) and 4(d), a peak of carrier density I2dg that is about Di larger than that of the comparative example occurs at the interface between the first spacer layer 102b and the first barrier layer 104.

[0045] Thus, constructing the first spacer layer at the interface with the first barrier layer 104 as a u-In0.05GaN spacer layer makes it possible to generate two-dimensional electron gas (2DEG) with approximately twice the carrier density as constructing it as a u-GaN spacer layer. As can be seen from these, constructing a u-In0.05GaN spacer layer increases the concentration of two-dimensional electron gas (2DEG), increasing the number of electrons that escape from the emitter layer 100 side to the collector layer 112 side by resonant tunneling, and increasing the resonant tunneling current. This is thought to be because the band gap of u-In0.05GaN is smaller than that of u-GaN, making the negative triangular potential smaller and increasing the concentration of two-dimensional electron gas (2DEG).

[0046] FIG. 5 shows the relationship between the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode device 10 according to this embodiment, as well as the width and carrier density. FIG. 5(a) shows the band gap when the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode device 10a is changed when no bias is applied. The horizontal axis represents the distance from the edge of the upper surface of the collector layer 112, and the vertical axis represents energy. The first spacer layer 102b is made of u-InGaN. The graph shows the potential change when the thickness is set to 5 nm and the In composition is changed to 2%, 5%, and 10%. The first spacer layer 102a is made of u-GaN. The thickness is fixed at 10 nm, and the composition is fixed. FIG. 5(b) shows the carrier density I2dg at the band gap of FIG. 5(a). The horizontal axis represents the distance from the edge of the upper surface of the collector layer 112, and the vertical axis represents carrier density.

[0047] FIG. 5(c) shows the band gap when the thickness of the first spacer layer 102b in the resonant tunneling diode device 10a is changed when no bias is applied. The horizontal axis represents the distance from the upper edge of the collector layer 112, and the vertical axis represents the energy. The first spacer layer 102b is made of u-InGaN. The first spacer layer 102b is made of u-In0.05GaN, and the first spacer layer 102a is made of u-GaN. The total thickness of the first spacer layer 102 is set to 15 nm, and the thickness of the first spacer layer 102b is changed to 2, 5, and 10 nm. FIG. 5(d) shows the carrier density I2dg in the band gap of FIG. 5(c). The horizontal axis represents the distance from the upper edge of the collector layer 112, and the vertical axis represents the carrier density.

[0048] As shown in Figures 5(a) and (b), the carrier density I2dg increases as the In composition increases from 2%, 5%, to 10%. As can be seen from these figures, increasing the u-InGaN composition increases the concentration of two-dimensional electron gas (2DEG), increasing the number of electrons that escape from the emitter layer 100 to the collector layer 112 by resonant tunneling, thereby increasing the resonant tunneling current. This is thought to be because increasing the u-InGaN composition narrows the band gap of u-InGaN, making it possible to further reduce the negative triangular potential. On the other hand, increasing the u-InGaN composition increases the lattice constant difference with u-GaN, increasing the possibility of crystal breakdown. For this reason, it is thought that an In composition ranging from 2% to 30% is an appropriate range for increasing the concentration of two-dimensional electron gas (2DEG).

[0049] 5(c) and 5(d), when the In composition is fixed and the thickness of the first spacer layer 102b is changed to 2, 5, or 10 nm, the effect on the potential profile and carrier density I2dg is limited. In other words, even if the thickness of the first spacer layer 102b is 2 nm, the same effect can be obtained as when the thickness is 10 nm or more. Conversely, if the thickness of the first spacer layer 102b is made too thick, lattice mismatch with u-GaN may cause crystal damage such as cracks, and a thickness of approximately 2 nm to 10 nm is considered to be an appropriate range.

[0050] Here, the operating characteristics of the resonant tunneling diode element 10 according to this embodiment will be described with reference to FIGS. 6 and 7. FIG. 6 is a diagram showing the conduction band structure Ec20 when a forward bias is applied between the emitter layer 100 and the collector layer 112 of the resonant tunneling diode element 10. The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the energy. FIG. 6(a) shows the case with no bias, FIG. 6(b) shows the case with a forward bias of 3.5 volts (V), and FIG. 6(c) shows the case with a forward bias of 5 volts (V). These are simulation results.

[0051] 7 is a diagram showing the voltage-current characteristics of the resonant tunneling diode element 10. The horizontal axis represents the voltage between the emitter layer 100 and the collector layer 112, and the vertical axis represents the current flowing between the emitter layer 100 and the collector layer 112.

[0052] 6(a) to 6(c) and 7, when no bias is applied, no current flows between the emitter layer 100 and the collector layer 112. On the other hand, as the applied voltage increases, the positive triangular potential (see FIG. 2) on the second spacer layer 110 side gradually falls below the Fermi level, and the barrier layer becomes thinner. As a result, at around 3.4 volts (V), the first state coincides with the Fermi level, and the current peaks.

[0053] After reaching the peak, as the forward bias voltage is further increased, the first state and the Fermi level gradually separate, and the current gradually decreases. At around 4.3 volts (V), a predetermined level between the first state and the second state becomes the Fermi level, and the current reaches its minimum value. This characteristic in region A10, where the current decreases even with increasing voltage, is called a negative differential resistance characteristic. Furthermore, as the forward bias voltage is further increased, the second state and the Fermi level become closer, and the current begins to increase. As explained with reference to FIGS. 5( a) and 5(b), the peak current at 3.4 volts can be increased as the In composition of u-InGaN increases to 2%, 5%, and 10%.

[0054] As described above, the resonant tunneling diode element 10 according to this embodiment is configured so as to reduce the band gap in the boundary region of the first spacer layer 102 formed between the emitter layer 100 and the first barrier layer 104. This makes it possible to increase the concentration of two-dimensional electron gas (2DEG) generated at the boundary surface between the first spacer layer 102 and the first barrier layer 104, thereby suppressing a decrease in the negative conductance of the resonant tunneling diode element 10.

[0055] (First Modification of First Embodiment) The resonant tunneling diode element 10 according to the first modification of the first embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that the In composition of the u-InGaN of the first spacer layer 102b is changed with respect to the film thickness. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0056] The resonant tunneling diode element 10 according to the first modification of the first embodiment is configured with three layers in which the In composition of u-InGaN in the first spacer layer 102b decreases to 7%, 5%, and 3% with increasing distance from the first barrier layer 104.

[0057] FIG. 8 is a graph showing the relationship between the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode element 10 according to this embodiment and the carrier density. FIG. 8(a) shows the band gap when the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode element 10a is u-In0.05GaN and the thickness is 6 nm in an unbiased state. The first spacer layer 102a is made of u-GaN. The thickness is fixed at 9 nm, and the composition is fixed. FIG. 8(b) shows the carrier density I2dg in the band gap of FIG. 8(a). The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the carrier density.

[0058] FIG. 8(c) shows the bandgap of the resonant tunneling diode device 10a when no bias is applied. The first spacer layer 102b is made of u-InGaN and is composed of three layers with In compositions decreasing from 7%, 5%, and 3% with increasing distance from the first barrier layer 104, with a thickness of 6 nm. Each of the three layers with In compositions of 7%, 5%, and 3% is 2 nm. The first spacer layer 102a is made of u-GaN. The composition is fixed at a thickness of 9 nm. FIG. 8(d) shows the carrier density I2dg at the bandgap of FIG. 8(c). The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the carrier density.

[0059] As shown in Figure 8, the average In composition of the first spacer layer 102b of the resonant tunneling diode device 10 according to this embodiment is 5%, but a higher carrier density I2dg is obtained than in the case of a single film with an In composition of 5%. This is thought to be because the In composition on the side in contact with the first barrier layer 104 is increased, making it possible to reduce the positive triangular potential on the second spacer layer 110 side (see Figure 2). In this way, a greater effect can be achieved by increasing the In composition closer to the barrier layer and decreasing it with increasing distance. Furthermore, the In composition may change continuously rather than stepwise as shown in the figure.

[0060] Furthermore, by constructing it from three layers with decreasing lattice constants of 7%, 5%, and 3%, it is possible to gradually bring the lattice constant of InGaN closer to that of u-GaN, thereby suppressing crystal damage such as cracks due to lattice mismatch with u-GaN.

[0061] (Modification 2 of First Embodiment) The resonant tunneling diode element 10 according to Modification 2 of the first embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that impurities are doped into the first spacer layer 102. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0062] The resonant tunneling diode element 10 according to the second modification of the first embodiment has a first spacer layer 102b composed of a 3 nm thick film of u-InGaN with an In composition of 5% and a 3 nm thick film doped with impurities with an In composition of 5%.

[0063] 9 is a diagram showing the relationship between the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode element 10 according to this embodiment and the carrier density. Fig. 9(a) shows the band gap when the In composition of the InGaN of the first spacer layer 102b of the resonant tunneling diode element 10a in an unbiased state is set to u-In0.05GaN and the thickness is set to 6 nm. u-GaN is used as the first spacer layer 102a. The thickness is fixed at 9 nm, so the composition is fixed.

[0064] Fig. 9B shows the carrier density I2dg in the band gap of Fig. 9A. The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents the carrier density.

[0065] 9(c) shows the band gaps when the first spacer layer 102b in the resonant tunneling diode element 10a is a 3 nm thick film of u-InGaN with an In composition of 5% and a 3 nm thick film doped with impurities with an In composition of 5% in the unbiased state. The first spacer layer 102a is made of impurity-doped GaN. The thickness is set to 9 nm, and the composition is fixed. For example, Si is doped as the impurity, and the concentration is set to 1E+19 (cm-3).

[0066] 9, the same effect can be obtained regardless of whether the first spacer layer 102 is doped. However, if the entire first spacer layer 102 is doped, impurity diffusion into layers above the first barrier layer 104 may lead to deterioration of device characteristics. Therefore, the layer immediately before the barrier layer is left undoped or lightly doped to the extent that diffusion does not occur. In this way, by doping the first spacer layer 102, it is possible to adjust the lattice constant while maintaining the carrier density I2dg.

[0067] Second Embodiment The resonant tunneling diode element 10 according to the second embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that an intermediate layer 200 is further configured between the second spacer layer 110 and the collector layer 112. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0068] 10 is a cross-sectional view schematically illustrating an example of a resonant tunneling diode element 10 according to a third embodiment of the present disclosure. As shown in FIG. 10 , the resonant tunneling diode (RTD) element 10 is an element having negative differential resistance characteristics, and includes a layered structure in which, from bottom to top, an emitter layer 100, a first spacer layer 102, a first barrier layer 104, a well layer 106, a second barrier layer 108, a second spacer layer 110, an intermediate layer 200, and a collector layer 112 are stacked on a nitride semiconductor having a C-plane as its principal surface orientation. That is, the resonant tunneling diode element 10 according to the third embodiment further includes the intermediate layer 200.

[0069] In addition to GaN substrates, SiC, sapphire, Si, etc. can be used as the substrate. The substrate conductivity is n-type or semi-insulating. Crystal growth can be performed using MOCVD (metal organic chemical vapor deposition), a vapor phase growth method using organic metal raw materials. Alternatively, MBE (molecular beam epitaxy), sputtering, or other growth methods can be used.

[0070] The quantum well structure consisting of the first barrier layer 104, the well layer 106, and the second barrier layer 108 is formed, for example, by the first barrier layer 104 of u (undoped, hereinafter referred to as undoped)-AlN, the well layer 106 of u-GaN, and the second barrier layer 108 of u-AlN.

[0071] An emitter layer 100 is provided adjacent to the first barrier layer 104 on the emitter electrode side of one of the outer sides of the quantum well structure, with a first spacer layer 102 interposed therebetween. For example, an n-GaN emitter layer 100 can be used as the emitter layer 100. The n-GaN emitter layer 100 is doped with, for example, Si to a concentration of, for example, 1E+19 (cm-3) or more. Note that the doping concentration is an example and is not limited to this.

[0072] Furthermore, an intermediate layer 200 is provided adjacent to the second barrier layer 108 on the collector electrode side of the other outer side of the quantum well structure, with a second spacer layer 110 interposed therebetween. A u-GaN spacer layer can be used as the second spacer layer 110, and an n-GaN collector layer 112 can be used as the collector layer 112. The n-GaN collector layer 112 is doped with, for example, Si to a concentration of, for example, 1E+19 (cm-3) or more. Note that the doping concentration is an example and is not limited to this.

[0073] The intermediate layer 200 is formed of a material with a larger band gap than the second spacer layer 110. For example, aluminum gallium nitride (AlGaN) is used for the intermediate layer 200. The intermediate layer 200 is formed directly on the second spacer layer 110 and is doped with the same impurities as the emitter layer 100 and the collector layer 112. For example, the intermediate layer 200 is doped with Si, and the doping concentration is set to, for example, 1E+19 (cm-3) or more. Note that the doping concentration is an example and is not limited to this. Ohmic electrodes are formed on the emitter layer 100 and the collector layer 112, respectively, and a bias voltage can be applied between the emitter layer 100 and the collector layer 112.

[0074] FIG. 11 is a diagram showing the conduction band structure Ec30 of the resonant tunneling diode element 10 according to this embodiment when no bias is applied. The horizontal axis represents the distance from the end of the upper surface of the collector layer 112, and the vertical axis represents energy. In the resonant tunneling diode element 10 according to this embodiment, the intermediate layer 200 is provided to adjust the band gap of the first spacer layer 102 and change the positive triangular potential shape. For example, in the resonant tunneling diode element 10 according to this embodiment, as described above, the intermediate layer 200 is formed of a material with a larger band gap than the second spacer layer 110. Note that FIG. 11 also shows the conduction band structure Ec20 when the intermediate layer 200 is not provided.

[0075] A two-dimensional electron gas (2DEGb) can be formed at the interface between the intermediate layer 200 and the second spacer layer 110. This two-dimensional electron gas suppresses the rise in the potential of the second spacer layer, and by reducing the effective thickness of the second barrier layer, the electron tunneling probability can be further increased. In addition, the potential of the well layer 106 can be simultaneously reduced, which makes it possible to further reduce the operating voltage of the resonant tunneling diode element 10.

[0076] (Third Embodiment) The resonant tunneling diode element 10 according to the third embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that an oscillator is formed by connecting the tunnel diode element 10 to an external resonator. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0077] FIG. 12 is a perspective view showing an example of the configuration of a terahertz oscillator 15. FIG. 13 is a cross-sectional view taken along the line AA′ in FIG. 12. As shown in FIG. 12, the terahertz oscillator 15 has gain in the frequency band of terahertz waves. This terahertz oscillator 15 includes an oscillator 20 and a power supply circuit 30. The oscillator 20 is an oscillator having a compact solid-state device structure formed by connecting and integrating a resonant tunneling diode element 10 and a patch antenna (microstrip antenna) 22 serving as an external resonator. The patch antenna (microstrip antenna) 22 is, for example, a 170 μm square (when the effective wavelength is 340 μm). The terahertz oscillator 15 according to this embodiment corresponds to an electronic device.

[0078] The power supply circuit 30 is a circuit that applies a DC voltage to the emitter layer 100 and the collector layer 112 of the resonant tunneling diode element 10. The power supply circuit 30 includes, for example, a metal-insulator-metal filter (MIM filter) 32 and a power supply pad 34. A dielectric 24 is disposed around the patch antenna 22 and the power supply circuit 30.

[0079] 13 , the oscillator 20 is configured such that a dielectric layer 24 is sandwiched between two conductors, a signal electrode 114 and a ground electrode 90, and is disposed on a GaN-based substrate 80. In this embodiment, the patch antenna 22 (114) is connected to the emitter layer 100 of the resonant tunneling diode element 10, but it may also be connected to the emitter layer 100 via an electrode 118. Furthermore, although a patch antenna is used as the antenna in this embodiment, the antenna is not limited to this and may also be a slot antenna, a slot ring antenna, a tapered slot antenna, a ring antenna, or the like.

[0080] 14 is a circuit diagram of the terahertz oscillator 15. The resonant tunneling diode element 10 of the oscillator 20 has a negative resistance (-crtd) and a capacitance Crtd. The patch antenna 22 has a capacitance Cant, an inductor Lant, and a resistance Gant (Glos+Grad). More specifically, a negative resistance (-crtd), a capacitance Crtd connected in parallel thereto, a capacitance Cant, and an inductor Lant constitute the circuit of the terahertz oscillator 15. When a predetermined voltage is applied to the resonant tunneling diode element 10 by the power supply circuit 30, the terahertz oscillator 15 oscillates an electromagnetic wave of a predetermined frequency f in the microwave to terahertz band, as shown in equation (1).

[0081] As described above, it is possible to fabricate a high-frequency oscillator and a receiver by combining the resonant tunneling diode 10 having the structure of this embodiment with the patch antenna 22 and the power feed circuit 30. By utilizing the negative resistance of the resonant tunneling diode 10 obtained by the present invention, it is possible to realize a high-output oscillator and a high-sensitivity receiver that operate in the microwave to terahertz bands.

[0082] The present technology can be configured as follows:

[0083] (1) A resonant tunneling diode element comprising: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein the average value of the band gap of the first spacer layer is smaller than the average value of the band gap of the emitter layer.

[0084] (2) The resonant tunneling diode element according to (1), wherein the first spacer layer is in contact with the first barrier layer.

[0085] (3) The resonant tunneling diode element according to (2), wherein a two-dimensional electron gas is generated at an interface between the first spacer layer and the first barrier layer based on a band gap of the first spacer layer and a band gap of the first barrier layer.

[0086] (4) The resonant tunneling diode element according to (3), wherein the band gap of the first spacer layer is configured to be smallest in a region in contact with the first barrier layer.

[0087] (5) The resonant tunneling diode element according to (3), wherein the band gap of the first spacer layer increases with increasing distance from the region in contact with the first barrier layer.

[0088] (6) The resonant tunneling diode element according to (1), wherein the first spacer layer has an average impurity concentration lower than an average impurity concentration of the emitter layer.

[0089] (7) The resonant tunneling diode element according to (1), wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

[0090] (8) The resonant tunneling diode element according to (7), wherein the first barrier layer and the second barrier layer of the quantum well layer are at least one of aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlInGaN), and the well layer is at least one of gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlInGaN), and aluminum indium nitride (AlInN), corresponding to the first barrier layer and the second barrier layer.

[0091] (9) The resonant tunneling diode element according to (7), wherein the emitter layer is formed of gallium nitride (GaN) doped with impurities, and the collector layer is formed of gallium nitride (GaN) doped with impurities.

[0092] (10) The resonant tunneling diode element according to (7), wherein the collector layer is formed of gallium nitride (GaN) doped with impurities.

[0093] (11) The resonant tunneling diode element according to (9), wherein the first spacer layer is made of indium gallium nitride (InGaN), and the indium (In) composition is configured to increase toward the first barrier layer.

[0094] (12) The resonant tunneling diode element according to (10), wherein a second spacer layer made of gallium nitride (GaN) is formed between the collector layer and the second barrier layer.

[0095] (13) The resonant tunneling diode element according to (1), further comprising: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, wherein the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

[0096] (14) The resonant tunneling diode element according to (13), wherein the intermediate layer is doped with an impurity.

[0097] (15) The resonant tunneling diode element according to (14), wherein the second spacer layer and the intermediate layer are in contact with each other.

[0098] (16) The resonant tunneling diode element according to (15), wherein the intermediate layer contains at least aluminum (Al) as a constituent element.

[0099] (17) The resonant tunneling diode element according to (15), wherein the intermediate layer is made of aluminum gallium nitride (AlGaN).

[0100] (18) An electronic device comprising: a resonator having a resonant tunneling diode element; and a power supply circuit that applies a voltage to the resonant tunneling diode element, wherein the resonant tunneling diode element comprises: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein an average value of the band gap of the first spacer layer is smaller than an average value of the band gap of the emitter layer.

[0101] (19) The electronic device according to (18), wherein the resonator has an antenna, and the antenna is connected to the resonant tunneling diode element.

[0102] (20) The electronic device according to (19), wherein the resonant tunneling diode element further includes: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, and the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

[0103] The aspects of the present disclosure are not limited to the individual embodiments described above, but include various modifications that may be conceived by those skilled in the art, and the effects of the present disclosure are not limited to the above-described contents. In other words, various additions, modifications, and partial deletions are possible within the scope of the conceptual idea and spirit of the present disclosure, which is derived from the contents defined in the claims and their equivalents.

[0104] REFERENCE SIGNS LIST 10 resonant tunneling diode element 15 terahertz oscillator 20 oscillator 30 power supply circuit 100 emitter layer 102, 102a, 102b first spacer layer 104 first barrier layer 106 well layer 108 second barrier layer 110 second spacer layer 112 collector layer 200 intermediate layer.

Claims

1. A resonant tunneling diode element comprising: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein the average band gap of the first spacer layer is smaller than the average band gap of the emitter layer.

2. The resonant tunneling diode device according to claim 1, wherein said first spacer layer is in contact with said first barrier layer.

3. The resonant tunneling diode element according to claim 2, wherein a two-dimensional electron gas is generated at the interface between the first spacer layer and the first barrier layer based on the band gap of the first spacer layer and the band gap of the first barrier layer.

4. The resonant tunneling diode element according to claim 3, wherein the band gap of said first spacer layer is smallest in a region in contact with said first barrier layer.

5. The resonant tunneling diode element according to claim 3, wherein the band gap of said first spacer layer increases with increasing distance from the region in contact with said first barrier layer.

6. The resonant tunneling diode element according to claim 1, wherein the average impurity concentration of said first spacer layer is configured to be lower than the average impurity concentration of said emitter layer.

7. The resonant tunneling diode element according to claim 1, wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

8. The resonant tunneling diode element according to claim 7, wherein the quantum well structure of the quantum well layer is formed by a laminated structure of aluminum nitride (AlN), gallium nitride (GaN), and aluminum nitride (AlN).

9. The resonant tunneling diode element according to claim 7, wherein the first barrier layer and the second barrier layer of the quantum well layer are at least one of aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlInGaN), and the well layer is at least one of gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlInGaN), and aluminum indium nitride (AlInN), corresponding to the first barrier layer and the second barrier layer.

10. The resonant tunneling diode element according to claim 7, wherein the emitter layer is formed of gallium nitride (GaN) doped with impurities, and the collector layer is formed of gallium nitride (GaN) doped with impurities.

11. The resonant tunneling diode element according to claim 9, wherein the first spacer layer is made of indium gallium nitride (InGaN) and the indium (In) composition increases toward the first barrier layer.

12. The resonant tunneling diode device according to claim 10, wherein a second spacer layer of gallium nitride (GaN) is formed between the collector layer and the second barrier layer.

13. The resonant tunneling diode element according to claim 1, further comprising: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, wherein the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

14. The resonant tunneling diode element according to claim 13, wherein the intermediate layer is doped with an impurity.

15. The resonant tunneling diode device according to claim 14, wherein the second spacer layer and the intermediate layer are in contact with each other.

16. The resonant tunneling diode element according to claim 15, wherein the intermediate layer contains at least aluminum (Al) as a constituent element.

17. The resonant tunneling diode device according to claim 15, wherein the intermediate layer is made of aluminum gallium nitride (AlGaN).

18. An electronic device comprising: a resonator having a resonant tunneling diode element; and a power supply circuit that applies a voltage to the resonant tunneling diode element, wherein the resonant tunneling diode element comprises: a quantum well layer stacked on a nitride semiconductor and having at least a first barrier layer, a well layer, and a second barrier layer; an emitter layer capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; and a first spacer layer stacked between the first barrier layer and the emitter layer, wherein the average value of the band gap of the first spacer layer is smaller than the average value of the band gap of the emitter layer.

19. The electronic device according to claim 18, wherein the resonator has an antenna, and the antenna is connected to the resonant tunneling diode element.

20. The electronic device according to claim 19, wherein the resonant tunneling diode element further comprises: a second spacer layer stacked between the second barrier layer and the collector layer; and an intermediate layer stacked between the second spacer layer and the collector layer, and the band gap of the intermediate layer is configured to be larger than the band gap of the second spacer layer.

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