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; a first spacer layer laminated between the first barrier layer and the emitter layer; a second spacer layer laminated between the second barrier layer and the collector layer; and an intermediate layer laminated between the second spacer layer and the collector layer. The bandgap of the intermediate layer is set to be greater than the bandgap of the second spacer 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 GaN-based materials, it is possible to form well / barrier 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 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 oscillation element that can suppress a decrease in negative conductance.

[0006] In order to solve the above 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 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; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

[0007] The intermediate layer may be doped with impurities.

[0008] The second spacer layer and the intermediate layer may be in contact with each other.

[0009] The intermediate layer may contain at least aluminum (Al) as a constituent element.

[0010] The intermediate layer may be made of aluminum gallium nitride (AlGaN).

[0011] 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).

[0012] 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.

[0013] 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.

[0014] The second spacer layer may be formed of gallium nitride (GaN).

[0015] The intermediate layer may be made of AlxGa1-xN (0<x<1), and the second spacer layer may be made of AlyGa1-yN (0≦y≦0.5), where x>y.

[0016] The intermediate layer may be made of aluminum gallium nitride (AlGaN), and may be configured so that the aluminum (Al) composition increases toward the second barrier layer.

[0017] The first spacer layer may have a smaller average bandgap than the emitter layer.

[0018] The first spacer layer may be in contact with the first barrier layer.

[0019] 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.

[0020] At least a portion of the first spacer layer may be made of a nitride semiconductor containing indium (In).

[0021] 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 capable of applying a potential to the first barrier layer; a collector layer capable of applying a potential to the second barrier layer; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

[0022] The resonator may include an antenna, and the antenna may be connected to the resonant tunneling diode element.

[0023] The first spacer layer may have a smaller average bandgap than the emitter layer.

[0024] The first spacer layer may be in contact with the first barrier layer.

[0025] 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.

[0026] At least a portion of the first spacer layer may be made of a nitride semiconductor containing indium (In).

[0027] 13A and 13B are cross-sectional views each showing a schematic example of a resonant tunneling diode element; a cross-sectional view each showing a schematic example of a resonant tunneling diode element according to a comparative example; a diagram showing a conduction band structure of a resonant tunneling diode element when no bias is applied; a diagram showing a relationship between the carrier density according to the present embodiment and the carrier density according to a comparative example; a diagram showing a relationship between the carrier density according to the present embodiment and the carrier density according to a comparative example; a diagram showing a relationship between the film thickness of AlGaN of the intermediate layer and the carrier density; a diagram showing a relationship between the Al composition and the carrier density of the intermediate layer; a diagram showing a conduction band structure when a forward bias is applied; a diagram showing voltage-current characteristics of a resonant tunneling diode element; a diagram showing a conduction band structure when the Al composition decreases with increasing distance from the second barrier layer; a cross-sectional view each showing a schematic example of a resonant tunneling diode element according to a second embodiment; a diagram showing a conduction band structure when no bias is applied; a perspective view showing a configuration example of a terahertz oscillator; a cross-sectional view taken along line AA' of FIG. 13A; a circuit configuration diagram of a terahertz oscillator.

[0028] 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.

[0029] 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, an intermediate layer 112, and a collector layer 114 are layered, in this order from the bottom, on a nitride semiconductor having a C-plane as its principal surface orientation.

[0030] 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).

[0031] 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.

[0032] 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.

[0033] Furthermore, an intermediate layer 112 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 114 can be used as the collector layer 114. The n-GaN collector layer 114 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.

[0034] The intermediate layer 112 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 this intermediate layer 112. The intermediate layer 112 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 114. For example, the intermediate layer 112 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 114, respectively, and a bias voltage can be applied between the emitter layer 100 and the collector layer 114.

[0035] 2 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 102, a first barrier layer 104, a well layer 106, a second barrier layer 108, a second spacer layer 110, and a collector layer 114 are stacked on a nitride semiconductor having a C-plane as its principal surface orientation. That is, 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 the intermediate layer 112.

[0036] 3 shows the conduction band structure of the resonant tunneling diode device 10a according to the comparative example when no bias is applied. The horizontal axis indicates the distance from the end of the upper surface of the collector layer 114, and the vertical axis indicates 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 indicates the Fermi level E Ｆ Shows.

[0037] On the other hand, the conduction band structure Ec20 of the GaN-based RTD element 10 according to the comparative example 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 tunneling current to flow than in the conduction band structure Ec10 without spontaneous polarization and piezoelectric polarization. In this embodiment, in the conduction band structure, a potential change in which the band gap increases as the distance to the quantum well structure decreases may be referred to as positive, and a potential change in which the band gap decreases as the distance to the quantum well structure decreases may be referred to as negative.

[0038] 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 RTD 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.

[0039] The resonant tunneling current passing through the quantum well structure increases as the tunneling probability of electrons passing through the quantum well structure increases. To increase this tunneling probability, it is effective to reduce the effective barrier layer thickness due to the positive triangular potential on the second spacer layer 110 side.

[0040] 4 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 114, and the vertical axis represents energy. In the RTD element 10 according to this embodiment, the intermediate layer 112 is provided to adjust the band gap of the first spacer layer 102 and change the positive triangular potential profile. For example, in the RTD element 10 according to this embodiment, as described above, the intermediate layer 112 is formed of a material with a larger band gap than the second spacer layer 110.

[0041] A two-dimensional electron gas (2DEGb) can be formed at the interface between the intermediate layer 112 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 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.

[0042] 5 is a diagram showing the conduction band structure Ec30 when the Al composition of the AlGaN in the intermediate layer 112 is 20%. The horizontal axis represents the distance from the end of the upper surface of the collector layer 114, and the vertical axis represents energy. The intermediate layer 112 is doped with, for example, Si as an impurity, with a concentration of, for example, 1E+19 (cm-3). FIG. 5 also shows the conduction band structure Ec40 of u-Al0.2GaN.

[0043] As described above, in contrast to the conduction band structure Ec20 when the intermediate layer 112 is not introduced, the conduction band structure Ec30 when doped results in two-dimensional electron gas (2DEGb) being formed at the interface with the second spacer layer 110, lowering the potential of the second spacer layer 110. On the other hand, in the conduction band structure Ec40 when undoped, the potential is raised, and the effect of thinning the effective barrier layer thickness due to the positive triangular potential on the second spacer layer 110 side cannot be obtained.

[0044] As can be seen from this, doping into the intermediate layer 112 is necessary to reduce the effective barrier layer thickness. The higher the doping concentration into the intermediate layer 112, the greater the effect that can be obtained. However, high doping concentrations pose the risk of crystal destruction and impurity diffusion into the well / barrier layer, so it is considered appropriate to limit the doping concentration to 1E+20 (cm-3) or less.

[0045] 6A and 6B are diagrams showing the relationship between the thickness of the AlGaN of the intermediate layer 112 according to this embodiment and the carrier density. For example, the carrier density is a current density. FIG. 6A shows the conduction band structure Ec30 when the Al composition of AlGaN is 20% and the film thickness is 5 nm, FIG. 6B shows the conduction band structure Ec30 when the film thickness is 10 nm, and FIG. 6C shows the conduction band structure Ec30 when the film thickness is 10 nm. The horizontal axis represents the distance from the end of the upper surface of the collector layer 114, and the vertical axis represents energy.

[0046] 6(d), (e), and (f) show the carrier densities in FIGS. 6(a), (b), and (c), respectively. The horizontal axis represents the distance from the end of the upper surface of the collector layer 114, and the vertical axis represents the carrier density. As can be seen from FIG. 6, the thickness of the intermediate layer 112 has little effect on the potential profile, and even a thickness of about 5 nm can achieve the same effect as a thickness of 10 nm. Conversely, if the thickness of the intermediate layer 112 is made too thick, lattice mismatch with GaN may cause crystal damage such as cracks, and therefore a thickness of about 5 nm to 20 nm is considered desirable.

[0047] 7A and 7B are graphs showing the relationship between the Al composition and carrier density of AlGaN in the intermediate layer 112 according to this embodiment. The thickness of the intermediate layer 112 is fixed at 10 nm. In FIG. 7A, the Al composition of AlGaN is 10%, in FIG. 7B, the Al composition of AlGaN is 15%, in FIG. 7C, the Al composition of AlGaN is 20%, and in FIG. 7D, the Al composition of AlGaN is 30%. The horizontal axis represents the distance from the end of the upper surface of the collector layer 114, and the vertical axis represents energy.

[0048] 7(e), (f), (g), and (h) show the carrier densities in FIGS. 7(a), (b), (c), and (d), respectively. The horizontal axis represents the distance from the top edge of the collector layer 114, and the vertical axis represents the carrier density. As can be seen from FIG. 7, the higher the Al composition, the higher the carrier density I2degb of the 2DEGb formed, and the stronger the effect of lowering the potential of the second spacer layer 110. On the other hand, at a composition of 10%, two-dimensional electron gas (2DEGb) is not formed, but the effect of lowering the potential is obtained, and it is presumed that the effects of the invention are achieved. Conversely, if the Al composition is too high, the lattice mismatch with GaN becomes large, which may lead to crystal damage such as cracks. Therefore, it is considered desirable to design the composition at approximately 10% to 30%.

[0049] Furthermore, in order to adjust the carrier density I2degb of 2DEGb, the intermediate layer 112 may be made of AlxGa1-xN (0<x<1) and the second spacer layer 110 may be made of AlyGa1-yN (0≦y≦0.5), where x>y. This allows the band gap of the intermediate layer 112 to be larger than the band gap of the second spacer layer 110, thereby lowering the potential of the second spacer layer 110.

[0050] Here, the operating characteristics of the resonant tunneling diode element 10 according to this embodiment will be described with reference to FIGS. 8 and 9. FIG. 8 is a diagram showing the conduction band structure Ec30 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 energy. FIG. 8(a) shows the case with no bias, FIG. 8(b) shows the case with a forward bias of 3.5 volts (V), and FIG. 8(c) shows the case with a forward bias of 5 volts (V). These are simulation results.

[0051] 9 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] 8(a) to 8(c) and 9, 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 described with reference to FIG. 7 , the peak current at 3.4 volts can be increased by increasing the Al composition of AlGaN.

[0054] As described above, according to this embodiment, the band gap of the intermediate layer 112 is configured to be larger than the band gap of the second spacer layer 110. This allows two-dimensional electron gas (2DEGb) to be formed at the interface between the intermediate layer 112 and the second spacer layer 110. This two-dimensional electron gas suppresses the rise in potential of the second spacer layer 110, reducing the effective barrier layer thickness and increasing the electron tunneling probability. Furthermore, since the potential of the well layer 106 can be simultaneously lowered, the operating voltage of the resonant tunneling diode device 10 can be further reduced.

[0055] (Modification of First Embodiment) The resonant tunneling diode element 10 according to the modification of the first embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that the Al composition of AlGaN in the intermediate layer 112 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 Al composition of AlGaN in the intermediate layer 112 decreases to 30%, 20%, and 10% with increasing distance from the second barrier layer 108 .

[0057] 10 is a diagram showing the conduction band structure Ec30 of a three-layer structure in which the Al composition of the AlGaN intermediate layer 112 decreases from 30%, 20%, to 10% with increasing distance from the second barrier layer 108. The horizontal axis represents the distance from the end of the upper surface of the collector layer 114, and the vertical axis represents energy. The intermediate layer 112 is doped with, for example, Si as an impurity, with a concentration of, for example, 1E+19 (cm-3). FIG. 10 also shows the conduction band structure Ec40 of u-Al0.2GaN.

[0058] As shown in Fig. 10, the average Al composition of the structure of the intermediate layer 112 is 20%, but the same effect as a single film with an Al composition of 20% can be obtained. In this way, by decreasing the Al composition of AlGaN to 30%, 20%, and 10% with increasing distance from the second barrier layer 108, it is possible to adjust the lattice constant while maintaining the shape of the barrier layer with a conduction band structure Ec30. Furthermore, instead of the stepwise change in Al composition shown in Fig. 10, a continuous change in Al composition is also possible.

[0059] Second Embodiment The first spacer layer 102 of 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 it further comprises a first spacer layer 102b made of u-InGaN. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0060] 11 is a cross-sectional view schematically illustrating an example of a resonant tunneling diode element 10 according to a second embodiment of the present disclosure. As shown in Fig. 11, 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, an intermediate layer 112, and a collector layer 114 are layered, in this order from the bottom, on a nitride semiconductor having a C-plane as its principal surface orientation.

[0061] The first spacer layer 102 is configured so that the average bandgap of the first spacer layer 102 is smaller than the average bandgap of the emitter layer 100 .

[0062] More specifically, 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 u-GaN spacer layer. The first spacer layer 102b is adjacent to the first barrier layer 104 and is, for example, a u-InGaN spacer layer. That is, the first spacer layer 102b is made of indium gallium nitride (InGaN). Thus, the first spacer layer 102 is made of a nitride semiconductor. 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] Furthermore, an intermediate layer 112 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 114 can be used as the collector layer 114. The n-GaN collector layer 114 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.

[0067] The intermediate layer 112 is formed of a material with a larger band gap than the second spacer layer 110. For example, AlGaN is used for this intermediate layer 112. The intermediate layer 112 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 114. For example, the intermediate layer 112 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 114, respectively, and a bias voltage can be applied between the emitter layer 100 and the collector layer 114.

[0068] 12 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 114, and the vertical axis represents energy. In the resonant tunneling diode element 10 according to this embodiment, 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 boundary between the first spacer layer 102 and the first barrier layer 104. This makes it possible to increase the carrier concentration on the emitter side of the first barrier layer 104, thereby increasing the tunneling probability of electrons passing through the quantum well structure.

[0069] As described above, the resonant tunneling diode element 10 according to this embodiment is configured so as to reduce the band gap of 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 between the first spacer layer 102 and the first barrier layer 104, thereby further suppressing a decrease in the negative conductance of the resonant tunneling diode element 10.

[0070] Third Embodiment The resonant tunneling diode element 10 according to the fourth embodiment differs from the resonant tunneling diode element 10 according to the first embodiment in that an oscillator is configured by connecting the resonant tunneling diode element 10 according to the fourth embodiment to an external resonator. The differences from the resonant tunneling diode element 10 according to the first embodiment will be described below.

[0071] FIG. 13 is a perspective view showing an example of the configuration of a terahertz oscillator 15. FIG. 14 is a cross section taken along the line AA′ in FIG. 13. As shown in FIG. 13, 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.

[0072] The power supply circuit 30 is a circuit that applies a DC voltage to the emitter layer 100 and the collector layer 114 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.

[0073] 14 , 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. 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. In addition, 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.

[0074] 15 is a circuit diagram of a 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).

[0075] 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.

[0076] The present technology can be configured as follows:

[0077] (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; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

[0078] (2) The resonant tunneling diode element according to (1), wherein the intermediate layer is doped with an impurity.

[0079] (3) The resonant tunneling diode element according to (1), wherein the second spacer layer and the intermediate layer are in contact with each other.

[0080] The resonant tunneling diode element according to (1), wherein the intermediate layer contains at least aluminum (Al) as a constituent element.

[0081] (4) The resonant tunneling diode element according to (2), wherein the intermediate layer is made of aluminum gallium nitride (AlGaN).

[0082] (5) The resonant tunneling diode element according to (4), wherein the quantum well structure of the quantum well layer is formed by a stacked structure of aluminum nitride (AlN), gallium nitride (GaN), and aluminum nitride (AlN).

[0083] (6) The resonant tunneling diode element according to (4), 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.

[0084] (7) The resonant tunneling diode element according to (5), 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.

[0085] (8) The resonant tunneling diode element according to (7), wherein the second spacer layer is made of gallium nitride (GaN).

[0086] (9) The resonant tunneling diode element according to (1), wherein the intermediate layer is made of AlxGa1-xN (0<x<1), the second spacer layer is made of AlyGa1-yN (0≦y≦0.5), and x>y.

[0087] (10) The resonant tunneling diode element according to (1), wherein the intermediate layer is made of aluminum gallium nitride (AlGaN) and the aluminum (Al) composition is increased toward the second barrier layer.

[0088] (11) The resonant tunneling diode element according to (1), 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.

[0089] (12) The resonant tunneling diode element according to (11), wherein the first spacer layer is in contact with the first barrier layer.

[0090] (13) The resonant tunneling diode element according to (12), 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.

[0091] (14) The resonant tunneling diode element according to (13), wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

[0092] (15) 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; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

[0093] (16) The electronic device according to (15), wherein the resonator has an antenna, and the antenna is connected to the resonant tunneling diode element.

[0094] (17) The electronic device according to (16), 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.

[0095] (18) The electronic device according to (17), wherein the first spacer layer is in contact with the first barrier layer.

[0096] (19) The electronic device according to (18), 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.

[0097] (20) The electronic device according to (19), wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

[0098] 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.

[0099] 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 intermediate layer 114 collector 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 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; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

2. The resonant tunneling diode element according to claim 1, wherein the intermediate layer is doped with an impurity.

3. The resonant tunneling diode element according to claim 1, wherein the second spacer layer and the intermediate layer are in contact with each other, and the intermediate layer contains at least aluminum (Al) as a constituent element.

4. The resonant tunneling diode device according to claim 2, wherein the intermediate layer is made of aluminum gallium nitride (AlGaN).

5. The resonant tunneling diode element according to claim 4, 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).

6. The resonant tunneling diode element according to claim 4, 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.

7. The resonant tunneling diode element according to claim 5, 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.

8. The resonant tunneling diode device according to claim 7, wherein the second spacer layer is formed of gallium nitride (GaN).

9. The resonant tunneling diode device according to claim 1, wherein the intermediate layer is made of AlxGa1-xN (0<x<1), the second spacer layer is made of AlyGa1-yN (0≦y≦0.5), and x>y.

10. The resonant tunneling diode element according to claim 1, wherein the intermediate layer is made of aluminum gallium nitride (AlGaN) and the aluminum (Al) composition increases toward the second barrier layer.

11. The resonant tunneling diode device according to claim 1, wherein the average value of the band gap of said first spacer layer is smaller than the average value of the band gap of said emitter layer.

12. The resonant tunneling diode device according to claim 11, wherein the first spacer layer is in contact with the first barrier layer.

13. The resonant tunneling diode element according to claim 12, 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.

14. The resonant tunneling diode element according to claim 13, wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

15. 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; a first spacer layer stacked between the first barrier layer and the emitter layer; 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.

16. The electronic device according to claim 15, wherein the resonator has an antenna, and the antenna is connected to the resonant tunneling diode element.

17. The electronic device according to claim 16, wherein the average band gap of the first spacer layer is configured to be smaller than the average band gap of the emitter layer.

18. The electronic device of claim 17, wherein the first spacer layer is in contact with the first barrier layer.

19. The electronic device of claim 18, wherein a two-dimensional electron gas based on the band gap of the first spacer layer and the band gap of the first barrier layer is generated at the interface between the first spacer layer and the first barrier layer.

20. The electronic device according to claim 19, wherein at least a portion of the first spacer layer is made of a nitride semiconductor containing indium (In).

Images