GUN DIODE AND METHOD FOR GENERATING TERAHERTZ RADIATION

Description

[0001] Embodiments of the present invention relate to a Gunn diode, a switch for generating terahertz radiation (THz radiation) and a method for generating THz radiation, and in particular to a GaN Gunn diode with laser irradiation and field plate technology. BACKGROUND

[0002] The Gunn effect has been successfully used in GaAs- or InP-based semiconductor devices to generate high-frequency signals. These semiconductor materials exhibit material properties such as energy band distributions, charge carrier velocities, and charge carrier mobilities that trigger the Gunn effect's electron transfer.

[0003] Gunn diodes utilize this effect by using suitable circuitry (e.g., by applying a corresponding supply voltage) to cause electrons to accumulate and migrate through the diode in bursts (like waves). This, in turn, results in the generation and subsequent emission of electromagnetic waves corresponding to this frequency. Known Gunn diodes are disclosed in AE Belyaev et al. Semicond.Sci. Technol. 28 (2013) 105011 or in US 3,573,469 or in US 3,440,425 or in WO 2008 / 095639 A1. Especially for very high frequencies (e.g., in the terahertz range), known GaAs-based semiconductor components have a number of disadvantages. These are due to the fact that the saturation speed for electrons and electron transfer times are too low for these high frequencies. Thus, these semiconductor components are hardly suitable for frequencies in the terahertz range. Furthermore, the electric threshold field strength for the so-called "electron transfer effect" orthe energy band gap is too small for high output power.

[0004] On the other hand, since there is a growing demand for THz radiation sources, it is desirable to find alternatives to GaAs-based semiconductor devices. BRIEF DESCRIPTION OF THE INVENTION

[0005] At least part of the above-mentioned problems is solved by a Gunn diode according to claim 1, by a switch according to claim 8 and a method according to claim 12. The dependent claims define further advantageous embodiments for the subject matters of the independent claims. The present invention relates to a Gunn diode comprising a first contact layer, a second contact layer, an active layer, a substrate and an optical input. The active layer is based on a gallium nitride (GaN)-based semiconductor material (e.g. Al x In y Ga (1-xy) N) and is formed between the first contact layer and the second contact layer. The active layer comprising the first contact layer and the second contact layer are formed on the substrate.The optical input is configured to receive laser light to facilitate or trigger charge carrier transport between energy bands of the active layer by laser irradiation.

[0006] The GaN-based semiconductor materials used for the active layer can, in particular, comprise a binary compound semiconductor (i.e., GaN), a ternary compound semiconductor (e.g., AlGaN, InGaN), or a quaternary compound semiconductor (e.g., AlInGaN), or other compound semiconductors with even more components but containing GaN as one component. Optionally, the substrate comprises one of the following materials: gallium nitride, silicon, or silicon carbide.

[0007] Optionally, the Gunn diode further comprises an anode contact (anode electrode) and a cathode contact (cathode electrode). The first contact layer and / or the second contact layer can be more highly doped regions of the same base material than the active layer. The anode contact can optionally be formed on a rear side of the substrate such that it is (electrically) connected to the first contact layer via the substrate. The cathode contact can electrically contact the second contact layer. For example, the first contact layer and the second contact layer comprise a doping in a range from 1 × 10 18< cm -3< to 5 × 10 18< cm -3<, and the anode contact a doping of at least 10 20< cm -3<. The active layer can also have a doping to create the desired band structure. Optionally, the first contact layer can be formed by the substrate.Without limiting the invention to this, the doping can be done with silicon, for example (but many other materials are also possible).

[0008] Optionally, the Gunn diode further comprises a heat sink (active or passive) with a higher thermal conductivity than that of the substrate to form a heat sink. A thermal connection between the active layer and the heat sink can be established via the substrate.

[0009] Optionally, the Gunn diode further comprises a field plate, which in particular comprises a metal, wherein the active layer is formed between the field plate and the substrate without protruding laterally beyond the field plate. The field plate homogenizes the field in the active region of the Gunn diode. In particular, this prevents field peaks at corners and edges (e.g., at the edge of the Gunn diode). The field plate can, for example, comprise chromium or gold or another material or multiple layers thereof.

[0010] The Gunn diode further comprises a passivation layer arranged such that the active layer with the first contact layer and the second contact layer are formed between the passivation layer and the substrate.

[0011] The optical input is formed by a material transparent to the laser. At least one of the following components is formed by a material transparent to the laser: the first contact layer, the second contact layer, the passivation layer.

[0012] These layers or parts thereof can also be designed as waveguides to guide the laser radiation to the active region.

[0013] Embodiments also relate to a switch for generating terahertz radiation using a Gunn diode as described above. The switch can also include the laser (e.g., as an integral component), wherein the laser is coupled to the optical input and is configured to generate continuous laser radiation or pulsed laser radiation. Continuous or pulsed THz radiation can also be generated in this way. Likewise, a control unit for controlling the laser and / or the Gunn diode can be provided.

[0014] The laser can, for example, generate infrared or ultraviolet light. However, the laser can also operate in the visible spectral range. Optionally, the laser is pulsed and has a laser rise time in the nanosecond, picosecond, or femtosecond range to trigger the desired THz radiation.

[0015] Embodiments also relate to a terahertz radiation source with a previously described switch and an (integrated) antenna.

[0016] Embodiments also relate to a method for generating terahertz radiation. The method comprises: Applying a supply voltage to a Gunn diode as previously described; and irradiating the Gunn diode with laser radiation to initiate or assist charge carrier transfer in the active layer of the Gunn diode.

[0017] Optionally, the supply voltage can be applied continuously, for example, when the laser radiation is pulsed. The supply voltage can also have an operating frequency, for example, when continuous laser radiation is used. BRIEF DESCRIPTION OF THE CHARACTERS

[0018] The embodiments of the present invention will be better understood from the following detailed description and the accompanying drawings of the various embodiments, which, however, should not be construed as limiting the disclosure to the specific embodiments, but are for explanation and understanding only. Fig. 1 shows a Gunn diode according to an embodiment of the present invention. Fig. 2 illustrates the Gunn effect, which occurs in certain semiconductor materials and is used according to embodiments to generate THz radiation. Fig. 3 shows another embodiment of the Gunn diode with further optional components. Fig. 4 shows another embodiment of the Gunn diode. Fig. 5 shows an example of an array-wise fabrication of the Gunn diodes according to embodiments. Fig. 6 shows a flowchart for a method for generating THz radiation according to embodiments of the present invention. DETAILED DESCRIPTION

[0019] Fig. 1shows a Gunn diode according to an embodiment of the present invention. The Gunn diode comprises a first contact layer 110, a second contact layer 120, and an active layer 130 based on a gallium nitride (GaN)-based semiconductor material formed between the first contact layer 110 and the second contact layer 120. Furthermore, the Gunn diode comprises a substrate 140 on which the active layer 130 with the first contact layer 110 and the second contact layer 120 are formed, and an optical input 150 for a laser 50 to facilitate or trigger charge carrier transfer between extremes (minima for electrons; maxima for holes) of energy bands of the active layer 130 by laser irradiation.

[0020] The first contact layer 110 or the second contact layer 120 can form the anode contact or be electrically connected to it. The cathode contact is then electrically connected to the other contact layer or forms it. The active layer 130 and optionally also the contact layers 110, 120 can be formed with a more or less strong doping (p-type or n-type doping). The GaN-based semiconductor material can contain additional elements that are deliberately introduced to further enhance the effect described below.

[0021] Fig. 2illustrates the Gunn effect, which occurs in certain semiconductor materials and is used according to exemplary embodiments to generate THz radiation. The energy bands of the corresponding semiconductor materials have relative maxima and minima at a small energetic distance. Electrons (or holes), which have been excited, for example, from a valence band into a conduction band, are initially in a first minimum 210 (e.g., an absolute minimum of the so-called Γ-valley) of the conduction band. If the exemplary electrons reach an energy in an electric field that lies in the range of the energy difference between the first minimum 210 and the second (relative) minimum 220 (e.g., the so-called X-valley), they are transferred by scattering (e.g., by optical phonons) into the neighboring minimum 220 (see first transition 211). The electric field can be generated, for example, by applying a voltage. Since, for these materials, the exemplary electrons have a high effective mass in the neighboring minimum, the so-called side valley (due to the energy band curvature), they have less mobility there. For this reason, the electric current drops significantly - despite the increasing voltage. This means that a negative differential resistance results.

[0022] The transition from the Γ-valley 210 into the X-valley 220 is further facilitated according to embodiments by irradiating the Gunn diode with a laser. This gives the charge carriers in the Γ -valley 210 absorbs additional energy proportional to the frequency f of the laser radiation (E photon = h*f). This energy absorption 250 facilitates the transition into the X-valley 220. If the first transition 211 represents a transition without laser excitation, the second, third, and fourth transitions 212, 213, and 214 are facilitated by the absorbed energy 250 with increasing frequency.

[0023] Embodiments use this to trigger the transition by the laser radiation or at least to support it so that the transition is carried out for as many charge carriers as possible in a short time.

[0024] Due to the rapid current decay, these components can be used as switches. In contrast to conventional GaAs switches (e.g., GaAs photoswitches or GaAs photoconductors), the exemplary electrons are not transferred from the valence band or "deep levels" between the valence and conduction bands to the conduction band, according to the exemplary embodiments. Rather, the laser irradiation transfers the electrons from Γ -Trough 210 (first minimum) in the conduction band to satellite valley 220 (again in the conduction band).

[0025] The key advantage of this approach is that the transfer can occur in (sub-)picoseconds (< 10 ps or < 1 ps). This results in very rapid current changes, making the devices suitable for generating THz radiation.

[0026] Laser irradiation according to embodiments thus serves to trigger and / or promote the THz emission of GaN Gunn diodes. This increases stability and enables the generation of broadband THz radiation (50 GHz - several THz). Continuous laser irradiation and / or pulsed irradiation with a nanosecond, picosecond, or femtosecond cycle can be used to effect / promote the electron transfer effect (Gunn effect). Pulsed irradiation therefore offers the advantage of achieving a very fast electron transfer effect.

[0027] Fig. 3shows a further embodiment in which the layer stack of the Gunn diode (first contact layer 110, second contact layer 120, active layer 130) on the substrate 140 is contacted by a (cathode) electrode 125. The electrode 125, in turn, contacts a field plate 170, which extends over the layer stack 110, 120, 130 of the Gunn diode, so that the layer stack 110, 120, 130 is formed between the substrate 140 and the field plate 170 and is exposed to the electric field due to the applied voltage.

[0028] The electrode 125 and the field plate 170 comprise, for example, a metal (one or more layers). The field plate 170 can comprise, for example, chromium or gold. A passivation layer 160 is formed between the field plate 170 and the layer stack 110, 120, 130, which achieves electrical insulation between the field plate 170 and the layers 110, 120, 130 of the Gunn diode.

[0029] Optionally, it is possible for the passivation layer 160 to be used as an optical waveguide to guide the laser radiation along the passivation layer 160 to the layer stack 110, 120, 130 of the Gunn diode. Optionally, it is also possible for either the substrate 140 or one or more contact layers 110, 120, or parts thereof, to be transparent in order to guide the laser light along these layers. One of these layers can thus be part of the optical input 150 or represent it.

[0030] For the generation of THz radiation, it is particularly advantageous that nitride materials are suitable for much higher frequencies and power levels. These materials can achieve the following effects, for example: a high saturation velocity for electrons (for GaN > 2 times higher than in GaAs), much higher electric threshold field strength for the so-called "electron transfer effect" (for GaN > 50 times higher than in GaAs), a large energy band gap, while signal sources based on GaAs and InP devices each have cutoff frequencies of 100 GHz and 200 GHz (for the fundamental mode), respectively, the calculated limits for GaN diodes, for example, are over 700 GHz.

[0031] There are many materials suitable for substrate 140. While Gunn diodes on sapphire substrates are possible, they lead to various effects and problems. These include, for example, the occurrence of electromigration effects and high series resistances. Furthermore, the low thermal conductivity of sapphire often makes the implementation of heat sinks difficult. This leads to high DC losses and reduces reliability. More suitable substrates (e.g., due to their good thermal conductivity) are GaN, SiC, or silicon. Particularly in combination with field plate 170, stable negative differential resistances can be achieved.

[0032] Fig. 4 shows an embodiment of the present invention in which the active layer 130 is formed directly on the substrate 140 and the substrate 140 itself or regions thereof act as the first contact layer 110. In addition, in the embodiment of the Fig. 4 An exemplary anode electrode 115 is provided below the substrate 140 (opposite the active layer 130). The current is conducted through the substrate 140 to the Gunn diode. Likewise, an exemplary cathode electrode 125 is formed on the second contact layer 120. Above the cathode electrode 125 and the protruding areas of the substrate 140, as shown in the Fig. 3 also, a passivation layer 160 is formed. In this embodiment, no field plate is provided, but it can also be formed above the passivation layer 160.

[0033] The embodiment of the Fig. 4further comprises an optional heat sink 180, which is formed laterally adjacent to the substrate 140 and represents a cooling sink. The heat sink 180 can provide active cooling (e.g., a fan or a Peltier element) or passive cooling (e.g., cooling fins). The heat flow occurs via the substrate 140 to the Gunn diode or to the active layer 130. For this purpose, it is particularly advantageous if the substrate 140, according to embodiments, comprises GaN, Si, or SiC (and not sapphire).

[0034] Fig. 5 shows an example of the production of the Gunn diode from an array of Gunn diodes on a single substrate. This example shows many separate field plates 170 formed on the underlying substrate 140. The Gunn diodes themselves each extend as a raised portion from the substrate 140.

[0035] Fig. 6shows a flowchart for a method for generating terahertz radiation. The method comprises the following steps: Applying S110 a supply voltage to a Gunn diode as previously described; and irradiating S120 the Gunn diodes with laser radiation to initiate or assist a charge carrier transfer in the active layer 130 of the Gunn diode.

[0036] The supply voltage can be constant or clocked at an operating frequency. According to embodiments, it is also possible for the Gunn effect to be triggered by pulsed laser radiation, in which case the supply voltage can be applied continuously.

[0037] The advantages of implementation examples can be summarized as follows: Compared to conventional GaAs switches, much larger current changes can be switched stably and quickly. Since the output power is proportional to the square of the current change (P ~ Δ I 2< ), high-power THz radiation can be generated. Furthermore, broadband THz radiation can be generated (bandwidths from 50 GHz to several THz), with an integrated broadband antenna sufficient for THz radiation. Field-plate technology enables a uniform supply to the Gunn diode because parasitic electric field peaks at the diode edge and electromigration effects are minimized. Rapid component destruction due to electromigration effects from the anode to the cathode at high field strengths can be avoided, thus significantly increasing reliability. Field-plate technology on the diode allows the required high electric field strengths to be applied to the Gunn diode more stably.GaN Gunn diodes on substrates with better thermal conductivity, such as GaN, Si, and SiC (higher than on sapphire), can be easily manufactured. The high power dissipation typical of Gunn diodes can thus be more effectively dissipated. Gunn diodes on the new substrates (GaN, Si, SiC) exhibit a stable current-voltage characteristic with a well-developed negative differential resistance. Due to the high thermal conductivity, heat sinks with larger external contacts can be better implemented. Field strengths much higher than the threshold field strength can be used. The use or integration with the appropriate resonator, as well as appropriate biasing and signal coupling devices, can lead to the generation of millimeter-wave frequencies up to high THz frequencies with high power levels.

[0038] Advantages of laser irradiation: With continuous laser irradiation (infrared to ultraviolet), the electrons in the conduction band can have higher energy and more easily undergo the electron transfer effect (Gunn effect). This reduces the required operating voltage of the Gunn diode. THz generation is facilitated and stabilized, but the output power is reduced. With pulsed laser irradiation (infrared to ultraviolet) in the nanosecond range, heat generation can be significantly reduced. Again, THz generation is facilitated and stabilized. GaN Gunn diodes can be powered by a specially manufactured high-voltage source with pulse widths of 40 nanoseconds. The supply voltage can be applied continuously during pulsed laser irradiation.

[0039] Both methods stabilize the original GaN Gunn diode, and the operating frequency can be set using an external resonator. The pulsed lasers can be used with a laser rise time in the picosecond or femtosecond range, enabling a very fast electron transfer effect.

[0040] Embodiments can thus be used in particular for THz switches which, similar to THz switches based on GaAs photoswitches or GaAs photoconductors, generate THz radiation through rapid current changes.

[0041] Since embodiments for the fabrication and use of GaN Gunn diodes enable the generation of extremely high THz frequencies and high output powers (much higher than for GaAs and InP Gunn diodes), a wide range of imaging and spectroscopic applications in the THz frequency range become possible.

[0042] The features of the invention disclosed in the description, the claims and the figures may be essential for the realization of the invention both individually and in any combination. LIST OF REFERENCE SYMBOLS

[0043] 50Laser 110, 120Contact layers 115, 125Electrode(s) (cathode contact / anode contact) 130Active layer 140Substrate 150Optical input 160Passivation 170Field plate 180Heat sink 210, 220Conduction band minima 211, 212, ...Transitions between the conduction band minima

Claims

1. A Gunn diode comprising: a first contact layer (110) and a second contact layer (120); an active layer (130), based on a gallium nitride (GaN) semiconductor material, which is formed between the first contact layer (110) and the second contact layer (120); a substrate (140) on which the active layer (130) is formed together with the first contact layer (110) and the second contact layer (120); and an optical input (150) for a laser (50) in order to facilitate or trigger a charge carrier transfer between extrema (210, 220) of the energy bands of the active layer (140) by means of laser irradiation, wherein the optical input (150) is formed by a material that is transparent to the laser for at least one of the following components: - the first contact layer (110), - the second contact layer (120), - a passivation layer (160), wherein the passivation layer (160) is arranged such that the active layer (130) is formed, together with the first contact layer (110) and the second contact layer (120), between the passivation layer (160) and the substrate (140).

2. The Gunn diode according to claim 1, wherein the substrate comprises one of the following materials: gallium nitride, silicon, silicon carbide.

3. The Gunn diode according to claim 1 or claim 2, further comprising an anode contact and a cathode contact, wherein the first contact layer (110) and / or the second contact layer are, compared to the active layer (130), more highly doped regions of the same base material, and the anode contact is electrically connected to the first contact layer (110) via the substrate (140) and the cathode contact is electrically connected to the second contact layer, and wherein the first contact layer (110) and the second contact layer (120) have a doping in a range of 1 x 1018 cm-3 up to 5 x 1018 cm-3 and the anode contact has a doping of at least 1020 cm-3.

4. The Gunn diode according to any of the preceding claims, in which the first contact layer (110) is formed by the substrate (140).

5. The Gunn diode according to any of the preceding claims, further comprising a heat sink (180) which has a higher thermal conductivity than that of the substrate (140).

6. The Gunn diode according to claim 5, in which a thermal connection of the active layer (130) to the heat sink (180) is established via the substrate (140).

7. The Gunn diode according to any of the preceding claims, further comprising a magnetoresistor (170), in particular made of a metal, wherein the active layer (130) is formed between the magnetoresistor (170) and the substrate (140) without projecting laterally beyond the magnetoresistor (170).

8. A switch for generating terahertz radiation, comprising a Gunn diode according to any of the preceding claims.

9. The switch according to claim 8, further comprising the laser (50) which can be coupled to the optical input (150) and is designed to generate continuous laser radiation or pulsed laser radiation.

10. The switch according to claim 9, wherein the pulsed laser (50) has a laser ramp-up time in the nanosecond range, picosecond range, or femtosecond range.

11. A terahertz radiation source comprising a switch according to any of claims 8 to 10 and an integrated antenna.

12. A method for generating terahertz radiation, comprising: applying (S110) a supply voltage to a Gunn diode according to any of claims 1 to 7; and irradiating (S120) the Gunn diode with laser radiation in order to trigger or support a charge carrier transfer in the active layer (130) of the Gunn diode.

13. The method according to claim 12, wherein the supply voltage is applied permanently if the laser radiation is pulsed, or is applied at an operating frequency if it is continuous laser radiation.