The prospects of wurtzite phase single-drift-region (SDR), flat and single-low-high-low (SLHL) type GaN IMPATT devices as terahertz sources are studied through a simulation experiment. The study indicates that GaN IMPATT diodes are capable of generating high RF power (at least 2.5 W) at around 1.45 THz with high efficiency (17–20%). The superior electronic properties of GaN make this a promising candidate for IMPATT operation in the THz regime, unapproachable by conventional Si, GaAs and InP based IMPATT diodes. The effect of parasitic series resistance on the THz performance of the device is further simulated. It is interesting to note that the presence of a charge bump in a flatly doped SDR structure reduces the value of parasitic series resistance by 22%. The effects of photo- illumination on the devices are also investigated using a modified double iterative simulation technique. Under photo-illumination (i) the negative conductance and (ii) the negative resistance of the devices (both flat and SLHL) decrease, while the frequency of operation and the device quality factor shift upwards. However, the upward shift in operating frequency is found to be more (~16 GHz) in the case of the SLHL SDR IMPATT device. The study indicates that GaN IMPATT is a promising opto-sensitive high power THz source.

http://iopscience.iop.org/article/10.1088/0268-1242/22/12/003/pdf

GaN IMPATT diode: a photo-sensitive high power terahertz source

Reliability of terahertz-frequency (~1.0 THz) characteristics of wide-bandgap (WBG) wurtzite (Wz)-GaN- and 4H-SiC-based p++nn++-type single-drift-region (SDR) impact avalanche transit time (IMPATT) devices (normal and photoilluminated) is compared through a simulation scheme. The simulation experiment reveals that an RF power density of 3.37 times 1011 W middotm-2 (efficiency of 18.2%) at around 1.126 THz may be realized from the optimized unilluminated GaN IMPATT device, whereas the unilluminated 4H-SiC IMPATT device is expected to generate an RF power density of 1.35 times 1011 W middotm-2 (efficiency of 9%) at 1.05 THz. However, the parasitic series resistance reduces the maximum exploitable power density from the terahertz devices. Under optical illumination, additional photogenerated carriers are created in the devices, and these carriers change the admittance and negative resistance properties of the terahertz IMPATT diodes. The performance modulation of the terahertz devices is simulated, and the results are compared in this paper. Under external radiation, the operating frequencies of the GaN- and SiC-based diodes are found to shift upward by 6.0 and 40.0 GHz, respectively, with degradation of maximum output-power density level and device negative resistance. The extensive simulation experiments establish that, although the photosensitivity of the 4H-SiC-based IMPATT device is better than its GaN counterpart, the overall terahertz performance of the unilluminated GaN IMPATT device is far better than the 4H-SiC-based device, particularly in terms of output power and efficiency. The simulation results and the proposed experimental methodology presented here can be used for realizing optically tuned WBG IMPATT oscillators for terahertz communication.

Photosensitivity Analysis of Gallium Nitride and Silicon Carbide Terahertz IMPATT Oscillators: Comparison of Theoretical Reliability and Study on Experimental Feasibility

A generalized method of DC and high-frequency analysis for microwave transit time diodes in mixed tunnelling and avalanche mode, which can be applied to any type of diode structure is reported. Taking a purely field-dependent tunnel generation rate for electrons, the same is computed for holes from a simulated energy band diagram within the depletion layer of the diode. The method has been applied to a variety of Si, GaAs and InP diode structures. The results show a substantial degradation of IMPATT properties due to phase distortion caused by the tunnelling current.

https://iopscience.iop.org/article/10.1088/0268-1242/7/2/008/pdf

A generalized simulation method for MITATT-mode operation and studies on the influence of tunnel current on IMPATT properties

The results of accurate and realistic high-frequency numerical analysis of a silicon DAR (double avalanche region) diode indicate some unique and useful microwave characteristics. The DAR diode under any structural condition exhibits multiband microwave negative resistance characteristics between 8 and 350 GHz which would make it possible to realize wide-band microwave oscillations (8 to 350 GHz) from any single DAR diode with a multituning facility. The negative resistance space distribution profiles of the diode at high frequencies of operation shows near sinusoidal variation, which suggests that a DAR diode can have several optimum diode widths for generation of a particular frequency. The results have been explained on the basis of computation of the total avalanche delay produced in both avalanche regions and transmit time delay produced in the drift region.

Computer-aided studies on the wide-band microwave characteristics of a silicon double avalanche region (DAR) diode

Theoretical and experimental investigations have been carried out for V-band (50–75 GHz) silicon double drift flat profile (DD) and double low high low (DLHL) IMPATT diodes. The theoretical designs have been used for the experimental realisation of the diodes for CW operation. The epitaxial layers were grown by silicon molecular beam epitaxy which enabled the realisation of the complex DLHL profile at millimetre-wave frequencies in the silicon material system for the first time. The maximum obtained conversion efficiency is 14.3%. A comparison of theoretical and experimental results for both types of diodes shows general agreement and the superiority of the DLHL structure.

Comparison of theoretical and experimental 60 GHz silicon IMPATT diode performance

Computer studies have been made on the high frequency characteristics of symmetrically doped flat profile double drift region (DDR) InP and GaAs impatts in the mm-wave (60–100 GHz) frequency band The spatial distribution of high frequency negative resistance and reactance in the depletion layer of DDR devices and their admittance properties have been investigated The results indicate that DDR InP impatts should have higher drift zone voltage drop, higher negative resistance and higher negative conductance compared to their GaAs counterparts designed for the same range of mm-wave frequencies Furthermore, the negative resistance peak is larger in magnitude for InP impatts compared to that for GaAs impatts It is thus observed that DDR InP impatts should be superior to their GaAs counterparts as regards mm-wave power generation with high conversion efficiency

High frequency characterisation of double drift region InP and GaAs diodes

Computer studies are carried out on the effect of electron and hole reverse saturation currents on i) the negative resistance profiles, ii) the admittance characteristics, and iii) device quality factor (Q) of silicon n+pp+ and GaAs p+nn+ IMPATTs operating in the Ku-band frequencies. The results show that the diode negative resistance decreases first appreciably and then increases again in the case of GaAs p+nn+ diode with the increase of electron reverse saturation current i.e., with the decrease in electron multiplication factor. In the case of silicon n+pp+ diode, the negative resistance increases first and then starts decreasing with the lowering of the hole multiplication factor. It is further observed that for both, Si and GaAs diodes the frequency of operation shifts upwards and the device Q factor increases gradually while the device negative conductance decreases with increasing reverse saturation current.



Es werden Computeruntersuchungen uber den Einflus von Elektronen- und Lochersattigungssperrstrome auf 1) die Profile des negativen Widerstands, 2) der Admittanzcharakteristiken und 3) den Gutefaktor (Q) von Silizium-n+pp+- und GaAs-p+nn+- und GaAs-p+nn+-IMPATTs, die bei Ku-Bandfrequenzen arbeiten, durchgefuhrt. Die Ergebnisse zeigen, das der negative Diodenwiderstand erst betrachtlich abnimmt und dann im Falle der GaAs-p+nn+-Diode mit dem Anwachsen des Elektronensattigungssperrstromes, d. h. mit dem Abfall des Elektronenmultiplikationsfaktors wieder ansteigt. Im Falle der Silizium n+pp+-Diode steigt der negative Widerstand zuerst an und beginnt dann mit der Verringerung des lochermultiplikationsfaktors abzunehmen. Es wird weiter beobachtet, das sowohl fur Si- als auch GaAs-Dioden die Arbeitsfrequenz sich aufwarts verschiebt und der Gutefaktor Q graduell zunimmt, wahrend der negative Bauelementeleitwert mit wachsendem Sattigungssperrstrom abnimmt.

Variation of high frequency negative resistance of silicon N+pp+ and GaAs p+nn+ IMPATT diodes with enhancement of reverse saturation current

The role of electron and hole saturation currents in controlling the millimeter-wave (mm-wave) properties of silicon and GaAs double drift region (DDR) high-efficiency (low-high-low type) IMPATTs is investigated through computer simulation. The results indicate that with increasing saturation current (due to electrons/holes), the device negative conductance as well as the device negative resistance decrease, while the device quality factor increases. It is also observed that a 15 GHz upward shift in the optimum frequency from a normal oscillation frequency around 100 GHz is possible by the enhancement of the saturation current in the high-efficiency (low-high-low ) silicon and GaAs diodes. The frequency shift with saturation current is found to be much larger in high-efficiency (low-high-low) IMPATTs compared to their flat profile counterparts.

Control of Millimeter-Wave Properties of High-Efficiency Double Drift Region IMPATTs through Enhancement of Saturation Current

Computer simulation of the high frequency properties of flat profile double drift region (DDR) impatts based on indium phosphide is carried out for mm-wave V-band (≈ 60 GHz) frequency. The profiles of negative resistance in the depletion layer of the DDR diode and their admittance characteristics are analysed for three current levels. The results show that the total negative resistance which is an useful parameter to indicate the output power from the device does not degrade appreciably due to mobile space charge even if a high current viz. 3 × 108 Am−2 is pushed through these devices.



Computersimulation der Hochfrequenzeigenschaften von Doppeldriftbereich (DDR)-Impattdioden mit flachem Profil auf der Grundlage von Indiumphosphid wird fur das mm-Wellen-V-Band (≈ 60 GHz) durchgefuhrt. Die Profile des negativen Widerstands in der Raumladungsschicht der DDR-Diode und ihre Admittanzcharakteristiken werden fur drei Stromniveaus analysiert. Die Ergebnisse zeigen, das der gesamte negative Widerstand, der ein nutzlicher Parameter fur die Ausgangsleistung des Bauelements ist, durch bewegliche Raumladung nicht wesentlich verschlechtert wird, sogar wenn ein hoher Strom, z. B. 3 × 108 Am−2, durch diese Bauelemente getrieben wird.

Computer simulation experiment on the mm‐wave properties of indium phosphide double drift impatts

High-frequency analysis has been carried out to predict the rf performance of 〈111〉, 〈110〉 and 〈100〉 oriented p+nn+, n+pp+ (single drift region) and n+npp+ (double drift region) GaAs IMPATT diodes for opertion at 35 and 60 GHz. The microwave performance is observed to be highly sensitive to crystal orientation in case of p+nn+ and n+npp+ diodes whereas orientation of the substrate has negligible effect on n+pp+ avalanche diodes. The calculation shows that 〈111〉 oriented GaAs IMPATT diode would provide the largest magnitude of negative resistance and negative conductance for both SDR p+nn+ and DDR n+npp+ diodes which indicates that high microwave power with high conversion efficiency can be realised from these 〈111〉 oriented GaAs devices. This result can be explained from the experimental data of electron and hole ionization rates for different orientations in GaAs.

S. K. Roy

Papers

High frequency characterisation of double drift region InP and GaAs diodes

Variation of high frequency negative resistance of silicon N+pp+ and GaAs p+nn+ IMPATT diodes with enhancement of reverse saturation current

Control of Millimeter-Wave Properties of High-Efficiency Double Drift Region IMPATTs through Enhancement of Saturation Current

Computer simulation experiment on the mm‐wave properties of indium phosphide double drift impatts

Studies on the high-frequency properties of 〈111〉, 〈110〉 and 〈100〉 oriented GaAs IMPATT diodes