TCAD simulation was performed to investigate the material properties of an AlGaN/GaN structure in Deep Acceptor (DA)-rich and Deep Donor (DD)-rich GaN cases. DD-rich semi-insulating GaN generated a positively charged area thereof to prevent the electron concentration in 2DEG from decreasing, while a DA-rich counterpart caused electron depletion, which was the origin of the current collapse in AlGaN/GaN HFETs. These simulation results were well verified experimentally using three nitride samples including buffer-GaN layers with carbon concentration ([C]) of 5×1017, 5×1018, and 4×1019 cm-3. DD-rich behaviors were observed for the sample with [C]=4×1019 cm-3, and DD energy level EDD=0.6 eV was estimated by the Arrhenius plot of temperature-dependent IDS. This EDD value coincided with the previously estimated EDD. The backgate experiments revealed that these DD-rich semi-insulating GaN suppressed both current collapse and buffer leakage, thus providing characteristics desirable for practical usage.

Current collapse of AlGaN/GaN heterostructure field-effect transistors (HFETs) formed on qualified epitaxial layers on Si substrates was successfully suppressed using graded field-plate (FP) structures. To improve the reproducibility of the FP structure manufacturing process, a simple process for linearly graded SiO2 profile formation was developed. An HFET with a graded FP structure exhibited a significant decrease in an on-resistance increase ratio of 1.16 even after application of a drain bias of 600 V.

We performed the (NH4)2S surface treatments before Al2O3 deposition to improve the Al2O3/III-Nitride interface quality in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). Interface state density at the Al2O3/GaN interface was decreased by the (NH4)2S treatment. The hysteresis width in ID-VGS and gm-VGS characteristics of the Al2O3/AlGaN MOSHFETs with the (NH4)2S treatment was smaller than that without the (NH4)2S treatment. In addition, transconductance (gm) decrease at a large gate voltage was relaxed by the (NH4)2S treatment. We also performed ultraviolet (UV) illumination during the (NH4)2S treatment for further improvement of the Al2O3/III-Nitride interface quality. Interface state density of the Al2O3/GaN MOS diodes with the UV illumination was smaller than that without the UV illumination.

High output power AlGaN/GaN metal-insulator-semiconductor (MIS) hetero-junction field effect transistor (HFET) on Si substrate for millimeter-wave application has developed. High temperature chemical vapor deposition (HT-CVD) grown SiN as a gate insulator improves the breakdown characteristics which enables the operation at high drain voltage of 55 V. The device exhibits high drain current of 1.1 A/mm free from the current collapse and high RF gain of 10.4 dB. The amplifier module developed AlGaN/GaN MIS-HFET with the gate width of 5.4 mm exhibits an output power of 10.7 W and a linear gain of 4 dB at 26.5 GHz. The resultant high output power is very promising for long-distance communication at millimeter-wave in the future which would enable high speed and high density data transmission.

Drain collapse in AlGaN/GaN HFET is analyzed using a two-dimensional device simulator. Two-step saturation is obtained, assuming hole-trap type surface states on the AlGaN surface and a short negative-charge-injected region at the drain side of the gate. Due to the surface electric potential pinning by the surface traps, the negative charge injected region forms a constant potential like in a metal gate region and it acts as an FET with a virtual gate. The electron concentration profile reveals that the first saturation occurs by pinch-off in the virtual gate region and the second saturation occurs by the pinch-off in the metal gate region. Due to the short-channel effect of the virtual gate FET, the saturation current increases until it finally reaches the saturation current of the intrinsic metal gate FET. Current collapses with current degradation at the knee voltage in the I-V characteristics can be explained by the formation of the virtual gate.

To improve the high voltage performance of AlGaN/GaN heterojunction field effect transistors (HFETs), we have fabricated AlGaN/GaN HFETs with p-GaN epi-layer on sapphire substrate with an ohmic contact to the p-GaN (p-sub HFET). Substrate bias dependent threshold voltage variation (VT-VSUB) was used to directly determine the doping concentration profile in the buffer layer. This VT-VSUB method was developed from Si MOSFET. For HFETs, the insulator is formed by epitaxially grown and heterogeneous semiconductor layer while for Si MOSFETs the insulator is amorphous SiO2. Except that HFETs have higher channel mobility due to the epitaxial insulator/semiconductor interface, HFETs and Si MOSFETs are basically the same in the respect of device physics. Based on these considerations, the feasibility of this VT-VSUB method for AlGaN/GaN HFETs was discussed. In the end, the buffer layer doping concentration was measured to be 21017 cm-3, p-type, which is well consistent with the Mg concentration obtained from secondary ion mass spectroscopy (SIMS) measurement.

The paper presents a systematic study of in-situ passivated AlN/GaN Metal Insulator Semiconductor Field Effect Transistors (MISFETs) with submicron gates. DC, high frequency small signal, large signal and low frequency dispersion effects are reported. The DC characteristics are analyzed in conjunction with the power performance of the device at high frequencies. Studies of the low frequency characteristics are presented and the results are compared with those of AlGaN/GaN High Electron Mobility Transistors (HEMTs). Small signal measurements showed a current gain cutoff frequency and maximum oscillation frequency of 49.9 GHz and 102.3 GHz respectively. The overall characteristics of the device include a peak current density of 335 mA/mm, peak extrinsic transconductance of 130 mS/mm, a maximum output power density of 533 mW/mm with peak power added efficiency (P.A.E.) of 41.3% and linear gain of 17 dB. The maximum frequency dispersion of transconductance and output resistance of the fabricated MISFETs is 20% and 21% respectively.

Heterostructure field-effect transistors (HFETs) composed of antimonide-based compound semiconductor (ABCS) materials have intrinsic performance advantages due to the attractive electron and hole transport properties, narrow bandgaps, low ohmic contact resistances, and unique band-lineup design flexibility within this material system. These advantages can be particularly exploited in applications where high-speed operation and low-power consumption are essential. In this paper, we report on recent advances in the design, material growth, device characteristics, oxidation stability, and MMIC performance of Sb-based HEMTs with an InAlSb upper barrier layer. The high electron mobility transistors (HEMTs) exhibit a transconductance of 1.3 S/mm at VDS = 0.2 V and an fTLg product of 33 GHz-µm for a 0.2 µm gate length. The design, fabrication and improved performance of InAlSb/InGaSb p-channel HFETs are also presented. The HFETs exhibit a mobility of 1500 cm2/V-sec, an fmax of 34 GHz for a 0.2 µm gate length, a threshold voltage of 90 mV, and a subthreshold slope of 106 mV/dec at VDS = -1.0 V.

AlN/GaN Metal Insulator Semiconductor Field Effect Transistors (MISFETs) were designed, simulated and fabricated. DC, S-parameter and power measurements were also performed. Drift-diffusion simulations using DESSIS compared AlN/GaN MISFETs and Al32Ga68N/GaN Heterostructure FETs (HFETs) with the same geometries. The simulation results show the advantages of AlN/GaN MISFETs in terms of higher saturation current, lower gate leakage and higher transconductance than AlGaN/GaN HFETs. First results from fabricated AlN/GaN devices with 1 µm gate length and 200 µm gate width showed a maximum drain current density of 380 mA/mm and a peak extrinsic transconductance of 85 mS/mm. S-parameter measurements showed that current-gain cutoff frequency (fT) and maximum oscillation frequency (fmax) were 5.85 GHz and 10.57 GHz, respectively. Power characteristics were measured at 2 GHz and showed output power density of 850 mW/mm with 23.8% PAE at VDS = 15 V. To the authors knowledge this is the first report of a systematic study of AlN/GaN MISFETs addressing their physical modeling and experimental high-frequency characteristics including the power performance.

This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30 nm had a maximum drain current density of 1.6 A/mm and a maximum transconductance of 402 mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181 GHz and a maximum oscillation frequency of 186 GHz.

In this paper, the thermal characteristic of the GaN HFETs has been analyzed using the hybrid finite element method (FEM). Both the steady and transient state thermal operations are quantitatively studied with the effects of temperature-dependent thermal conductivities of GaN and the substrate materials properly treated. The temperature distribution and the maximum temperatures of the HFETs operated under excitations of continuous-waves (CW) and pulsed-waves (PW) including double exponential shape PW such as electromagnetic pulse (EMP) and ultra-wideband (UWB) signal are studied and compared.

This investigation of the temperature and illumination effects on the AlGaN/GaN HFET threshold voltage shows that it shifts about -1 V under incandescent lamp or blue LED illumination, while almost no shift takes place under red LED illumination. The temperature coefficient for the threshold voltage shift is +3.44 mV/deg under the illuminations and +0.28 mV/deg in darkness. The threshold voltage variation can be attributed to a virtual back-gate effect caused by light-generated buffer layer potential variations. The expressions for the potential variation are derived using Shockley-Read-Hall (SRH) statistics and the Maxwell-Boltzmann distribution for the carriers and deep traps in the buffer layer. The expressions indicate that large photoresponses will occur when the electron concentration in the buffer layer is extremely small, that is, highly resistive. In semi-insulating substrates, the substrate potential varies so as to keep the trap occupation function constant. The sign and the magnitude of the threshold voltage variation are explained by the shift of the pinning energy calculated from the Fermi-Dirac distribution function.

A model for the enhancement-mode operation of an AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor (MIS-HFET) under DC and AC conditions is proposed. In DC operation at positive gate voltages, the MIS-HFET can be divided into a transistor area and a resistor area due to the diode nature of the insulator/AlGaN interface. The transistor area shrinks with the increases in gate voltage. The intrinsic-transistor gate-length reduction causes a drain current increase. The I-V characteristics based on the gradual channel approximation are derived. The ID hysteresis of the MIS-HFET is investigated by a circuit simulation using SPICE. We have confirmed that the hysteresis was caused by the phase difference between the potential variation of the gate insulator/AlGaN interface and that of the gate electrode due to CR components in the gate structure.

We present ultra low noise- and wide dynamic range performances of an AlGaN/GaN heterostructure FET (HFET). An HFET fabricated on a high quality epitaxial layers grown on a semi-insulating SiC substrate exhibited impressively low minimum noise figure (NF min ) of 0.4 dB with 16 dB associated gain at 2 GHz. The low NF (near NF min ) operation was possible in a wide drain bias voltage range, i.e. from 3 V to 15 V. At the same time, the device showed low distortion character as indicated by the high third order input intercept point (IIP3), +13 dBm. The excellent characteristics are attributed to three major factors: (1) high quality epitaxial layers that realized a high transconductance and very low buffer leakage current; (2) excellent device isolation made by selective thermal oxidation; (3) ultra low gate leakage current realized by Pd based gate. The results demonstrate that the AlGaN/GaN HFET is a strong candidate for front-end LNAs in various mobile communication systems where both the low noise and the wide dynamic range are required.

We report in this paper, the performance of AlGaN/GaN HFETs in the context of high power, low noise and high temperature operations, along with a comparison of their characteristics with other conventional technologies. Finally, a single stage modulator driver for long haul optical communications is presented as an example of application of the GaN-based devices high power handling capabilities.

We report the investigation of major dispersion mechanisms such as self-heating, trapping, current collapse, and floating-body effects present in AlGaN/GaN HFETs. These effects are analyzed using DC/Pulsed IV, load-pull, low-frequency noise systems, and a cryogenic probe station. This study leads to a better understanding of the device physics, which is critical for accurate large-signal modeling and device optimization.

Silicon Germanium Heterostructure field effect transistors have been proposed as a promising extension to the CMOS technologies affording enhanced performance at relaxed geometries. Particularly promising is the potential of SiGe Heterostructure MOS and Heterostrucure FET at the low power operating regime. We discuss circuit design techniques applicable in the micropower regime which can be applied to SiGe HMOS technologies. We then review recent results in HMOS both from the material and the applications point of view. We conclude by reporting simulation results indicating the potential of SiGe HMOS in radiofrequency micropower applications.

In this paper we summarize the actual status of the GaN/AlGaN HFET power technology at DaimlerChrysler using high quality structures grown by MOCVD on sapphire or semi-insulating SiC supplied by Epitronics. High mobilites and record extrinsic transconductance and current values were achieved on devices with sapphire and SiC substrate. Small area devices with 0.3 µm gate length yield fT=43 GHz and fmax=78 GHz (s.i. SiC substrate). Load-pull characterisation have been performed on multifinger HFETs up to 20 GHz with e.g. output power levels above 3 Watt cw at 15 GHz for a single 1.6 mm device. Though the achieved results are very promising, the performance of these devices is still hampered by transient effects on different time scales. We will show in this paper that passivation of the devices by SiN could considerably improve the RF power performance as well as reduce long time constant effects present before passivation.

A 100 W, low distortion AlGaAs/GaAs heterostructure FET has been developed for CDMA cellular base stations. This FET employs the longest gate finger ever reported of 800 µm to shrink the chip size. The size of the chip and the package are miniaturized to 1.242.6 mm2 and 17.4 24.0 mm2, respectively. The developed FET exhibits 100 W (50 dBm) saturation output power, and 11.5 dB power gain at 1 dB gain compression at 2.1 GHz. The third-order intermodulation distortion and the power-added efficiency under the two-tone test condition (Δf=1 MHz) are -35 dBc and 24%, respectively at 42 dBm output power, that is 8 dB back off from the saturation power.

This paper describes a high-power and low-distortion AlGaAs/GaAs HFET amplifier developed for digital cellular base station system. We proved experimentally that distortion characteristics such as IMD (Intermodulation Distortion) or NPR (Noise Power Ratio) are drastically degraded when the absolute value of the drain bias circuit impedance at low frequency are high. Based on the experimental results, we have designed the drain bias circuit not to influence the distortion characteristics. The developed amplifier employed two pairs of pre-matched GaAs chips mounted on a single package and the total output-power was combined in push-pull configuration with a microstrip balun circuit. The push-pull amplifier demonstrated state-of-the-art performance of 140 W output-power with 11.5 dB linear gain at 2.2 GHz. In addition, it exhibited extremely low distortion performance of less than 30 dBc at two-tone total output-power of 46 dBm. These results indicate that the design of the drain bias circuit is of great importance to achieve improved IMD characteristics while maintaining high power performance.