We demonstrate the use of high-quality thin InGaN films as the reactive ion etching (RIE) stop layer for fabrication of recessed gate high-microwave-power AlGaN∕InGaN∕AlGaN∕GaN heterostructure field-effect transistors. We used migration-enhanced-metalorganic-chemical-vapor-deposition grown InGaN layer sandwiched in AlGaN barrier yields better than 10 times RIE selectivity of AlGaN and InGaN compared to our conventional standard AlGaN∕GaN high electron mobility transistors. The fabricated devices exhibited a 50% increase in the breakdown voltage, which is attributed primarily due to the improved electric field distribution at the gate edge and to the increased distance between the AlGaN surface and the device channel. The continuous wave microwave power was measured at the drain bias as high as 60V. The maximum output power and power added efficiency were 8.9W∕mm and 40%, respectively. The obtained results demonstrate a potential of this technique for development of the next-generation high-power transistors.

A comparative study of the dynamic current-voltage (DI-V) characteristics of III-N heterojunction and double heterojunction field-effect transistors (HFETs and DHFETs) reveals that the current and RF power collapse in HFETs arise from modulation of device series resistances under large input signal. A model based on space-charge limited current through the depletion regions formed at the gate edges due to the charge trapping explains the DI-V behavior and other observations related to the RF current collapse in III-N HFETs.

We describe the properties of novel III-N-based insulating gate heterostructure field-effect transistors (HFETs). For the gate isolation, these devices use either SiO2 layer (in metal-oxide-semiconductor HFET (MOSHFET) structures) or Si3N4 layer (in metal-insulator-semiconductor HFET structures). These insulating gate HFETs have the gate-leakage currents 4-6 orders of magnitude lower than HFETs, even at elevated temperatures up to 300 degreesC. A double-heterostructure MOSHFET with SiO2 gate isolation exhibits current collapse-free performance with extremely low gate-leakage current. Insulating gate devices, including large periphery multigate structures, demonstrate high-power stable operation and might find applications in high-performance power amplifiers and microwave and high-power switches with operating temperatures up to 300 degreesC or even higher.

In this paper, we report the high performance quarter micron gate Si/sub 3/N/sub 4//AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MISHFET) with a 5 W/mm CW power at 26 GHz at drain bias of 35 V. The MISHFETs to have the same power gain and power added efficiency as the HFETs, in spite of lower transconductance. An improved RF linearity of MISHFETs is attributed to more linear transconductance-gate bias dependence.

The authors report the output RF signal distortions in the novel SiO/sub 2/-AlGaN-InGaN-GaN metal-oxide-semiconductor double heterostructure FET (MOSDHFET) device structure. Their comparative studies of MOSDHFETs and Schottky gate type DHFETs fabricated on the same wafer show significantly improved RF output signal linearity in MOSDHFETs at high-input signals. At the RF output powers close to saturation, the relative level of the second and third harmonic powers in MOSDHFETs was found to be less than -30 dB, which is about 15-20 dB lower as compared to identical geometry DHFET. This improvement is attributed to a better linearity of the MOSDHFET current-gate voltage characteristics.

We present the characteristics of a quarter-micron gate metal-insulator-semiconductor heterostructure field-effect transistor (MISHFET) with Si/sub 3/N/sub 4/ film as a gate insulator. A detailed comparison of the MISHFET and an identical geometry HFET shows them to have the same radio frequency (RF) power gain and cut-off frequency, while the MISHFET has much lower gate-leakage currents and higher RF powers at operating frequencies as high as 26 GHz. The MISHFET gate-leakage currents are well below 100 pA at gate bias values from -10 V to +8 V. At zero gate bias, the drain saturation current is about 0.9 A/mm and it increases to 1.2 A/mm at +8 V gate bias. The output RF power of around 6 W/mm at 40 drain bias was found to be frequency independent in the range of 2 to 26 GHz. This power is 3 dB higher than that from HFET of the same geometry. The intrinsic cutoff frequency is /spl sim/63 GHz for both the HFET and the MISHFET. This corresponds to an average effective electron velocity in the MISHFET channel of 9.9/spl times/10/sup 6/ cm/s. The knee voltage and current saturation mechanisms in submicron MISHFETs and heterostructure field-effect transistors (HFET) are also discussed.

The characteristics of a novel nitride based field-effect transistor combining SiO/sub 2/ gate isolation and an AlGaN/InGaN/GaN double heterostructure design (MOSDHFET) are reported. The double heterostructure design with InGaN channel layer significantly improves confinement of the two-dimensional (2-D) electron gas and compensates strain modulation in AlGaN barrier resulting from the gate voltage modulations. These decrease the total trapped charge and hence the current collapse. The combination of the SiO/sub 2/ gate isolation and improved carrier confinement/strain management results in current collapse free MOSDHFET devices with gate leakage currents about four orders of magnitude lower than those of conventional Schottky gate HFETs.

The characteristics of a novel nitride based field-effect transistor combining SiO2 gate isolation and an AlGaN/InGaN/GaN double heterostructure design (MOSDHFET) are reported. The double heterostructure design with InGaN channel layer significantly improves confinement of the two-dimensional (2-D) electron gas and compensates strain modulation in AlGaN barrier resulting from the gate voltage modulations. These decrease the total trapped charge and hence the current collapse. The combination of the SiO2 gate isolation and improved carrier confinement/strain management results in current collapse free MOSDHFET devices with gate leakage currents about four orders of magnitude lower than those of conventional Schottky gate HFETs.

We present experimental and modeling results on the gate-length dependence of the maximum current that can be achieved in GaN-based heterostructure field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs (MOSHFETs). Our results show that the factor limiting the maximum current in the HFETs is the forward gate leakage current. In the MOSHFETs, the gate leakage current is suppressed and the overflow of the two dimensional electron gas into the AlGaN barrier region becomes the most important factor limiting the maximum current. Therefore, the maximum current is substantially higher in MOSHFETs than in HFETs. The measured maximum current increases with a decrease in the gate length, in qualitative agreement with the model that accounts for the velocity saturation in the channel and for the effect of the source series resistance. The maximum current as high as 2.6 A/mm can be achieved in MOSHFETs with a submicron gate.

The DC and RF-characteristics of novel AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) were studied at elevated temperatures up to 300 °C, after a 36 h continuous operation at 200 °C and after a 1 min thermal stress at temperatures up to 850 °C. At 300 °C, the gate-leakage current remains about four orders of magnitude lower than that for regular HFETs. At zero gate-bias, the saturation current decreased by only about 20% after 36 h of continuous operation at 200 °C. After a 700 °C, 1 min thermal stress, the gate leakage remained as low as 5 nA/mm, whereas the peak current and DC transconductance showed a 20% reduction. In spite of the decrease in the peak-current, the RF saturation power remained nearly constant for operation at temperatures up to 200 °C. We attribute this to a reduction in the current collapse.

The transient characteristics of GaN highly doped thin channel metal–semiconductor and metal-oxide–semiconductor field effect transistors were investigated by switching the transistors from the OFF to the ON state. Transient gated transmission line model measurements showed that the channel resistance under the gate remains constant and the current collapse effects are linked to the transient variations of the series source–gate and gate–drain resistances, similar to what was previously reported for GaN/AlGaN heterostructure field effect transistors. The transient temperature measurements revealed that trapping processes responsible for the transient behavior cannot be described by the activation mechanism.

The mechanism of radio-frequency current collapse in GaN–AlGaN heterojunction field-effect transistors (HFETs) was investigated using a comparative study of HFET and metal–oxide–semiconductor HFET current–voltage (I–V) and transfer characteristics under dc and short-pulsed voltage biasing. Significant current collapse occurs when the gate voltage is pulsed, whereas under drain pulsing the I–V curves are close to those in steady-state conditions. Contrary to previous reports, we conclude that the transverse electric field across the wide-band-gap barrier layer separating the gate and the channel rather than the gate or surface leakage currents or high-field effects in the gate–drain spacing is responsible for the current collapse. We find that the microwave power degradation in GaN–AlGaN HFETs can be explained by the difference between dc and pulsed I–V characteristics.

Gated transmission line model pattern measurements of the transient current–voltage characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs were made to develop a phenomenological model for current collapse. Our measurements show that, under pulsed gate bias, the current collapse results from increased source–gate and gate–drain resistances but not from the channel resistance under the gate. We propose a model linking this increase in series resistances (and, therefore, the current collapse) to a decrease in piezoelectric charge resulting from the gate bias-induced nonuniform strain in the AlGaN barrier layer.

The characteristics of novel AlGaN/GaN Metal–Oxide–Semiconductor Heterostructure Field Effect Transistors (MOSHFETs) were measured in the temperature range of 20–300 °C. At 300 °C the leakage current of MOSHFET remains four orders of magnitude lower than that of regular HFET. The saturation current and transconductance for both types of transistors follow the temperature dependence of electron velocity in the channel. The recovery of the current collapse at elevated temperatures compensates the effect of the decrease of the steady-state saturation current with temperature. As a consequence, the saturation microwave power remains fairly constant in the temperature range 20–200 °C, varying only by about 20% or so. These results show high potential of MOSHFETs for high-temperature microwave, digital and switching applications.

Novel, current collapse free, double heterostructure AlGaN/InGaN/GaN field effect transistors (DHFETs) are fabricated on the insulating SiC substrates. The simulations show that a combined effect of the bandgap offsets and polarization charges provides an excellent 2D carrier confinement. These devices demonstrate output RF powers as high as 4.3 W/mm in CW mode and 6.3 W/mm in the pulsed mode, with the gain compression as low as 4 dB.

The dependence of the 1/f noise on 2D electron concentration in the channel n Ch of AlGaN/GaN Heterostructure Field Effect Transistors and Metal Oxide Semiconductor Heterostructure Field Effect Transistors has been studied and compared. The dependencies of Hooge parameter αCh for the noise sources located in the channel of the transistors on sheet electron concentration are found identical for both types of devices. The increase of the Hooge parameter αCh with the decrease of the channel concentration observed in both types of devices confirms that the noise sources are located in the region under the gate in the AlGaN/GaN heterostructure and that electron tunneling from the 2D electron gas into the traps in GaN or AlGaN layers is a probable noise mechanism.

We report on AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor (HFET) over SiC substrates with peripheries from 0.15 to 6 mm, These multigate devices with source interconnections were fabricated using a novel oxide-bridging approach. The saturation current was as high as 5.1 A for a 6 mm wide device with a gate leakage of 1 muA/cm(2) for 1.5 mum gate length in a 5 mum source-drain opening. The cutoff frequency of around 8 GHz was practically independent of the device periphery, Large-signal output rf-power as high as 2.88 W/mm was measured at 2 GHz, Both the saturation current and the rf-power scaled nearly linearly with the gate width.