A. Tarakji

AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors on SiC substrates

We report on AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) grown over insulating 4H–SiC substrates. We demonstrate that the dc and microwave performance of the MOS-HFETs is superior to that of conventional AlGaN/GaN HFETs, which points to the high quality of SiO2/AlGaN heterointerface. The MOS-HFETs could operate at positive gate biases as high as +10 V that doubles the channel current as compared to conventional AlGaN/GaN HFETs of a similar design. The gate leakage current was more than six orders of magnitude smaller than that for the conventional AlGaN/GaN HFETs. The MOS-HFETs exhibited stable operation at elevated temperatures up to 300 °C with excellent pinch-off characteristics. These results clearly establish the potential of using AlGaN/GaN MOS-HFET approach for high power microwave and switching devices.

Si3N4/AlGaN/GaN–metal–insulator–semiconductor heterostructure field–effect transistors

We report on a metal–insulator–semiconductor heterostructure field-effect transistor (MISHFET) using Si3N4 film simultaneously for channel passivation and as a gate insulator. This design results in increased radio-frequency (rf) powers by reduction of the current collapse and it reduces the gate leakage currents by four orders of magnitude. A MISHFET room temperature gate current of about 90 pA/mm increases to only 1000 pA/mm at ambient temperature as high as 300 °C. Pulsed measurements show that unlike metal–oxide–semiconductor HFETs and regular HFETs, in a Si3N4 MISHFET, the gate voltage amplitude required for current collapse is much higher than the threshold voltage. Therefore, it exhibits significantly reduced rf current collapse.

Induced strain mechanism of current collapse in AlGaN/GaN heterostructure field-effect transistors

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.

SiO/sub 2//AlGaN/InGaN/GaN MOSDHFETs

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.

Mechanism of radio-frequency current collapse in GaN–AlGaN field-effect transistors

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.

Insulating gate III-N heterostructure field-effect transistors for high-power microwave and switching applications

Describes the properties of novel III-N-based insulating gate heterostructure field-effect transistors (HFETs). For the gate isolation, these devices use either SiO/sub 2/ layer (in metal-oxide-semiconductor HFET (MOSHFET) structures) or Si/sub 3/N/sub 4/ 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/spl deg/C. A double-heterostructure MOSHFET with SiO/sub 2/ 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/spl deg/C or even higher.

Submicron gate Si 3 N 4 /AlGaN/GaN-metal-insulator-semiconductor heterostructure field-effect transistors

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.

Dynamic current-voltage characteristics of III-N HFETs

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.

Maximum Current in Nitride-Based Heterostructure Field-Effect Transistors

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.

DC and Microwave Performance of a GaN/AlGaN MOSHFET under High Temperature Stress

Abstract 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.