This work reports on the dc performance of AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) grown on Si (111) substrate and the study of current dispersion in these devices using various widely adopted methods. The MOSHEMTs were fabricated using a very thin (4.2 nm) SiO2 film as the gate insulator and were subsequently passivated with about 30 nm thick Si3N4 layer. For devices with 2.5 mu m long gates and a 4 mu m drain-to-source spacing, the maximum saturation drain current density was 822 mA mm-(1) at + 4 V gate bias and the peak external transconductance was similar to 100 mS mm(-1). Furthermore, the oxide layer successfully suppressed the drain and gate leakage currents with the subthreshold current and the gate diode current levels exceeding by more than three orders of magnitude the levels found in their Schottky gate counterparts. Capacitance-voltage and dynamic current-voltage measurements were carried out to assess the oxide quality as well as the devices' surface properties after passivation. The efficacy of each of these characterization techniques to probe the presence of interface traps and oxide charge in the nitride-based transistors is also discussed.

Abstract : Fabrication and characteristics of high voltage, normally-on junction field effect transistors (JFETs) in 4H-silicon carbide (4H-SiC) are presented. The devices were built on 5x10(exp 15) cm(exp -3) doped, 12 micron thick n-type epilayer grown on a n+ 4H-SiC substrate. A specific on-resistance of 10 mOmega-square cm and a blocking voltage of 1.8 kV were measured. Device characteristics were measured for temperatures up to 300 deg C. An increase of specific on-resistance by a factor of 5 and a decrease in transconductance were observed at 300 deg C, when compared to the value at room temperature. This is due to a decrease in bulk electron mobility at elevated temperature. A slight negative shift in pinch-off voltage was also observed at 300 deg C. The devices demonstrated robust DC characteristics for temperatures up to 300 deg C, and stable high temperature inverter operation in a power DC-DC converter application, using these devices, is reported in this paper.

Record high RF power densities up to 30 W/mm have been recently achieved using AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs). These results exhibit the promise of III-Nitride HFETs for high-power microwave applications. However the output RF power of these devices was found to degrade with the operation time. Gate leakage current in conventional HFETs is a major contributor to device instability and failure. We previously reported on the development of Metal-Oxide-Semiconductor Insulated gate HFETs (MOSHFETs) with gate leakage currents four to six orders of magnitude lower than those of the HFETs. The MOSHFETs has demonstrated a significant performance improvement as compared to conventional HFETs resulting in higher channel currents and RF powers. In this study, we present the first comparative experimental evaluation of the HFET and MOSHFET lifetime at room and elevated temperatures.The room temperature HFET stability lifetime as high as 2.4 years is estimated. We also present the detailed study of the HFETs stabilization and failure mechanisms. The pronounced similarity of the mechanisms of device degradation and current collapse is demonstrated. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This work introduces the first step in a version of a wide-bandwidth, frequency-agile power interface that can sit between a simulation environment and real electrical hardware. Silicon (Si) technology is incapable of meeting the extreme switching demands of such a power interface, while gallium-nitride (GaN) technology is best suited for this application. A GaN power cell, in the form of an integrated H-bridge power block, is used as the core element in this new interface to take advantage of the III-V semiconductor material properties, resulting in enhanced operating characteristics. The GaN integrated H-bridge transistors are constructed out of AIGaN/GaN, MOS-Hetero-junction FETs (MOSHFETs). The H-bridge is mounted to an aluminum nitride (AIN) substrate for heat removal via a thermally conducting, electrically insulating, epoxy. This wide bandgap power converter utilizes a high- and low-side driver to modulate the gate-source voltage of each device between +5 V and -12 V. Control for the power converter is provided via a dual output pulse generator. The pulse generator operates open-loop with two outputs to experimentally test the H-bridge in a half-bridge converter topology under different loading conditions.

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 MOSHFET design which combines the advantage of the MOS structure, which suppresses the gate leakage current, and an AlGaN/GaN heterointerface that provides high density, high mobility two-dimensional electron gas channel. This article presents a comparative review of the I-V characteristics, cut-off frequencies, RF output powers, power gain, and nonlinear distortions of AlGaN/GaN MOSHFET, and HFET device. The MOSHFETs possess significant advantages for the monolitic IC design. They sustain very high input impedance at elevated temperatures, even above 300 °C. The results show that the MOSHFET based ICs are extremely promising for a large variety of high-power high-temperature applications.

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy underfill. They also show that in sapphire-based flip-chip mounted devices the heat spread from the active region under the gate along the GaN buffer and the substrate is the key contributor to the overall thermal impedance.

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 propose and demonstrate an AlGaN/GaN/AlGaN double heterostructure (DH) with significantly improved two-dimensional (2D) confinement for high-power III-N heterostructure field-effect transistors (HFETs). The DH was grown directly on an AlN buffer over i-SiC substrate. It enables an excellent confinement of the 2D gas and also does not suffer from the parasitic channel formation as experienced in past designs grown over GaN buffer layers. Elimination of the GaN buffer modifies the strain distribution in the DH, enabling Al contents in the barrier region well over 30%. For the AlGaN/GaN/AlGaN DH design, the 2D electron gas mobility achieved was 1150 cm2/V s at room temperature and 3400 cm2/V s at 77 K, whereas the temperature independent sheet carrier density was NS≈1.1×1013 cm−2. Compared to a regular AlGaN/GaN structure, the channel mobility-concentration profiling shows significant improvement in the carrier confinement. Sample DHFETs with 1-μm long gates demonstrate the threshold voltage of 3.5 V, wit...

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.