Prashant Chavarkar

30-W/mm GaN HEMTs by field plate optimization

GaN high-electron-mobility-transistors (HEMTs) on SiC were fabricated with field plates of various dimensions for optimum performance. Great enhancement in radio frequency (RF) current-voltage swings was achieved with acceptable compromise in gain, through both reduction in the trapping effect and increase in breakdown voltages. When biased at 120 V, a continuous wave output power density of 32.2 W/mm and power-added efficiency (PAE) of 54.8% at 4 GHz were obtained using devices with dimensions of 0.55/spl times/246 /spl mu/m/sup 2/ and a field-plate length of 1.1 /spl mu/m. Devices with a shorter field plate of 0.9 /spl mu/m also generated 30.6 W/mm with 49.6% PAE at 8 GHz. Such ultrahigh power densities are a dramatic improvement over the 10-12 W/mm values attained by conventional gate GaN-based HEMTs.

Bias-dependent performance of high-power AlGaN/GaN HEMTs

Large-signal behavior with a fixed load and varying supply voltages was proposed for characterizing the quality of AlGaN/GaN HEMTs. Improved devices demonstrated constantly high PAEs of 56-62% at 8 GHz throughout a wide voltage range from 10 to 40 V. These 300-/spl mu/m-wide devices also generated 3.1-W output power with only 3.4-dB gain compression at 45 V, which translates to 10.3-W/mm power density; the highest for any FET of the same size.

High-gain microwave GaN HEMTs with source-terminated field-plates

GaN HEMTs with field-plates connected to the source terminal have been developed for high-gain, high-voltage operation at microwave frequencies. Due to the reduced feedback capacitance compared to the gate-terminated field-plate structures, improvement in large-signal gain of 5-7 dB is obtained. Superior performance including 21-dB associated gain, 20-W/mm output power and 60% power-added-efficiency at 4 GHz and 118V bias, is achieved simultaneously. This translates to an extremely high voltage-frequency-gain product approaching 10 kV-GHz.

3.5-watt AlGaN/GaN HEMTs and amplifiers at 35 GHz

Sub-0.2-/spl mu/m AlGaN/GaN HEMTs were successfully scaled to 1.05 mm gate-width with minor gain reduction. On-chip single-stage amplifiers exhibited gains of 8 dB and 7.5 dB, as well as output powers of 3.6 W and 3.5 W, at 30 GHz and 35 GHz, respectively. This multi-watt output power at millimeter-wave frequencies well exceeded previous state-of-the-art for a GaN HEMT and is comparable to that from 6-7 times larger GaAs-based devices.

GaAs MESFET's on a truly insulating buffer layer: demonstration of the GaAs on insulator technology

We demonstrate for the first time a GaAs on insulator (GOI) technology, with aluminum oxide (Al/sub 2/O/sub 3/) formed by the wet oxidation of AlAs as the insulating buffer layer. The insulating buffer gives excellent charge control and eliminates substrate leakage current. The first results of GOI technology include 1.5-/spl mu/m gate length GOI MESFETs with f/sub /spl tau//=9 GHz and f/sub max/=45 GHz.

Very high voltage operation (>330 V) with high current gain of AlGaN/GaN HBTs

N-p-n Al/sub 0.05/GaN/GaN heterojunction bipolar transistors with a common emitter operation voltage higher than 330 V have been demonstrated using selectively regrown emitters. Devices were grown by metalorganic chemical vapor deposition on sapphire substrates. The n-type emitter was grown selectively on a 100-nm-thick p-base with an 8 /spl mu/m n-collector structure using a dielectric mask. The shallow etch down to the collector mitigates damages induced in the dry etch, resulting a low leakage and a high breakdown. The graded AlGaN emitter results in a common emitter current gain of /spl sim/18 at an average collector current density of up to 1 kA/cm/sup 2/ at room temperature.

Strain relaxation of InxGa1−xAs during lateral oxidation of underlying AlAs layers

Strain relaxation of hypercritical thickness InxGa1−xAs layers has been observed during lateral oxidation of underlying AlAs layers. Strain relaxation of InxGa1−xAs layers was studied as a function of indium composition and the AlAs oxidation temperature. It is proposed that the enhanced strain relaxation is due to two factors. The first is enhanced motion of threading dislocations due to stresses generated during the lateral oxidation process. The second is the porous nature of the InxGa1−xAs/Al2O3 interface that minimizes the interaction of threading dislocations with existing misfit dislocation segments. The extent of strain relaxation increases with increasing oxidation temperature, whereas the efficiency of strain relaxation was found to decrease with increasing indium composition.

A 50-W AlGaN/GaN HEMT amplifier

A high-power amplifier IC employing an 8-mm-periphery AlGaN/GaN HEMT was successfully demonstrated. The GaN-based device maintained a power density of 6.4 W/mm, enabling a total output power of 51 W at 6 GHz under pulsed operation. This is 6-10 times as high as GaAs-based devices of the same size.

Linearity and gain characteristics of AlGaN/GaN HEMTs

AlGaN/GaN HEMTs exhibited high cut-off frequencies at low current levels, which enabled linear operation at close to class-B biases. When biased at 35V and 40mA/mm (4% of channel current), excellent linearity performance including simultaneous 3/sup rd/ order intermodulation (IM3) of -30dBc and PAE of 40% was obtained at 4 GHz, with only -2.6dB back off. However, the gain reduction at high current levels made class-A operation less favorable. These gain characteristics are believed to be related to the ratio of the effective electron mass (m/sub e/) over the conduction band discontinuity (/spl Delta/E/sub c/) of AlGaN/GaN.

Linearity of high Al-content AlGaN/GaN HEMTs

This work will address the advancements in the linearity of AlGaN/GaN HEMTs. This is the first reported linearity result for high-Al content devices, and it is the first reported result of linearity for nitride-based HEMTs under both transmitter and receiver types of conditions. When the device technology matures, it will have commercial and military applications in wireless base stations, satellite communications, and radar.