Koji Hataya

High-power recessed-gate AlGaN-GaN HFET with a field-modulating plate

Recessed-gate AlGaN-GaN heterojunction field-effect transistors (FETs) with a field-modulating plate (FP) have been successfully fabricated for high-voltage and high-power microwave applications. The developed recessed-gate FP-FET with a gate length of 1 /spl mu/m exhibited an increased transconductance of 200 mS/mm. A series of current collapse measurements revealed that the recessed-gate FP-FET is highly desirable for collapse-free high-voltage power operation. Equivalent circuit analysis demonstrated that the gain loss due to the additional gate feedback capacitance associated with the FP electrode is considerably compensated by increasing the drain bias voltage to more than 30 V. A 48-mm-wide recessed-gate FP-FET biased at a drain voltage of 48 V exhibited a record saturated output power of 197 W with a linear gain of 10.1 dB and a power-added efficiency of 67% at 2 GHz.

12 W/mm recessed-gate AlGaN/GaN heterojunction field-plate FET

A recessed-gate structure was introduced to improve transconductance (gm) and gain characteristics in AlGaN/GaN field-plate (FP) FETs. A maximum gm was improved from 130 to 200 mS/mm by introducing gate recess. Recessed FP-FETs exhibited 3-7 dB higher linear gain as compared with planar FP-FETs. A 1 mm-wide recessed FP-FET biased at a drain voltage of 66 V demonstrated 12.0 W output power, 21.2 dB linear gain, and 48.8 % power added efficiency at 2 GHz. To our knowledge, the power density of 12.0 W/mm is the highest ever achieved for GaN-based FETs.

Low-contact-resistance and smooth-surface Ti∕Al∕Nb∕Au ohmic electrode on AlGaN∕GaN heterostructure

We have achieved both low contact resistance and smooth surface morphology by using Ti∕Al∕Nb∕Au formed on an Al0.3Ga0.7N∕GaN heterostructure. A low contact resistance of less than 0.6Ω∕mm was reproducibly recorded after thermal treatment with a wide range of annealing temperature from 830 to 1000°C and annealing time from 10 to 300s. It was found that root-mean square surface roughness of under 35nm was obtained with all the annealing conditions investigated. In consideration of surface roughness and reproducibility, the most suitable annealing condition was 850°C for 100s, which leads to a contact resistance and a specific contact resistivity of 0.48Ω∕mm and 5×10−6Ω∕cm2, respectively.

A 149W recessed-gate AlGaN/GaN FP-FET

A recessed-gate AlGaN/GaN field-modulating plate (FP) FET was successfully fabricated on a SiC substrate. By employing recessed-gate structure for an FP-FET, the transconductance (gm) was increased from 130 mS/mm to 220 mS/mm, leading to an improvement in gain characteristics. The gate breakdown voltage (BV/sub gd/) was improved from 160V for the planar FP-FET to 200V for the recessed FP-FET, resulting from one-order reduction in gm and BV/sub gd/, current collapse was suppressed by introducing the recessed-gate structure. At 2GHz, a 32mm-wide recessed FP-FET exhibited an output power of 149 W (4.7W/mm) with 64% power-added efficiency and 8.7 dB linear gain with a drain bias of 47 V.

High power AlGaN/GaN heterojunction FETs for base station applications

This work describes high power AlGaN/GaN heterojunction FETs on SiC substrates which we have developed. To our knowledge, Psat values of 203 W and 4 W are the highest ever achieved at 2 GHz and 30 GHz, respectively, for GaN FETs. We believe that GaN FET technology will play a dominant role in future base station systems.