Y. Okamoto

10-W/mm AlGaN-GaN HFET with a field modulating plate

AlGaN-GaN heterojunction field-effect transistors (HFETs) with a field modulating plate (FP) were fabricated on an SiC substrate. The gate-drain breakdown voltage (BV/sub gd/) was significantly improved by employing an FP electrode, and the highest BV/sub gd/ of 160 V was obtained with an FP length (L/sub FP/) of 1 /spl mu/m. The maximum drain current achieved was 750 mA/mm, together with negligibly small current collapse. A 1-mm-wide FP-FET (L/sub FP/=1 /spl mu/m) biased at a drain voltage of 65 V demonstrated a continuous wave saturated output power of 10.3 W with a linear gain of 18.0 dB and a power-added efficiency of 47.3% at 2 GHz. To our knowledge, the power density of 10.3 W/mm is the highest ever achieved for any FET of the same gate size.

A normally-off GaN FET with high threshold voltage uniformity using a novel piezo neutralization technique

In this paper, we successfully demonstrate a recessed gate normally-off GaN FET on a silicon substrate with high threshold voltage (V<inf>th</inf>) uniformity and low on-resistance. In order to realize high V<inf>th</inf> uniformity, a novel V<inf>th</inf> control technique is proposed, which we call “piezo neutralization technique”. This technique includes a piezo neutralization (PNT) layer formed at the bottom of the gate recess. Since the PNT layer neutralizes the polarization charges under the gate, the V<inf>th</inf> comes to be independent of the gate-to-channel span. The fabricated normally-off GaN FET with PNT structure exhibits an excellent V<inf>th</inf> uniformity (σ(V<inf>th</inf>)=18 mV) and a state-of-the-art combination of the specific on-resistance (R<inf>on</inf>A=500 mΩmm²) and the breakdown voltage (V<inf>B</inf>≫1000 V). The normally-off GaN FETs wtih PNT structure show great promise as power devices.

A 110-W AlGaN/GaN heterojunction FET on thinned sapphire substrate

SiN-passivated AlGaN/GaN heterojunction FETs (HJFETs) were fabricated on a thinned sapphire substrate. A 16 mm-wide HJFET on a 50 /spl mu/m-thick sapphire exhibited 22.6 W (1.4 W/mm) CW power, 41.9% PAE, and 9.4 dB linear gain at 26 V drain bias. Also, a 32 mm-wide device, measured under pulsed operation, demonstrated 113 W (3.5 W/mm) pulsed power at 40 V drain bias. To our best knowledge, 113 W total power is the highest achieved for GaN on any substrate, establishing the validity of the GaN-on-thinned-sapphire technology.

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.

Ka-band 2.3W power AlGaN/GaN heterojunction FET

Describes the first successful watt-level Ka-band power operation of an AlGaN/GaN heterojunction FET fabricated on a SiC substrate. Taking the advantage of high breakdown voltage, high-current, and high-gain characteristics of the short-channel GaN-based FET, state-of-the-art high-power performance of >2W has been achieved at 30GHz from a single chip having a gate width of 0.36mm. The developed device with a gate length of 0.25/spl mu/m exhibited a linear gain of 8.8dB at 30GHz, indicating that the short-channel AlGaN/GaN FET is promising for a variety of high-power applications at Ka-band and above.

Highly efficient double-doped heterojunction FET's for battery-operated portable power applications

Microwave power performance of double-doped AlGaAs-InGaAs-AlGaAs heterojunction field-effect transistors (HJFETs) operated at a drain bias of 3 V is described. The fabricated 1.0 /spl mu/m gate-length HJFET exhibited a maximum drain current of 500 mA/mm, a transconductance of 300 mS/mm, and a gate-to-drain breakdown voltage of 10 V. Power performance for a 14 mm gate periphery device demonstrated a maximum output power of 1.7 W with a 66% power-added efficiency (PAE) at 900 MHz. When the device was tuned for a maximum PAE, it delivered a 71% PAE with an output power of 1.2 W. The results indicate that the developed HJFET has great potential for 3 V battery-operated portable power applications.<<ETX>>

C-band single-chip GaN-FET power amplifiers with 60-W output power

A C-band high power amplifier was successfully developed with a single-chip GaN-based FET. At 4.0GHz, the fabricated 24-mm wide FET delivers 62 W and 156W under CW and pulsed operating conditions, respectively with a universal test fixture. The internal matching circuit was designed to be set up in a half-size package as compared to that for GaAs-based comparable- power-level amplifiers. The developed GaN-FET amplifier with 24-mm gate periphery delivers a 61W output power with 10.2dB linear gain and 42% power-added efficiency under CW operating conditions. To the best of our knowledge, this is the highest CW output power achieved from a single-chip FET power amplifier at C-band.

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.

Novel InGaP/AlGaAs/InGaAs heterojunction FET for X-Ku band power applications

We have successfully fabricated a novel heterojunction FET (HJFET) with an InGaP surface layer for high yield X-Ku band power applications. Standard deviation in the threshold voltage of 60 mV was achieved by using a highly-selective wet recess etching technique. The fabricated HJFET (Wg=16.8 mm) delivered an output power of 9.5 W and a power-added efficiency of 35% with a low carrier-to-third-order intermodulation distortion ratio of -29.5 dBc at 12 GHz. Moreover, the 25.2 mm HJFET delivered an output power of 12.2 W.