Koji Hirata

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.

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.

Correlation between micropipes on SiC substrate and dc characteristics of AlGaN∕GaN high-electron mobility transistors

We report the influence of the size of hollow cores that extend from micropipes in SiC substrates on the dc characteristics of AlGaN∕GaN high-electron mobility transistors (HEMTs) fabricated on the substrates. Significant deterioration of the dc characteristics of HEMTs fabricated in the vicinity of hollow cores with a diameter of 5μm was observed, while no major deterioration was observed for HEMTs fabricated around hollow cores whose diameters were 1.5 and 3μm. A clear correlation between the size of hollow cores and free carrier densities at the peripheries of the cores was observed using micro-Raman imaging. The high densities of free carriers around relatively large hollow cores were suggested to be the cause of deterioration of the dc characteristics of HEMTs fabricated in the vicinity of the hollow cores. The device deterioration length with respect to the hollow core size was empirically deduced, and the effective decrease in the wafer yield was estimated as well.

Influence of Micropipe and Domain Boundary in SiC Substrate on the DC Characteristics of AlGaN/GaN HFET

AlGaN/GaN HFETs were fabricated around micropipes and on a domain boundary in a semi-insulating silicon carbide (SI-SiC) substrate and the DC characteristics of the fabricated devices were measured. Devices around micropipe showed no pinch-off or large gate leakage. The devices on the domain boundaries showed no degradation in the performances, even though an X-ray topographic analysis indicated that crystal imperfections, due to the defects, propagated to the GaN layer across the hetero interface. Based on these results, we concluded that micropipe degrades the DC characteristics and that the domain boundary does not affect the DC characteristics. From Raman analysis on the devices around the micropipes, these degradations could be attributed to the free carriers introduced into the GaN crystal by the micropipes.

Effect of Reducing Thickness of TiN Buffer Layers on Epitaxial Growth of GaN Layes

We investigated effect of reducing thickness of TiN buffer layers on growth of the smooth GaN layers. The sputtered TiN layers with thicknesses in the range of 2 to 100 nm were deposited on sapphire substrates. The sputtered TiN layers were exposed NH3 + H2 mixed gas atmosphere at about 1000°C to enrich nitrogen concentration of the layers. GaN layers were deposited on the nitrogen-enriched TiN layer using a MOCVD method. Average grain size of the nitrogen-enriched TiN layer was minimized at the thickness of 5 nm. In the initial stages of GaN growth, density of GaN hexagons grown on the 5nm-thick TiN layers was the highest. The 2μm-thick GaN layers grown on the 5nm-thick TiN layers exhibited the smoothest surface. Thus, the 5nm thickness is believed to be the best thickness for the smooth GaN growth on the sapphire/TiN substrates.