Dc and Microwave Characteristics of High Transconductance AlGaN/GaN Heterostructure Field Effect Transistors on Sic Subs
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ABSTRACT High quality AJGaN/GaN heterostructures have been successfully deposited on both nand p-type SiC substrates. Heterostructure field effect transistors fabricated using these layers exhibited high channel current density (1.71 A/mm), well behaved pinch-off characteristics, and excellent extrinsic transconductance (G,, = 229 mS/mm). There is negligible channel current degradation up to a source to drain bias of 20 V as opposed to devices grown on sapphire substrates. The 0.25 pm gate-length devices fabricated on the heterostructures grown on p-type SiC has allowed us to extract a cutoff frequency of 53 GHz. The cutoff frequency showed little deterioration with increasing drain bias voltage. These results demonstrate for the first time the high frequency and high power operation potential of the heterostructure field effect transistors based on AIGaN grown on SiC.
INTRODUCTION Wide band-gap semiconductor GaN has been recognized as a promising candidate for high power electronic device applications due to its high breakdown field and saturated electron drift velocity. Excellent DC and RF performances have been reported for heterostructure field effect transistors (HFETs) fabricated on AIGaN/GaN heterostructures grown on sapphire substrates [15]. Given the several watts/mm power density already demonstrated on AJGaN/GaN based HFET [4,5], the low thermal conductivity (0.5 W/cm K [6]) of sapphire is likely to become the limiting factor for practical power amplifier performance. While flip-chip approach that dissipates the heat generated by the device from the top surface can alleviate this problem to some extent, it is advantageous to use a substrate with high thermal conductivity. A potential semiconductor for power electronics in itself, SiC has a high thermal conductivity (4.9 W/cm K for 6H SiC) and is closely lattice matched to GaN. AIGaN/GaN high electron mobility transistors (HEMTs) have been reported [7] using SiC as the substrate with a transconductance of 70 mS/mm and ft and f,,,x of 6 GHz and 11 GHz, respectively. It has also been shown that the two-dimensional electron gas (2DEG) transport property from the AIGaN/GaN heterostructures grown on SiC is comparable to or even better than those grown on sapphire substrates [8] in terms of the electron mobility and the sheet carrier density-times-mobility (n,[t) product. Better HFET performance should be expected. In this presentation, we report on the DC and RF characteristics of the greatly improved high transconductance HFETs fabricated with AIGaN/GaN grown on both n- and ptype SiC substrates.
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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
EXPERIMENT The layer structures used in this work consisted of a 0. 15 pam AIN buffer on either n- and p-type c-plane SiC. This was followed by a nominally 1 pm semi-insulating GaN, 50 nm of n-GaN (n= 5-7x1017 cm 3), 3 nm undoped Al 0.2Gao. 8N spacer, and 30 nm of doped Al0 .2Ga0 .8 N. The doping level in the Al0. 2Gao.sN had resulted in measured nap products of 2.7x10'6/V-s and 1.5xl0 1 6/
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