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K5.19.1

The Effect of Channel Recess and Passivation on 4H-SiC MESFETs Ho-Young Cha, Christopher I. Thomas, Goutam Koley, Lester F. Eastman, and Michael G. Spencer Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, U.S.A.

ABSTRACT Channel-recessed 4H-SiC MESFETs were fabricated and demonstrated excellent small signal characteristics. A saturated current of 250 – 270 mA/mm at Vgs = 0 V and a maximum transconductance of 40 – 45 mS/mm were measured for channel-recessed devices with a gate length of 0.45 µm. The three-terminal breakdown voltages (Vds) range from 120 V to 150 V. The Ft and Fmax of the 2 × 200 µm devices were measured to be 14.5 GHz and 40 GHz, respectively. The channel recess technique results in a lower saturation current but higher breakdown voltage which makes it possible for the devices to operate at high voltages. Si3N4 passivation suppresses the instability in DC characteristics and improves CW power performance by reducing the surface effects. Less dispersion in the drain current during a power sweep was observed after passivation.

INTRODUCTION Silicon carbide (SiC), a wide bandgap semiconductor, has unique material properties, such as high saturated electron velocity, high breakdown field, and high thermal conductivity. Because of these properties, SiC MESFETs have been developed for high power microwave applications. Recently, good progress in material growth has been achieved and SiC MESFETs have demonstrated great potential for high power applications. With high quality materials, the device structure is an important factor in determining the electrical characteristics of SiC MESFETs. It is well known in GaAs MESFETs that the recess technique increases the device breakdown voltage due to gradual field distribution at the edge of the gate. Likewise, the channel-recessed SiC MESFETs show higher breakdown voltages and excellent small signal characteristics compared to non-recessed devices. The highest Fmax ever reported in SiC MESFETs is 42GHz with an Ft of 13.2 GHz [1]. In this work, the channel recessed devices showed a desirable combination of small signal characteristics, with an Ft and Fmax of 14.5GHz and 40GHz for 2 × 200 µm devices. SiC MESFETs have been reported to have a current instability problem due to surface and substrate trapping effects [2]. Reducing the number of deep traps in the substrate is essential for solving the instability problem. Recently, high purity semi-insulating substrates have been developed, and SiC MESFETs fabricated on a Vanadium-free substrate showed significant improvement in device performance [3]. In this work, we studied surface effects on the devices by comparing electrical characteristics before and after passivation. It is suggested that the passivation reduces the current instability phenomenon and consequently improves the power performance.

K5.19.2

EXPERIMENT The epitaxial layers used in this work consisted of a semi-insulating 4H-SiC substrate, a 0.25 µm17p-type buffer layer doped < 5 × 1015 cm-3, and a 0.