Chin. Phys. B Vol. 20, No. 1 (2011) 017304 Improved performance of 4H-SiC metal-semiconductor field-effect transistors with step p-buffer layer * Deng Xiao-Chuan(邓小川) † , Zhang Bo(张 波), Zhang You-Run(张有润), Wang Yi(王 易), and Li Zhao-Ji(李肇基) State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China (Received 23 July 2010; revised manuscript received 19 August 2010) An improved 4H-SiC metal-semiconductor field-effect transistors (MESFETs) with step p-buffer layer is proposed, and the static and dynamic electrical performances are analysed in this paper. A step p-buffer layer has been applied not only to increase the channel current, but also to improve the transconductance. This is due to the fact that the variation in p-buffer layer depth leads to the decrease in parasitic series resistance resulting from the change in the active channel thickness and modulation in the electric field distribution inside the channel. Detailed numerical simulations demonstrate that the saturation drain current and the maximum theoretical output power density of the proposed structure are about 30% and 37% larger than those of the conventional structure. The cut-off frequency and the maximum oscillation frequency of the proposed MESFETs are 14.5 and 62 GHz, respectively, which are higher than that of the conventional structure. Therefore, the 4H-SiC MESFETs with step p-buffer layer have superior direct-current and radio-frequency performances compared to the similar devices based on the conventional structure. Keywords: 4H-SiC, metal-semiconductor field-effect transistors, step buffer layer PACS: 73.40.Qv, 72.80.Ey DOI: 10.1088/1674-1056/20/1/017304 1. Introduction Silicon carbide (SiC) is one of the materials for choice in advanced next-generation power electronics both for high frequency, high temperature and high voltage power-switching applications due to its ex- cellent electrical and physical properties, i.e., wide band gap, high electron saturation velocity, high critical electric field, high radiation stability, and high thermal conductivity. [1-3] These properties have made SiC metal-semiconductor field-effect transistors (MESFETs) a promising candidate for microwave power applications. In recent years, SiC MESFETs are in the market, and the application areas are the commercial and military communication and radars. As is known to all, the use of a p-buffer layer that isolates the semi-insulating substrate from the active channel is of vital importance and can significantly re- duce the substrate trapping effects associated with the channel-substrate interface for MESFETs. [4] The per- formance and reliability correlate closely to the critical characteristics of off-state breakdown voltage and par- asitic capacitance that are strongly dependent on the quality and behaviour of the p-buffer layer. Since the SiC material growth and the device fabrication tech- nology are becoming quite mature, substrate trapping effects of SiC MESFETs can be alleviated by the in- troduction of high-purity semi-insulating substrate. [5] Hence the p-buffer layer of the device will play a crit- ical role to improve the device performances, which needs to be carefully designed in order to provide the needed channel isolation while not hindering high- frequency device operation by introducing significant parasitic capacitance and drain leakage current. Al- though a large number of SiC MESFETs structures and various optimisations have been reported, [6,7] few studies of p-buffer layer configuration in SiC MES- FETs have been performed up to now. In this paper, an improved 4H-SiC MESFETs with step p-buffer layer is investigated using a two- dimensional (2D) device simulation program, ATLAS. The thinner p-buffer layer in ungate channel region is employed to obtain low parasitic series resistance, while the thicker p-buffer layer underneath the gate terminal is adopted to maintain a larger aspect ra- tio of gate length to channel thickness (L g /a). The direct-current (DC) and radio-frequency (RF) perfor- mances of the proposed 4H-SiC MESFETs have been studied in detail, and the simulation results are com- pared with those obtained from the conventional p- * Project supported by the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2009J029). † Corresponding author. E-mail: xcdeng@uestc.edu.cn © 2011 Chinese Physical Society and IOP Publishing Ltd http://www.iop.org/journals/cpb http://cpb.iphy.ac.cn 017304-1