Suppressed surface‐recombination structure and surface passivation for improving current gain of 4H‐SiC BJTs

Abstract

4H‐SiC bipolar junction transistors (BJTs) are one of the promising candidates for next‐generation power devices. 4H‐SiC BJTs have the advantages of low on‐resistance and high temperature capability. On the other hand, high common emitter current gain is required from a practical use point of view because BJTs are current‐controlled devices. In order to improve the current gain of 4H‐SiC BJTs, we have concentrated on suppressing surface recombination on the SiC surface, which is a critical limiting factor of the current gain and can be suppressed by reducing carrier density and/or trap density. We have proposed novel 4H‐SiC bipolar junction transistors with suppressed surface‐recombination structure: SSR‐BJTs that have the SiC surface with low carrier density, characterized by a lightly doped n‐type layer (LDN layer) and a highly resistive p‐type region (HRP region). In addition, surface passivation suitable for reducing surface traps has been investigated by applying a new characterization method of surface‐recombination current in which the sp · Ls value, a product of a surface‐recombination velocity (sp) and a surface‐diffusion length (Ls), is derived from an analysis of forward I–V characteristics of SiC pn diodes. We have experimentally demonstrated that the sp · Ls value is a practical indicator to evaluate surface passivation. By using the characterization, an effective passivation method with the combination of a pyrogenic oxidation, a post‐oxidation anneal in H2 ambient, and an anneal in NH3 ambient has been developed. SSR‐BJTs with a variety of device structures and process conditions have been fabricated to investigate their characteristics. A fabricated SSR‐BJT showed a recorded maximum current gain of 134 at room temperature with a specific on‐resistance of 3.2 mΩ cm2 and a blocking voltage VCEO of 950 V. The SSR‐BJT kept a current gain of 60 °C at 250 °C with a specific on‐resistance of 8 mΩ cm2. These results have demonstrated the outstanding current‐gain capability of the SSR‐BJTs compared to conventional BJTs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of ultrathin AlN spacer on electronic properties of GaN/SiC heterojunction bipolar transistors

“…Bipolar junction transistors (BJTs) in 4H-SiC are very attractive candidates for high-power switching devices owing to their high breakdown voltage, low on-resistance, and high current density. [1][2][3][4][5][6][7][8] In comparison with metal-oxide-semiconductor field-effect transistors (MOSFETs), BJTs are free of oxide-related problems such as channel mobility and oxide reliability. However, SiC BJTs have so far suffered from a limited current gain.…”

Defect Electronics in SiC and Fabrication of Ultrahigh-Voltage Bipolar Devices

“…On the (0001) face, a fabricated BJT also exhibited a high current gain of 257. These values are twice as large as the previous record gain (35). The breakdown voltage of these BJTs was approximately 600 V, due to the lack of edge termination.…”

Current Transport Characteristics of Quasi-Al_xGa_1-xN/SiC Heterojunction Bipolar Transistors with Various Band Discontinuities

“…Since BJTs are current-controlled devices, high common-emitter current gain (β) over 100 is required to use simple drive circuits. A number of studies have been devoted to improve the current gain [2][3][4], and a high current gain of 257 was achieved recently [5]. In SiC BJTs, however, current-gain reduction with elevating the temperature is inevitable because injection efficiency decrease by increasing hole density in the base region [6].…”