The graded doped trench MOS Barrier Schottky rectifier: a low forward drop high voltage rectifier Srikanth Mahalingam, B. Jayant Baliga Power Semiconductor Research Center, North Carolina State University, Raleigh, NC 27695-7924, U.S.A. Received 8 December 1997; received in revised form 23 June 1998 Abstract A novel high voltage Schottky rectifier, called the Graded Doped Trench MOS Barrier Schottky (GD-TMBS) rectifier, is described in this paper. It is shown to have very low forward drop with excellent reverse blocking characteristics through device simulation and electrical characterization of fabricated devices. A linearly graded drift region doping profile is shown to result in an uniform electric field in the drift region resulting in the ability to support blocking voltages proportional to the trench depth. Two-dimensional device simulations have shown that breakdown voltages of upto 200 V can be achieved with a very low forward drop of 0.54 V. The measured on-state drop of fabricated 60 and 100 V GD-TMBS are about half those of conventional Schottky rectifiers. Power dissipation analysis indicates higher operating temperatures (1508C) with reduced heat sink sizes when compared to conventional Schottky barrier diodes. # 1998 Elsevier Science Ltd. All rights reserved. 1. Introduction Schottky power rectifiers find application in low-vol- tage switching power supplies for integrated circuits owing to their low forward voltage drop and faster reverse recovery compared to their P–N junction counterparts. The need for reduced power consump- tion and increased energy eciency calls for a low on- state drop and reduced leakage current in a rectifier. Unfortunately, the tradeo between the forward bias drop and the reverse leakage current in a Schottky rec- tifier makes it dicult to reduce both characteristics simultaneously [1]. Moreover the reverse leakage cur- rent of a Schottky rectifier increases with increase in reverse bias due to image force barrier lowering eect leading to soft breakdown characteristics [1, 2]. Attempts to improve the tradeo between the forward drop and reverse leakage current have resulted in novel Schottky rectifier structures, such as the JBS (Junction Barrier Schottky) [3–6] and the TMBS (Trench MOS Barrier Schottky) [7] rectifier. In the JBS rectifier, the dead-zone presented by the junction regions imposes a limit on the forward drop for a given breakdown voltage. Moreover the susceptibility of shallow junctions to breakdown in this structure limits the breakdown voltage capability of the JBS rec- tifiers to less than 50 V. In the TMBS structure [7], two dimensional charge coupling between the mesa region and the trench sidewall metal reduces the elec- tric field at the Schottky interface thereby reducing the reverse leakage current and improving the reverse blocking capability. However, the high value of electric field at the bottom corner of the trenches in this struc- ture has been found to limit their reverse blocking capability to about 25 V. In this paper, an improved TMBS rectifier structure that employs a nonuniformly doped drift region is described. It is demonstrated, through two dimensional numerical simulations, to have enhanced breakdown voltage capability (upto 200 V), while maintaining a very low forward drop. Experimental results that are in excellent agreement with the simulations are also presented to demonstrate the viability of the concept. 2. Device concept The basic structure of the proposed device, called the Graded Doped Trench MOS Barrier Schottky (GD-TMBS) rectifier, is shown in Fig. 1. This structure Solid-State Electronics 43 (1999) 1–9 0038-1101/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0038-1101(98)00250-0 PERGAMON