Diffusion Capacitance of an Epitaxial High Barrier Schottky Diode M. M. Shahidul Hassan 1 and Ashok Kumar Karmokar 1 1 Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology. Dhaka-1000, Bangladesh Abstract – A low barrier Schottky diode is a majority carrier device. But, a high barrier Schottky diode injects minority carrier at forward bias. Due to this minority carrier injection, minority carrier charge is stored in the drift region. This stored charge gives rise to diffusion capacitance and that is not negligible for high barrier Schottky diode. No literatures have yet been published on the diffusion capacitance of a high barrier Schottky diode. In this work, an analytical expression for diffusion capacitance of an epitaxial high barrier Schottky diode is developed assuming no recombination in the drift region. The expression is valid for all levels of injection. Both the drift and diffusion current is considered. The reflecting property of the n-n+ junction is described by an effective surface recombination velocity in this paper. The effects of different device parameters on the diffusion capacitance are studied. Keywords: high barrier, diffusion capacitance, effective surface recombination velocity. 1. INTRODUCTION High barrier Schottky diodes are today widely used for low voltage rectification [1], as a bipolar mode Schottky diode [2], in bipolar memory cells [3], in Integrated Schottky logic and Schottky transistor logic [4] and in high-speed devices [5]. For a low barrier Schottky diode, the total current is due to majority carriers only. But, for a high barrier Schottky diode, minority carrier injection takes place. Several authors studied the J-V characteristics of a high barrier Schottky diode considering both the majority and minority carrier currents [6-8]. The authors considered recombination in the drift region and used regional approximations to derive the expression for the hole profile. If the recombination in the drift region is considered, the analysis is rigorous and expressions become very complex. Practically, the length of the drift region is less than the hole diffusion length. For practical devices, the recombination in the drift region is insignificant and can, therefore, be neglected. If we neglect the recombination in the drift region a single expression for hole profile can be obtained and the resulting expressions become more compact and simple. 2. ANALYSIS The one-dimensional structure of an epitaxial Schottky barrier diode is shown in fig. 1. Fig. 1. A one-dimensional epitaxial Schottky barrier diode. The epitaxial layer of the Schottky barrier diode is made of lightly doped n-type silicon semiconductor of constant doping density N d and is bounded by a heavily doped n + substrate. W and L d are the length of the depletion and epitaxial layer respectively. When the SBD is forward biased, holes will be injected into the drift region. Electrons move towards the SB contact leading to a quasi-neutral situation accompanying the hole pile up within the drift region. Blocking of the holes by the low-high (n-n+) junction enhances its accumulation within the drift region and the drift region is conductivity modulated. This stored holes gives rise to diffusion capacitance. The one-dimensional transport equations within the drift region are given by the following equations: () () () () n n n dn x J x q nxEx qD dx µ = + . . . . . . . . . . . (1) () () () () p p p dp x J x q pxEx qD dx µ = − . . . . . . . . . . . . (2) where, J n (x) and J p (x) are the total drift and diffusion electron and hole current density, n(x) and p(x) are the electron and hole concentrations, E(x) is the electric field, µ n and µ p are the electron and hole mobilities, D n and D p are the electron and hole diffusion constants and q is the charge of electron. If we neglect the recombination in the drift region, both the electron and hole current density become constant throughout the drift region. 0 n d J (x) constant; x L = ≤ ≤ . . . . . . . . . . . . . . . . .(4) 0 p d J (x) constant; x L = ≤ ≤ . . . . . . . . . . . . . . . (5) Metal n-type n + L d W x Second International Conference on Electrical and Computer Engineering ICECE 2002, 26-28 December 2002, Dhaka, Bangladesh ISBN 984-32-0328-3 124