An improved model to explain ohmic contact resistance of f-GaAs and other semiconductors

Abstract

A model is proposed which explains the inverse proportionality between specific contact resistance ϱ c and the carrier concentration ( N D ) of n-GaAs ohmic contact obtained experimentally very well and can be extended to the ohmic contact of other semiconductors. This model assumes that ϱ c is composed of two parts ϱ c 1 and ϱ c 2. The contact resistivity ϱ c 1 is due to the contact between the alloy and the underlying heavily doped contact region ( N DC for n-type contact, N AC for p-type contact) formed by doping from the contact alloys during annealing. The contact resistivity ϱ c 2 is caused by the barrier height (Φ 2) of the high-low junction between the heavily doped contact region and the bulk material. There are two aspects: (1) If bulk material is degenerated, i.e. N D > N C ( N C : effective density of states in conduction band) or N A > N V ( N V : effective density of states in valence band), the barrier height Φ 2 vanishes and ifϱ (c) is mainly determined by ifϱ c 1 which depends solely on concentrations N DC and N AC . It is calculated theoretically. (2) When the bulk material is lightly doped, i.e. N D < N C or N A < N V , then Φ 2 appears and increases with the decreasing of N D ( N A ). If N DC or N AC are high enough, the field emission is the main mechanism to control the carrier transport between the contact alloy and the underlying layer. In this case ϱ c is predominantly determined by ϱ c 2 and an inverse proportionality between ϱ c and N D or N A can be found.