Models for ohmic contacts on graded crystalline or amorphous heterojunctions

Abstract

A one-dimensional model for sandwich-type graded heterojunctions is presented to explain the formation of contacts that cannot be based on highly doped contact layers, such as indium contacts to GaAs and GaSb. In this model a hyperbolic dependence on distance for the permittivity and a constant space charge density owing to filled interferance states is assumed. Algebraic solutions have been obtained for the barrier height and capacitance of a wide variety of heterojunctions. The conditions for zero barrier height, i.e. ohmic contacts, are discussed. The graded heterojunction model is also applied to heterojunctions formed by an amorphous and a crystalline semiconductor to explain the formation of ohmic contacts in cases where neither doping nor crystalline heterojunction effects can be assumed, such as Au, Ag, InAu, InAg and AuNi metal contacts to GaAs, GaSb or GaP.