Theory for nonequilibrium behavior of anisotype graded heterojunctions

An analysis for the terminal behavior and capacitance of anisotype graded heterojunctions is presented. A closed form expression for the I–V characteristics is derived using the depletion approximation and the rigid band model. The expression is valid for uniform nondegenerate doping and low-level injection. The transition region is modeled to extend beyond the space-charge region with a constant bandgap gradient. Variations of the dielectric constant, carrier lifetime and mobility are ignored. The nonsaturating nature of the reverse current is demonstrated. The effects of variations of bandgap and electron affinity on the I–V characteristics and junction capacitance are discussed. Numerical results for a graded nGe-pGaAs device are given.