High Electron Mobility Transistors (HEMTs)

The High Electron Mobility Transistor (HEMT) is a heterostructure field-effect transistor (FET). Its principle is based on a heterojunction which consists of at least two different semiconducting materials brought into intimate contact. Because of the different band gaps and their relative alignment to each other, band discontinuities occur at the interface between the two semiconducting materials.

Enlarged view: Diagram of the band structures of two InAlAs and InGaAs at the equilibrium. — Diagram of the band structures of InAlAs and InGaAs at equilibrium.

These discontinuities are referred to as the conduction and valence band offsets ΔE_c and ΔE_v. By choosing proper materials and compositions thereof, the conduction band offset can form a triangular shaped potential well confining electrons in the horizontal direction. Within the well the electrons can only move in a two-dimensional plane parallel to the heterointerface and are therefore referred to as a two-dimentional electron gas (2DEG).
To determine the exact shape of the conduction and valence bands, the Schrödinger and Poisson equations must be solved self-consistently.

Enlarged view: Semiconductors in contact at the equilibrium. A 2DEG is formed at the interface. — Semiconductors in contact at the equilibrium. A 2DEG is formed at the interface in the triangular shaped well.