Infrared Excitations in Electronic Systems with Reduced Dimensionality

Abstract

Modern semiconductor devices are increasingly based on structure in which the electronically active channel is an electron or hole system with reduced dimensionality. The classical example is the metal-oxide-semiconductor-field-effect-transistor (MOSFET) in which electrons can be confined at the semiconductor-oxide interface to form a quasi-two-dimensional electron system (2DES). Another more recently developed device is the high-electron-mobility transistor (HEMT) in which a suitable doping profile induces a 2D electron channel at the heterojunction potential barrier. The extraordinary electronic properties of such two-dimensional electron systems at interfaces are summarized in part in an extended review by Ando, Fowler and Stern1. Fig. 1 schematically pictures the band diagram in the confinement direction perpendicular to the interface for both devices. In the MOS-system (Fig. 1a) a positive gate voltage Vg is applied between the p-type semiconductor and the metal gate and causes bending of the conduction and valence bands near the semiconductor-oxide interface. The electrons are confined in a narrow potential well bounded by the oxide barrier and the conduction band edge. In the modulation doped heterojunction (Fig. 1b) n-type doping of the large gap semiconductor (e.g. AlxGa1-x As) causes electron transfer to the undoped p-type lower gap semiconductor (e.g. GaAs) with a larger electron affinity.