Chapter 4 - Heterostructure Device Physics: Band Discontinuities as Device Design Parameters

Abstract

This chapter discusses the physics of heterostructure devices. Heterostructures are semiconductor structures in which the basic semiconductor itself is different in different portions of the structure. There are several reasons why the incorporation of heterojunctions into device structures is advantageous. The dominant one, more important than all other reasons combined, is the fact that the abrupt change in the basic semiconductor implies an abrupt change in the energy band structure. This invariably leads to discontinuities, or offsets, in the conduction and valence band edges. These offsets act as potential steps, like abrupt electrostatic potential steps, but independently of and in addition to whatever purely electrostatic potential variations are present. Such quasielectric potential steps provide an extraordinarily powerful new design parameter to aid in the control of the flow and distribution of the mobile charge carriers in device structures. To be able to utilize heterostructure band offsets as device design parameters, it is necessary to know the magnitude of the band offsets. Therefore, the chapter presents the current status of the knowledge on this matter and it also illustrates the complete energy band diagram of a single heterojunction in the presence of the band bending caused by the space charges that accompany heterojunctions just as they accompany p–n homojunctions.