In this chapter aspects of the basic physics of heterojunctions are presented. The chapter deals mainly with basic ideas relating the electronic structure and the crystallography and is certainly not meant to be a comprehensive review but to serve as a building block for successive chapters. In particular, detailed discussion of experimental and theoretical techniques has been avoided in view of the limitations of space.

This chapter will review the simulation of semiconductor heterojunction devices. By simulation it is implied that a computer program has been used to mimic or predict the physical behaviour of a device in some way. There are almost as many different simulation tools as there are researchers in the area of heterojunction device simulation. This is mainly because people like to feel that they have a 'tailor-made' solution to their particular needs. More fundamental than this is the need for models of different levels of physical complexity to model different device structures. For example it is not possible to model the quantisation effects that occur in a quantum-well structure, with a basic drift diffusion simulation tool. On the other hand such a tool may be adequate to investigate the operation of physical structures such as the heteroj unction bipolar transistor (HBT) or solar cell. However, if simulation is to be used by engineers to develop products based around particular devices, the simulation tool must be computationally attractive (preferably running on a workstation platform1) and have an efficient user interface, whilst at the same time being based on sufficient physics to predict the device behaviour to some degree of accuracy. This chapter brings a diversity of models and modelling techniques together and attempts to give the reader a feeling for the circumstances under which any one model or technique is appropriate.

The High Electron Mobility Transistor (HEMT) has achieved its predicted performance goals, operating at high frequencies (>60 GHz) and high speeds (>10 Gbit/s). As a consequence, it is becoming the transistor of choice for millimetre wave and high speed applications; stimulating the development of new monolithic integrated electronic and optoelectronic analogue and digital circuits. These circuits will be utilised in radar, satellite communications and computing applications.

In this chapter, the realisation of semiconductor lasers which operate continuously at room temperature with reasonably low currents is not possible without the use of heterostructures. In a normal double heterostructure (DH) laser the hetero structures provide the refractive index step which is necessary to localise the light in a waveguide, and the difference in band gap which are necessary to confine both electrons and holes to the same region of the structure. The individual discontinuities in the conduction and valence bands are used further in a quantum well laser to produce beneficial 2-dimensional effects in the electronic structure of the active region material and to modify the operating wavelength by quantum size effects. While all these intrinsic aspects of heterostructures are of great benefit to the laser, it is important to avoid deleterious effects due to the extrinsic properties of the structure, such as poor quality interfaces between the heterostructure components and unwanted non radiative leakage currents, which can increase the total operating current.