GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

6

DEVICE-RELATED PHYSICS AND PRINCIPLES

6.1 SOME BASIC ELECTRONIC CONCEPTS IN LOW-DIMENSIONAL PHYSICS

In the last decade the technology drive toward smaller microelectronic devices has given rise to the concept of low-dimensional physics [Kel87]. Low-dimensional structures [Sak80; Pet82], such as the quantum-well wire (QWW) or the quantum-well box (QWB), have quantum confinement (QC) of two or three dimensions and have in the last few years attracted much attention not only for their potential in uncovering new phenomena in solid state physics but also for their potential application in semiconductor devices. Theoretically extremely high electron mobility in QWWs and high performance of QWW or QWB lasers [Ara82] and modulators [Sue88] can be expected. Recent experiments in QWB resonant-tunneling devices [Ree88] have further claimed to demonstrate new structures within a zero-dimensional system. This chapter will discuss the physics of low-dimensional devices.

In this section we consider the range of electronic states that are encountered in various microstructure systems. The ideal systems come straight from the quantum mechanics—potential wells of different shapes bounded by infinitely high barriers [Kel87]:

1. A three dimensional system without bounding (3D).
2. A purely two-dimensional system bounded by Z = 0 and L (2D), whenever L is small.
3. A purely one-dimensional system bounded by X, Y = 0 and L (1D), whenever L is small.
4. A quasi two-dimensional system bounded by Z = 0 and L