SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

490 CHAPTER 10. COHERENT TRANSPORT AND MESOSCOPIC DEVICES

PERFECT

CRYSTALLINE

MATERIALS WITH

RIGID LATTICE

“SMALL”

DEGREE OF

DISORDER

“LARGE”

DISORDER

VERY SMALL

STRUCTURES

No scattering:

Bloch

oscillations

Scattering, mobility,

velocity-field

relations

Trapping-hopping

conduction

• Electron-wave
  based quantized
  transport


• Single electron
  tranport
  (a) (b) (c) (d)

Figure 10.1: A schematic of how levels of structural disorder and size impact electronic proper-
ties of a material.

move according to the modified Newton’s equation; and ii) Bloch oscillations, where electrons
oscillate ink-space as they reach the Brillouin zone edge, as will be discussed in section 10.2.
In addition we can have tunneling type transport as well as quantum interference effects. These
are discussed in Sections section 10.3 and section 10.4. The wave nature of electrons and the
quantization of charge also leads to conductance quantization and Coulomb blockade effects.
Finally if the spin of electrons can be manipulated novel devices can result.
In figure 10.1b we show the case where there is a small degree of disorder. This is the situation
where Born approximation can be used and transport under these conditions has been discussed
in the previous chapters. In figure 10.1c we show the case where the structural disorder is large.
This happens in amorphous materials and leads to localized states (band tails) and transport
that is described by “hopping” behavior. Transport in disordered semiconductors (or amorphous
semiconductors) is relatively poor and used primarily for low cost applications such as thin film
transistors for display. Such devices are not useful for high performance devices which are the
primary focus of this text.
Finally in figure 10.1d we show the case for devices that are very small (several tens of atoms
across). Such structures are calledmesoscopicstructures and are increasingly becoming impor-
tant as fabrication technology improves. Mesoscopic structures have a number of very interesting
and potentially important transport properties. Single electron effects as well as spin effects are
manifested in such structures.