Electronic defects reduce the photosensitivity, suppress doping and impair the device performance of a-Si:H. Their high density in pure amorphous silicon makes this material of lesser interest and is the reason for the attention on the hydrogenated material, in which the defect density is greatly reduced. The remaining defects in a-Si:H control many electronic properties and are centrally involved in the substitutional doping process. The phenomena of metastability, which are described in Chapter 6, are caused by the defect reactions.

Defects are described by three general properties. First is the set of electronic energy levels of their different charge states. Those defects with states within the band gap are naturally of the greatest interest in understanding the electronic properties because of their role as traps and recombination centers. Second is the atomic structure and bonding of the defect, which determine the electronic states. Third are the defect reactions which describe how the defect density depends on the growth and on the treatment after growth. This chapter is mostly concerned with the first two properties, and the defect reactions are discussed in Chapters 5 and 6.

The defects are one of the more controversial aspects of a-Si:H. Many models have been proposed and there is active debate about the interpretation of several of the important experiments. This chapter concentrates on what is currently the most widely accepted model, but also compares the different possibilities and discusses the underlying reasons for disagreement.