**1. Introduction**

Deep Level Transient Spectroscopy (DLTS) is an efficient and powerful method used for observing and characterizing deep level impurities in semiconductors. The method was initially introduced by D. V. Lang [1] in 1974. DLTS is a capacitance transient thermal scanning technique, operating in the high frequency (Megahertz) range. It uses the capacitance of a pn junction or Schottky barrier as a probe to monitor the changes in charge state of a deep centre. The capacitance techniques [2-10] used before DLTS lacked either in sensitivity, speed, range of observable trap depths, or the spectroscopic nature, thus rendering the techniques inade‐ quate for a complete characterization of a deep level. DLTS has the advantage over all the techniques used to-date in that it fulfils almost all the requirements for a quick and complete characterization of a deep centre. DLTS is a technique, which is sensitive enough, rapid and easy to analyze. It is able to distinguish between majority- and minority-carrier traps [2]. DLTS can also give the concentrations, energy and capture rates of both kinds of traps. It is spectro‐ scopic in the sense that it can also resolve signals due to different traps. In the many variants of the basic DLTS technique the deep levels are filled with free carriers by electrical or optical methods. Subsequent thermal emission processes give rise to a capacitance transient. The transient is analyzed by signal processing while the temperature is varied at a constant rate. This results in a full spectroscopic analysis of the semiconductor band-gap.

For a complete understanding of DLTS we must have some basic knowledge of capacitance transients arising from the depletion region of a p-n junction. The use of capacitance transients for studying the properties of defect centers is well known [4-6]. These transients provide

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information about an impurity level in the depletion region by observation of the capacitance transient originating from the return to thermal equilibrium after a perturbation is applied to the system. A brief description of the capacitance change due to the change in occupancy of the deep levels in the depletion region is given below.
