**2. Deep level transient spectroscopy**

To characterize lattice defects in a semiconductor, several techniques were used during the second half of the last century. Between these methods, we cite the thermally stimulated current (TSC) (Leonard & Grossweiner, 1958; Bube, 1960), the admittance spectroscopy

(Losee, 1974), the increase or decay curves of photoconductivity (Rose, 1951; Devore, 1959), the optically stimulated conductivity (Lambe, 1955; Bube, 1956), and the analysis of spacecharge-limited currents as function of applied voltage (Smith & Rose, 1955; Rose, 1955; Lampert, 1956). The basic concept of using the change of capacitance under bias conditions by the filling and emptying of deep levels was already anticipated fifty years ago (Williams, 1966). The thermally stimulated capacitance (TSCAP), which gives the temperature dependence of junction capacitance (Sah et al., 1978; Sah & Walker, 1973), was used. By dressing the properties of all these techniques, D. V. Lang found that they lacked the sensitivity, the speed, the depth range of recorded trap, and the spectroscopic nature to make them practical for doing spectroscopy on non-radiative centers. For that, in 1974, he proposed DLTS as a characterization method of lattice defects that can overcome the disadvantages of the other methods (Lang, 1974). DLTS is based on the analysis of the change of capacitance due to a change in bias condition at different temperatures. It can be applied to Schottky contacts and *p-n* junctions. DLTS has advantages over TSC due to its better immunity to noise and surface channel leakage currents. It can distinguish between majority and minority carrier traps, unlike TSC, and has a strong advantage over admittance spectroscopy, which is limited to majority-carrier traps. Comparing with TSCAP, DLTS has much greater range of observable trap depths and improved sensitivity. Despite the success of optical techniques such as photoluminescence to characterize superficial levels, they are rarely used in the study of non-radiative deep levels. Furthermore, such experiences must be done in the infrared domain. However, sensors are less sensitive than in the visible domain. Thus, we need a technique, which can separate between minority and majority traps and evaluate their concentrations, their energies, and their capture cross sections.
