The transient exclusion effect in intrinsic semiconductors

Abstract

The process of formation and relaxation of charge-carrier exclusion in a structure with an h-l-junction has been studied both theoretically and experimentally. It is shown, for the first time, that temporal dependencies of the most important process characteristics, such as the exclusion length, the extent of integral charge-carrier depletion in the base, and the current establishment, may be expressed by simple analytical formulae. The modelling experiment has been carried out using Ge crystals with intrinsic conductivity (T 300 K). In order to 'visualize' the spatial distribution of the charge carriers, the transmission beyond the edge of fundamental absorption and non-equilibrium thermal emission of the structure base in the spectral range 8-12 µm have been investigated. It is shown that thermal generation of charge carriers by the ohmic contact and base surface plays a key role in the process of exclusion relaxation. The difference between process duration in the cases of establishment of a steady-state current and spatial-charge-carrier distribution is explained by the formation of a high electric field domain.

Negative contrast IR emitting device based on the carrier contact exclusion

“…In the range from 8 to 12 µm at T = 400 K one can achieve actually a level of 10 mW cm −2 . The device speed is determined, according to [9], by carriers lifetime (∼10 −4 s). The signal magnitude is easily controlled by applied voltage with a simple functional dependence.…”

“…As following from (6), in this case one may enlarge ℵ by increase of n 0 with temperature growth. In addition, one may enlarge ℵ by increase of N a as long as the condition N a n 0 remains valid.…”

“…If the structure is optically thin in this direction then its emissivity is ε = kd 1, where k = σ p p + σ n n is the absorption coefficient, d is the thickness along y, n and p are the free electron and hole concentrations, σ n , σ p are the cross sections of absorption by free electrons and holes, respectively. The spectral radiation density under such conditions is proportional to the concentration of carriers [6,7]. For simplicity, one may also neglect contribution of electrons to emissivity because their absorption cross section in the considered spectral range is small as compared to holes.…”

“…where L ex is the exclusion length, S is the base cross section area. On the other hand, the following approximate expression may be written for ℵ according to [5,9]…”

“…When comparing (4) and ( 5) with experiment one should take into account that assumption of n = 0 becomes rough at large distances from the p + contact (figure 1) that may cause some discrepancy [9].…”