Photomagnetoelectric Effect in Graded Band-Gap Semiconductors

Abstract

The equation of continuity has been written for a graded band-gap semiconductor taking into account the effect of band-edge gradients and the variation of the radiative recombination lifetime with position. The equation is then solved to give the value of excess minority carriers at a point in such a specimen. The effect of surface states is included through the appropriate boundary conditions. These expressions are deduced for a semiconductor which is space-charge free as a result of inhomogeneous doping, i. e., the doping being position dependent in the direction of band-edge gradients. The resulting expression shows that the $I_y^{\mathrm{sc}}$ consists of the usual photodiffusion term plus a component arising from a "quasifield" setup due to band-edge gradients. Simplified expressions are obtained under the following conditions: (a) The generation of carriers takes place only on the front surface, and (b) the variation of the radiative lifetime with position is neglected. In the latter case, the expressions for $I_y^{\mathrm{sc}}$ are further analyzed under the conditions of small and large band-edge gradients. In this case, the conditions for sign reversal of $I_y^{\mathrm{sc}}$ are discussed and it is found that spectral response of $I_y^{\mathrm{sc}}$ depends on whether the band-edge gradient helps or opposes the concentration gradient. A few curves are plotted to exhibit the spectral response and the sign reversal of $I_y^{\mathrm{sc}}$.