Photomagnetoelectric Effect in Graded Band-Gap Semiconductors

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 Iyˢᶜ 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 Iyˢᶜ are further analyzed under the conditions of small and large band-edge gradients. In this case, the conditions for sign reversal of Iyˢᶜ are discussed and it is found that spectral response of Iyˢᶜ 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 Iyˢᶜ.