Photoeffects in Nonuniformly Irradiated p-n  Junctions

GERALD LUCOVSKY 

A theoretical basis is provided for the interpretation of photoeffects observed in nonuniformly irradiated p-n junctions. Differential equations describing the junction photovoltage are developed through an application of the continuity and diffusion equations. Solutions of the small-signal steady-state photoeffect equation indicate that the effects of nonuniform irradiation become increasingly important as the ratio of the lateral to the transverse resistance increases. a, a parameter introduced in this paper and designated as the lateral-fall off parameter, is a measure of this resistance ratio. The lateral photovoltage resulting from nonuniform irradiation can be eliminated by reverse biasing the junction into saturation. Experimental curves in agreement with the predictions of the analysis are presented. 

INTRODUCTION 

It is well known that sufficiently energetic radiation normally incident on a p-n junction produces a photovoltage across the junction.  If the junction is nonuniformly irradiated, the photovoltage will, ingeneral, vary with position, producing an additional photovoltage parallel to the plane of the junction. Wallmark first recognized this effect and demonstrated how it could be utilized to produce a position sensitive photocell. [1] In this paper the restricted analysis of Wallmark is expanded and a theoretical basis is provided for the interpretation of the photoeffects observed in nonuniformly irradiated p-n junctions.  In a  uniformly  irradiated  junction,  the  absorption  of  radiation  results  in  the  creation  of hole-electron  pairs  in the  neighborhood  of  the  surface  upon which  the radiation is incident. If the radiation is incident on  the  surface  of  the  p  region  and  if  the  thickness of this  region  is  approximately  equal  to  or less  than  a  minority  carrier  diffusion  length,  then an appreciable number  of  optically  generated  hole-electron  pairs  diffuse  to the junction where they are separated by the  electrostatic  forces  of  the  transition  region.  The  electron is swept into the n region and the hole  remains  in the p region. The separation process therefore cancels  a  portion  of  the  barrier  space  charge  which  in  turn causes a  reduction of the internal barrier potential. In  an attempt  to  reestablish  the  equilibrium  barrier  configuration  a  net  current  of  negative  charge  now  flows  from  the n to the p region.  This current consists  of  electrons  being  reinjected  from  the  n  to  the  p  region  and  holes  being  injected  from  the  p to  the  n region. As long as a generating source of pairs is present,  the barrier does not return to its equilibrium configuration.  A  steady-state  condition  results  when  the  rate of hole-electron pair separation by the  barrier is  equal to the reinjected current.