▁desired ▁net ▁electron ▁motion ▁is ▁opposite ▁the ▁direction ▁predicted ▁by ▁the ▁dr ift - diff usion ▁equation . ▁ ▁For ▁example , ▁it ▁might ▁be ▁the ▁case ▁that ▁when ▁an ▁electron ▁absor bs ▁a ▁phot on , ▁it ▁is ▁dis pro port ion ately ▁likely ▁to ▁wind ▁up ▁in ▁a ▁state ▁where ▁it ▁is ▁moving ▁left ward . ▁And ▁perhaps ▁each ▁time ▁a ▁phot on ▁exc ites ▁an ▁electron , ▁the ▁electron ▁moves ▁left ward ▁a ▁bit ▁and ▁then ▁immediately ▁rela xes ▁into ▁(" gets ▁stuck ▁in ") ▁an ▁imm obile ▁state — until ▁it ▁absor bs ▁another ▁phot on ▁and ▁the ▁cycle ▁repe ats . ▁In ▁this ▁situation , ▁a ▁left ward ▁electron ▁current ▁is ▁possible ▁despite ▁an ▁electric ▁field ▁pushing ▁electrons ▁in ▁the ▁opposite ▁direction . ▁However , ▁if ▁when ▁a ▁phot on ▁exc ites ▁an ▁electron , ▁it ▁does ▁not ▁quickly ▁relax ▁back ▁to ▁an ▁imm obile ▁state , ▁but ▁instead ▁keeps ▁moving ▁around ▁the ▁cry stal ▁and ▁scatter ing ▁randomly , ▁then ▁the ▁electron ▁will ▁eventually ▁" for get " ▁that ▁it ▁was ▁moving ▁left , ▁and ▁it ▁will ▁wind ▁up ▁being ▁pulled ▁right ward ▁by ▁the ▁electric ▁field . ▁Again , ▁the ▁total ▁left ward ▁motion ▁of ▁an ▁electron , ▁per ▁phot on ▁absor bed , ▁cannot ▁be ▁much ▁larger ▁than ▁the ▁mean ▁free ▁path . ▁ ▁A ▁consequence ▁is ▁that ▁the ▁quantum ▁efficiency ▁of ▁a ▁thick ▁device ▁is ▁extremely ▁low . ▁It ▁may ▁require ▁millions ▁of ▁phot ons ▁to ▁bring ▁a ▁single ▁electron ▁from ▁one ▁electro de