and are collected the holes generated in the second concentration 17 part of the second p type semiconductor layer 16 travel to the anode 12 thus entering the first concentration 15 where the concentration of dopants is relatively high and where they are collected thus ending their transit time by way of comparison the light that is absorbed in the first concentration 15 part of the second p type semiconductor layer 16 also produces electrons and holes in this case however the holes are readily collected in the first concentration 15 and thus do not add substantially to the transit time of the carriers or reduce the bandwidth of the photodiode 10 accordingly insofar as the holes are concerned the graded doping concentration of the photodiode 10 does not add to their transit time or reduce the detector bandwidth in either in the first concentration 15 or the second concentration 17 another aspect of the graded doping concentration of the second p type semiconductor layer 16 is the creation of a pseudo electric field the electrons generated in the first concentration 15 region are subject to this pseudo field shown below as e kt q u2062 u2146 p u2146 x u2062 1 p 2 where k is boltzman 39 s constant t.