For a fast response time, silicon resistivity and operating voltage must be chosen to give a depletion layer width such that the majority of the carriers are generated within it at the operating wavelength. In this case transit time will be dependent on both electron and hole drift velocities. Figure 5 illustrates the way drift velocity increases with electric field and it will also be seen that, except at very high fields the electron velocity is 2.5 times that for holes. Hence, hole transit time tends to dominate.