The electronic properties of the material system should also be considered. Large carrier velocities directly reduce transit time delays. However, the device must be designed to withstand the necessary biases which produce high carrier velocities. Low recombination rates reduce the dark current and noise of a detector. As the device area is reduced to seek higher bandwidths, the surface recombination component increases in relative importance reflecting the larger surface area to device volume ratio.
Parasitic resistances can also limit device bandwidth.
It is important to choose materials on which low resistance ohmic contacts can be easily formed. The heterojunction band offsets and dopability of the mirror materials need to be considered for their effect on the device series resistance. If the mirror resistance cannot be suitably minimized, then an etched mesa contact can be used to circumvent conduction through the mirror stack, but only at the expense of added fabrication complexity and the introduction of a lateral access resistance.