Near-contact diffusion and compensation in extrinsic photoconductors

Carrier diffusion from heavily doped contact regions has been experimentally monitored with resistivity measurements of variable thickness Si: P extrinsic photoconductors (n⁺−n−n⁺). Modeling of free carrier diffusion at the interface of heavily doped contacts and highly resistive bulk at low temperatures predicts extended regions (5–30 μm) of excess carriers in high purity materials so that, in thin device structures, free carrier diffusion profiles from each contact will overlap and determine the resistivity of the device. In this work, a decrease in resistivity of four orders of magnitude was observed in a 5 μm thick structure compared to a 10 μm thick device. The resistivity of an ohmic structure in the thin limit is strongly dependent on the bulk and near-contact compensation, and resistivity measurements can be used as a sensitive measure of compensation at interfaces or in the tail of implanted layers.