There is provided a photodetector, comprising a semiconductor heterostructure having in sequence: a first collection layer having substantially uniform doping of a first doping type; a radiation-absorbing layer having substantially uniform doping of the first doping type and having a band gap less than or equal to that of the first collection layer; and a barrier layer having a band gap greater than that of the radiation-absorbing layer, the top of the valence band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is n-type or the bottom of the conduction band of the barrier layer being substantially equal in energy to that of the radiation-absorbing layer where the first doping type is p-type; wherein a first portion of the barrier layer is of the first doping type and a second portion of the barrier layer is of a second doping type, the first portion of the barrier layer being adjacent to the radiation-absorbing layer, forming a heterojunction within the barrier layer which gives rise to a depletion region within each portion of the barrier layer.