A photo-detector comprising: an n-type photon absorbing layer of a first energy bandgap; a barrier layer; an intermediate layer having a second energy bandgap and being a pseudomorphic semiconductor structure comprising a sub-band energy level near the valence band of the intermediate layer; and a contact layer of a third energy bandgap; said intermediate layer being located between the contact layer and the barrier layer; wherein the layer materials are selected such that the first energy bandgap of the photon absorbing layer is narrower than that of said barrier layer, and the barrier layer has a residual valence band energy offset with the photon absorbing layer allowing free passage of minority carrier holes, and a large conduction band energy offset with the photon absorbing layer, which blocks the flow of majority carrier electrons; wherein the material composition and thickness of said intermediate layer are selected such that the sub-band energy level of the intermediate layer lies higher in energy than the valence band of the barrier layer to enable efficient trapping of minority carrier holes from the valence band of the barrier layer and to allow efficient transfer of the minority carrier holes to a majority carrier energy band of the contact layer; wherein a tunnel current through the barrier layer from the contact layer to the photon absorbing layer is less than a dark current in the photo-detector, and wherein when an operating bias is applied across the detector making the contact layer negative biased with respect to the photon absorbing layer, the absorbing layer has sufficiently flat bands near the barrier layer so that any generation-recombination current of minority carrier holes from the photon absorbing layer to the barrier layer is below a diffusion dark current which is due to the unimpeded flow of minority carriers from the photon-absorbing layer to said barrier layer. an n-type photon absorbing layer of a first energy bandgap; a barrier layer; an intermediate layer having a second energy bandgap and being a pseudomorphic semiconductor structure comprising a sub-band energy level near the valence band of the intermediate layer; and a contact layer of a third energy bandgap; said intermediate layer being located between the contact layer and the barrier layer; wherein the layer materials are selected such that the first energy bandgap of the photon absorbing layer is narrower than that of said barrier layer, and the barrier layer has a residual valence band energy offset with the photon absorbing layer allowing free passage of minority carrier holes, and a large conduction band energy offset with the photon absorbing layer, which blocks the flow of majority carrier electrons; wherein the material composition and thickness of said intermediate layer are selected such that the sub-band energy level of the intermediate layer lies higher in energy than the valence band of the barrier layer to enable efficient trapping of minority carrier holes from the valence band of the barrier layer and to allow efficient transfer of the minority carrier holes to a majority carrier energy band of the contact layer; wherein a tunnel current through the barrier layer from the contact layer to the photon absorbing layer is less than a dark current in the photo-detector, and wherein when an operating bias is applied across the detector making the contact layer negative biased with respect to the photon absorbing layer, the absorbing layer has sufficiently flat bands near the barrier layer so that any generation-recombination current of minority carrier holes from the photon absorbing layer to the barrier layer is below a diffusion dark current which is due to the unimpeded flow of minority carriers from the photon-absorbing layer to said barrier layer.