NOVELTY - A radiation detector device comprises a blocking barrier layer in contact with the second side of the multi-quantum well structure and having a thickness that prohibits a tunneling; and a collector contact layer in contact with the blocking barrier layer.    USE - For low-background applications.    ADVANTAGE - The multi-quantum well detector structure in the blocked intersubband detector configuration is designed in part to reduce the background noise of the detector and to achieve high detection sensitivity.    DETAILED DESCRIPTION - A radiation detector device includes an emitter contact layer; a multi-quantum well structure having a first side in contact with the emitter contact layer and an opposing second side; a blocking barrier layer in contact with the second side of the multi-quantum well structure and having a thickness that prohibits a tunneling; and a collector contact layer in contact with the blocking barrier layer. The multi-quantum well structure is formed of alternating quantum well layers and barrier layers. Each barrier layer is of a thickness that allows for a spatial overlap of wave functions of adjacent quantum wells to permit a tunneling from one quantum well to an adjacent quantum well. An INDEPENDENT CLAIM is included for a method of forming a radiation detector device by configuring a multi-quantum well structure with barrier layers that allow for carriers to tunnel through one quantum well to an adjacent quantum well; coupling two ohmic contact layers electrically to the multi-quantum well structure and using the layer to supply a voltage to generate an electrical signal when the multi-quantum well structure absorbs radiation; and forming a blocking barrier layer between the multi-quantum well structure and one of the ohmic contact layers that prevents carriers from tunneling through the blocking barrier layer to reach one ohmic contact layer.    DESCRIPTION OF DRAWING(S) - The figure shows a multi-quantum well detector structure in a blocked intersubband detector configuration where the horizontal axis is the position along the growth direction of the semiconductor layers.