Optical Gain and Spatial Resolution in a Broad-Area Phototransistor

Abstract

A broad‐area phototransistor is considered as an image intensifier for a beam of nonuniform radiation incident in an axial direction at the emitter surface. Gain is related to a reduction in the emitter potential barrier by the presence of excess carriers generated optically in and around the base layer. Blurring or image degradation is the result of the lateral spreading of these carriers during their finite lifetime. The analysis demonstrates the essential difficulty in achieving high gain while preserving good image resolution, although both gain and resolution are enhanced by reducing the base width and increasing its purity. The results are applied to germanium at room temperature.