ABSTRACT

Photojunction sensors convert the electrons generated by the photoelectric effect into a detectable electronic signal. Photojunction sensors are an alternative to other light sensing technologies, such as vacuum tube detectors, for example, photomultiplier tubes. In general, semiconductor photojunction sensors produce more dark current than photomultiplier tubes, especially at room temperature. Upon cooling, the noise in semiconductors can be reduced significantly due to a reduction in dark current. Phototransistors are provided similar to transistors except that the signal amplified is the charge pairs generated by the optical input. Important requirements for photodiodes include good charge collection efficiency, and low noise. The low-noise requirement is satisfied by reducing detector capacitance and its dark current. A heterojunction exists at the interface of two different semiconductors. Photodiodes are frequently operated at fully depleted junction capacitance. Capacitance of the photodiode decreases as the depletion width increases.