Photodiodes & Phototransistors

Excelitas utilizes Silicon and InGaAs materials for their photodiodes to provide detection from 220 nm to 1700 nm. These devices are offered in a variety of sizes to meet customer sensitivity and speed requirements.

Phototransistors are examples of photodiode-amplifier combinations integrated within a single silicon chip.

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Photodiodes are unique among light detectors in that when illuminated, they generate an output which is proportional to light level. They are solid state light detector that consists of a shallow diffused P-N junction with connections provided to the outside world. When the top surface is illuminated, photons of light penetrate into the silicon to a depth determined by the photon energy and are absorbed by the silicon generating electron-hole pairs. The electron-hole pairs are free to diffuse (or wander) throughout the bulk of the photodiode until they recombine. The average time before recombination is the “minority carrier lifetime”.

Phototransistors are solid state light detectors that possess internal gain. This makes them much more sensitive than photodiodes of comparably sized area. These devices can be used to provide either an analog or digital output signal. The phototransistor can be viewed as a photodiode whose output photocurrent is fed into the base of a conventional small signal transistor. While not required for operation of the device as a photodetector, a base connection is often provided allowing the designer the option of using base current to bias the transistor. The typical gain of a phototransistor can range from 100 to over 1500.