Phototransistors are solid state light detectors that possess internal gain and are consequently much more sensitive than photodiodes of comparably sized area. These photodiode-amplifier combinations are put together to overcome the major limitation of photodiodes: unity gain. Many applications demand a greater output than can be generated by a photodiode alone, and even though the signal of a photodiode can be amplified through external circuitry (e.g. an operational amplifier), this is not always cost effective. In such cases, a phototransistor provides a lower cost alternative.

Phototransistors 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.

VTT Series and CR Series

This RoHS-compliant range from Excelitas is available in a wide range of packages.

  • • Low cost visible and near IR photo detection
  • • Low dark current
  • • Available in package with integrated visible filtering or with integrated IR filtering
VTP, VTB and VTT Ambient Light Sensors

Ambient Light Sensors from Excelitas are spectrally adapted photodiodes and phototransistors, featuring Excelitas' IR-BLOC technology to give a response approaching that of the human eye.

There are three main devices types, one being filtered photodiodes, the second filtered phototransistors and finally wavelength selective devices based on III-V material.

They are available in a number of standard packages, including surface mount for automated assembly.

Kodenshi phototransistors
Ushio phototransistors