A phototransistor is a bipolar semiconductor device that converts light energy into electric current.

Phototransistors can be used in any electronic device that senses light. For example, phototransistors are often used in smoke detectors, infrared receivers, and DVD players. Phototransistors can be sensible for infrared (IR), visible, or ultraviolet (UV) light although many applications use IR light.

Because phototransistors, like any transistor, amplify the signal, they produce a higher current than photodiodes. Phototransistors are quite fast and their output is practically instantaneous. They are relatively inexpensive, simple, and so small that several of them can fit onto a single integrated computer chip.

Packages that incorporate an IR LED and an IR phototransistor are also available. They can be used as proximity sensor.

Be aware: many photo transistors have two terminals only, not three as you might expect for a transistor. Do not confuse them with photo diodes.

How a Phototransistor Works

A phototransistor is a bipolar device that is completely made of silicon or another semi-conductive material and is dependent on light energy. Phototransistors are generally packaged in an opaque or clear container that helps focus light as it travels through it and allow the light to reach the phototransistor’s sensitive parts. A phototransistor generally has an exposed base that amplifies the light that it comes in contact with. This causes a relatively high collector current to pass through the phototransistor, which can be converted in a voltage difference with the help of a suitable resistor.

Most important specifications

Maximum ratings.

  • Vce,max: the maximum collector-emitter voltage (V).
  • Ic,max: the maximum collector current (mA).


  • Ic,0: Collector dark current (mA).
  • Ic,L: The collector light current (mA), usually specified for a given light intensity.
  • Vce(sat): collector-emitter saturation Voltage (V).
  • Spectral response: the output of a phototransistor as function of the wavelength of incident light, usually a range and a peak response (nm).
  • Sensitivity: the output of a phototransistor as function of light intensity.