What is a Phototransistor?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 24 November 2019
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A phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate. A phototransistor has exposed-base sections which, when exposed to light, activate the component rather than the electric current used in conventional examples. As with most regular transistors, the phototransistor's operating range is also base-input dependent. This means that the transistors range of operation may be controlled by the intensity of the applied light. The component is commonly used in devices such as optical remote controls, light pulse counters, and light measurement meters.

Bipolar transistors are one of the most commonly used forms of electronic semiconductor components. Typically consisting of a collector, emitter, and base sections, a regular transistor will remain inactive until it receives an appropriate electric pulse on its base input. This input switches the transistor on and allows a flow of current across the collector/emitter section of the component. The extent to which the transistor conducts or transmits this current is dependent on the size or amplitude of the base current. A phototransistor operates in exactly the same way except it relies on light falling on its base to activate it.


All transistors, and most semiconductor components in fact, are light sensitive. The phototransistor has been optimized to harness this characteristic. These components feature transparent base sections which allow unimpeded light gathering and, in most cases, do not have a base lead at all. Those that do have a base lead use it to bias or control the way the current flows rather than for activation. Apart from these differences, it is identical in construction and application to its conventional siblings.

The first phototransistors used single semiconductor materials such as germanium and silicone in their construction. Modern components use several differing material junctions including gallium and arsenide which lend the components far higher efficiency levels. The physical structure of the transistor is also optimized to allow for maximum light exposure. This usually entails placing the component contacts in an off-set configuration so as to avoid impeding light falling on the base.

The phototransistor's operational range is also base input dependent, i.e., the extent to which the component conducts can be controlled by varying the intensity of light it is exposed to. This makes a phototransistor ideal for light measuring instruments such as photographers' light meters. Many optical remote controls also use this characteristic to allow the system to transmit a range of instructions. Counters which use light pulses also utilize phototransistors in their circuitry as do several types of day/night switches. The infrared phototransistor is also frequently used in light-dependent proximity switches such as door closure sensors and security motion detectors.


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