Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. Phototransistors are used in almost all electronic devices that depend on light including smoke detectors, laser-ranging finding devices, and optical remote controls.
They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes. This category includes photodarlingtons.
Composition of Phototransistors
Bipolar transistors are the most commonly used transistors. Phototransistors are typically bipolar NPN devices and are made of three lead components:
- The base is the lead responsible for activating the transistor. It is the gate controller device for the larger electrical supply.
- The collector is the positive lead and the larger electrical supply.
- The emitter is the negative lead and the outlet for the larger electrical supply.
While ordinary transistors have photosensitive effects when exposed to light, phototransistors are optimized for use with light. Phototransistors have larger base and collector areas than ordinary transistors. They generally have a clear or opaque casing to enhance light exposure.
Most phototransistors are made of a single material although some others may be composed of multiple materials.
- Early phototransistors had a homo-junction structure made of germanium or silicon (see photo below).
- Modern phototransistors may be composed of multiple material junctions of type III-V materials like gallium and arsenide (see photo below, right). The physical structure can be optimized to allow higher levels of light exposure by using an offset configuration.
Phototransistors are made of semi-conductive materials. Although germanium has more desirable electrical properties, silicon is more commonly used because of its reliability and low cost.
How Phototransistors Work
A typical transistor consists of a collector, emitter, and base sections. The collector is biased positively with respect to the emitter and the base-collector junction is reverse biased. A phototransistor remains inactive until light falls onto the base. Light activates the phototransistor, allowing the formation of hole-electron pairs and the flow of current across the collector or emitter. As the current spreads it is concentrated and converted into voltage.
A phototransistor usually does not have a base connection (see diagram below). The base is left disconnected because the light is used to enable the current to flow through the phototransistor.