What is the difference between a photodiode and a phototransistor?

A photodiode is a semiconductor device that converts light into electrical current, while a phototransistor is a transistor that is sensitive to light.

Photodiodes and phototransistors are both used in electronic devices to detect light. However, they differ in their construction and operation. A photodiode is a p-n junction diode that is designed to operate in reverse bias. When light falls on the diode, it creates electron-hole pairs, which are then swept across the junction by the electric field. This results in a current flow through the diode, which can be measured and used to detect the light.

On the other hand, a phototransistor is a bipolar transistor that is designed to operate in the active region. When light falls on the base-collector junction, it creates electron-hole pairs, which then act as carriers in the transistor. This increases the collector current, which can be measured and used to detect the light.

One advantage of a phototransistor over a photodiode is that it provides a higher gain, which makes it more sensitive to light. However, it also has a slower response time and a higher capacitance, which can limit its use in high-speed applications.

A-Level Physics Tutor Summary: Photodiodes and phototransistors both detect light but work differently. A photodiode, made of a p-n junction, turns light into current when reversed biased. A phototransistor, a type of bipolar transistor, increases current with light at the base. Phototransistors are more sensitive due to higher gain, but are slower and not suited for fast applications compared to photodiodes.