A photodarlington transistor, also known as a photodarlington or photodarlington pair, is a type of optoelectronic device that combines the principles of a phototransistor and a Darlington transistor to achieve higher sensitivity and current gain in response to light. It is used for light detection and amplification applications.
To understand a photodarlington transistor, let's break it down into its two main components:
Phototransistor: A phototransistor is a light-sensitive semiconductor device that acts as a transistor. It consists of a semiconductor material (usually silicon) with a base, emitter, and collector regions. When photons (light particles) strike the base-emitter junction of the phototransistor, electron-hole pairs are generated, leading to an increase in current flow between the collector and emitter terminals.
Darlington Transistor: A Darlington transistor is a configuration that consists of two bipolar transistors connected in such a way that their current gains (also known as hFE or beta) multiply, resulting in a much higher overall current gain. This configuration allows for a significant increase in the current amplification capability of the transistor.
Combining the above two components, a photodarlington transistor uses a phototransistor as the input stage and a Darlington transistor as the output stage. When light falls on the phototransistor's base-emitter junction, it generates a small current, which is then amplified by the Darlington configuration, resulting in a much larger output current.
The photodarlington transistor's sensitivity to light is significantly higher than that of a regular phototransistor due to the Darlington configuration's ability to provide substantial current gain. As a result, photodarlingtons are commonly used in applications where low-level light detection and amplification are required, such as optical sensors, light barriers, encoders, and other light-controlled electronic circuits.
It's important to note that while photodarlington transistors offer higher sensitivity, they also have a slower response time compared to regular phototransistors due to the additional transistor stage. This characteristic is a trade-off to achieve increased current gain.