The phototransistor is a semiconductor light sensor formed from a basic transistor with a transparent cover that provides much better sensitivity than a photodiode. It is basically a 3-layer semiconductor device which has a light-sensitive base region. The base senses the light and converts it into the current which flows between the collector and the emitter region.
The symbol of Phototransistor (NPN & PNP configuration) is shown in fig. 1. The two arrows show the light incident on the base of the phototransistor. C denotes the collector and E denote the emitter.
Phototransistor Structure & Construction
The phototransistor has a much larger base and collector areas that would be used for a normal transistor. These devices were generally made using diffusion or ion implantation. Early phototransistors used germanium or silicon throughout the device giving a homo-junction structure. The basic structure is shown in fig.2.
Phototransistors are operated in their active region. For operation, the bias conditions are quite simple. The collector of an n-p-n transistor is made positive with respect to the emitter or negative for a p-n-p transistor. The light enters the base region of the phototransistor where it causes hole-electron pairs to be generated. This mainly occurs in the reverse biased base-collector junction. The hole-electron pairs move under the influence of the electric field and provide the base current, causing electrons to be injected into the emitter.
Collector Current of Phototransistor
Consider the conventional transistor. The collector base leakage current acts as a base current ICBO.
IC = βIB + (1+β) ICBO
As the base current in phototransistor is zero i.e. IB = 0, It acts as an open circuited. Therefore, the collector current is
IC = (1+β) ICBO
The above equation showed that the collector current is directly proportional to the current base leakage current, i.e., the collector current increases with the increases of the collector base region.
A further advantage of all phototransistors when compared to the avalanche photodiode, another device that offers gain, is that the phototransistor has a much lower level of noise.
One of the main disadvantages of the phototransistor is the fact that it does not have a particularly good high-frequency response. This arises from the large capacitance associated with the base-collector junction.
- The phototransistor is widely used in electronics devices likes smoke detectors, infrared receiver, CD players, lasers etc. for sensing light.
- They also find applications in Opto-isolators, Position sensing, Security systems, Coin counters, etc.