Differences between LDR (Photoresistor) and Photodiode.however other designs allow high light-conversion efficiency. Overall the homojunction design is economical. Such design is fabricated using epitaxial growth of materials that has matching lattice structures as shown in the figure below. It uses type III – V semiconductor material such as gallium Ga and arsenide As to manufacture. Such design is known as heterojunction design. Modern phototransistor is fabricated using different semiconductor material to increase efficiency.

Thus making it more sensitive than a photodiode. The larger area generates more base current, that is amplified. The physical area of the base and collector region is kept large to collect more light. The emitter is heavily doped as compared to the collector but the collector is relatively very large. And the base is covered with transparent epoxy resin and lens to prevent the base from contamination as well as focus light when entering the region. The base and collector region is larger as compared to a normal transistor. It has a similar design to a normal bipolar transistor. Instead of the base terminal, there are two pointing arrows representing incident light as shown in the figure below. The symbol of the phototransistor is very similar to any normal transistor except for the base terminal. Unlike a normal transistor, a phototransistor has only two terminals, an emitter and collector. It is similar to BJT except for the exposed base instead of a terminal.

They are used for sensing light pulses of high speed and small magnitude. The light striking the base converts into a base current that amplifies the current between the emitter and collector proportional to the intensity of light. It is a three-layer semiconductor device whose light-sensitive base is exposed.

Advantages & Disadvantages of PhototransistorsĪ phototransistor is a light-controlled switch that switches a circuit and amplifies the current when exposed to light.

Collector Characteristics of Phototransistor.Parameters & Characteristics of Phototransistor.

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