Transistor

The generic transistor is called a bipolar junction transistor and is formed by two p-n junctions formed with the same substrate material, indicated by the middle letter in pnp or npn transistors. The transistor acts as a current amplifier, having many applications for amplification and switching.

Testing junction transistors
Transistor maximum values
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Photodiodes

A photodiode consists of an active p-n junction which is operated in reverse bias. When light falls on the junction, a reverse current flows which is proportional to the illuminance. The linear response to light makes it an element in useful photodetectors for some applications. It is also used as the active element in light-activated switches.

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Photodiode

The mechanism of the photodiode is like that of a (miniaturized) solar cell. Their response time is fast, on the order of nanoseconds. As light detectors, they are reverse biased - the reverse current is linearly proportional to the illuminance striking the diode. They are not as sensitive as a phototransistor, but their linearity can make them useful in simple light meters.
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Photodiode Characteristics

Photodiode discussionMore detailed characteristic curves
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Photodiode Characteristics

The reverse current through a photodiode varies linearly with illuminance once you are significantly above the dark current region.

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