An analog compontent that acts like a transistor, except that instead of voltage or current, the gate reacts to light: Whenever a photon hits the gate, the transistor lets some number of electrons pass through its substrate. Phototransistors are rarely found in solar cells, because they require a bias voltage in order to work, unlke their cousins the photodiodes. They are more useful as elements in light sensors: Tie a voltage source and a resistor to the source, then measure the source voltage, which will go lower as light increases. Phototransistors are also used in opto-isolators.

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