A phototransistor is a form of transistor in which the base-emitter junction is not covered and can be affected by incident light. The base-emitter junction acts as a photodiode, and the current in this junction is then amplified by the normal transistor action so as to provide a much larger collector current, typically 1000 times greater than the output current of a photodiode. The penalty for this greatly increased sensitivity is a longer response time, measured in )ls rather than in ns, so that the device is not suitable for detecting light beams that have been modulated with high- frequency signals. Phototransistors are used to a smaller extent nowadays because it is just as simple to make a chip containing a photodiode inte- grated with an operational amplifier, and better response times can usually be obtained in this way. The phototransistor is still found in conjunction with an infrared emitter used for such purposes as punched tape readers, end of tape detectors, object counters and limit switches.
The first form of photovoltaic device was the selenium cell, as used in early types of photographic exposure meters. The principle is that the voltage across the cell is proportional to the illumination, and since for the selenium cell the voltage was of an appreciable size (of the order of 1 V in bright illumination), an exposure meter using this type of cell needed no amplification and could use a meter of reasonably rugged construction. The use of selenium is now only of historical interest – the properties of the metal were discovered by Wheatstone (of Wheatstone Bridge fame) when he was trying to construct high-value resistors to act as standards in a bridge circuit to measure the resistance of a trans-Atlantic telegraph cable.
Modern photovoltaic devices are constructed from silicon, and the con- struction method is as for a photodiode. A silicon photovoltaic device is a silicon photodiode with a large area junction and used without bias. It is connected into a large load resistance, and the typical voltage output is of the order of 0.25 V for bright artificial illumination of 1000 lux. The main application of such cells is in camera exposure controls, because with suitable filtering the peak response and the response curve can be made very close to that for the eye. For the same reasons, the device can be used for the monitoring and control of light levels in critical manufacturing processes, and in instruments used for checking light levels. The photovol- taic cell can also be used as a photodiode, and if amplification is to be used, this mode is generally preferred to its use as a photovoltaic device.