Consta de un sustrato usualmente silicio y tres partes dopadas artificialmente contaminadas con materiales especficos en cantidades especficas que forman dos uniones bipolares, el emisor que emite portadores, el colector que los recibe o recolecta y la tercera, que est intercalada entre las dos primeras, modula el paso de dichos portadores base. A diferencia de las vlvulas, el fototransistor es un dispositivo controlado por corriente y del que se obtiene corriente amplificada. En el diseo de circuitos a los fototransistores se les considera un elemento activo aquellos que son capaces de excitar circuitos o de realizar ganancias o control del mismo , a diferencia de los resistores, capacitores e inductores que son elementos pasivos. Actualmente se los encuentra prcticamente en todos los enseres domsticos de uso diario: radios, televisores, grabadoras, reproductores de audio y vdeo, hornos de microondas, lavadoras, automviles, equipos de refrigeracin, alarmas, relojes de cuarzo, computadoras, calculadoras, impresoras, lmparas fluorescentes, equipos de rayos X, tomgrafos, ecgrafos, reproductores mp3, celulares, etc.
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When a photon of sufficient energy strikes the diode, it creates an electron — hole pair. This mechanism is also known as the inner photoelectric effect. Thus holes move toward the anode , and electrons toward the cathode , and a photocurrent is produced. The total current through the photodiode is the sum of the dark current current that is generated in the absence of light and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode.
If the circuit is shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect , which is the basis for solar cells — a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent. The reverse bias also reduces the dark current without much change in the photocurrent.
Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects.
Other modes of operation[ edit ] Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown , resulting in internal gain within the photodiode, which increases the effective responsivity of the device. Electronic symbol for a phototransistor A phototransistor is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base—collector junction.
It was invented by Dr. John N. Shive more famous for his wave machine at Bell Labs in ,  but it was not announced until If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode.
While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Field-effect phototransistors, also known as photoFETs, are light-sensitive field-effect transistors.
Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage. A Solaristor is a two-terminal gate-less phototransistor.
A compact class of two-terminal phototransistors or solaristors have been demonstrated in by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.
Materials commonly used to produce photodiodes include:  Material.
Fototransistor PT331C Receptor