Photodiodes can be used in either zero bias or reverse bias. In zero bias, light falling on the diode causes a voltage to develop across the device, leading to a current in the forward bias direction. This is called the photovoltaic effect, and is the basis for solar cells - in fact a solar cell is just a large number of big, cheap photodiodes.
Diodes usually have extremely high resistance when reverse biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse biased diode can be used as a detector by monitoring the current running through it. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect.
A phototransistor is in essence nothing more than a normal bipolar transistor that is encased in a transparent case so that light can reach the Base-Collector diode. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that tunnel through the Base-Collector diode are amplified by the transistor function.
P-N photodiodes are used in applications similar to photoconductors.
Consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and street lights usually seem to use photoconductors rather than photodiodes, although in principle either could be used.
Receivers for remote controls in VCRs and televisions often use photodiodes.
Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified CCDs (ICCDss) or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night-vision equipment and laser range finding.
PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for communications.Applications