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When a phototransistor is reverse biased?
Photodiodes are reverse biased to compress the space-charge region and reduce the junction capacitance. This allows higher bandwidth. There’s no direct analogy to a phototransistor. Usually, phototransistors will be slow compared to photodiodes due to long minority-carrier recombination times.
Why does resistance increase in reverse bias?
If a diode is reverse biased, the voltage at the cathode is higher than that at the anode. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing only a very small electric current to leak across the PN junction.
When does reverse bias occur in a phototransistor?
When a photon of sufficient energy (hence, the dependency of the wavelength of the light) hits the photodiode, an electron is freed with an energy to pass the barrier i.e. during reverse bias, when light strikes the junction, there is an increase in the current. Keeping this mind, a Phototransistor is a Photojunction…
Can a photodiode be run in forward or reverse bias?
A photodiode in a real circuit can be run in photovoltaic mode (when run in forward bias) or in photodiode mode (when run in reverse bias). Photodiodes are run in reverse bias as this provides a linear response, and the responsivity range can be quite large.
Which is better a phototransistor or a photodiode?
Since a typical β is 100, a phototransistor has a higher gain from light to current than a photodiode. For example, the OSRAM SFH 310 NPN phototransistor creates emitter currents of up to a few milliamps, as compared to the microamps of a photodiode. This higher current makes phototransistors much easier to interface to than photodiodes.
What happens when there is no light falling on a phototransistor?
When there is no light falling on the phototransistor, a small amount of leakage current known as Dark Current flows from collector to emitter. When there is enough light falling on the base terminal, a base current is produced, which is proportional to the intensity of the light.