How is the phototransistor related to the transistor?
Below saturation, the phototransistor implements the equations IC = βIB, where IC is the collector current and β is the transistor’s gain, and IE = IC + IB, where IE is the emitter current. (See Appendix B.3 for more on bipolar junction transistors.)
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.
What kind of transistor has a photodiode junction?
A phototransistor is a type of bipolar junction transistor including a photodiode junction. An NPN phototransistor has a photodiode at its base-collector junction, and the photocurrent generated there acts as the base current IB.
Which is larger base collector junction or phototransistor junction?
In phototransistors, the size of base – collector junction is larger as it is a light sensitive region of the sensor. The larger size of the junction results in a significantly larger junction capacitance and as a result, phototransistors have low frequency response than photodiode in spite of the high gain.
What is the peak spectral response of a phototransistor?
The output of a phototransistor is dependent upon the wavelength of incident light. These devices respond to light over a broad range of wavelengths from the near UV, through the visible and into the near IR part of the spectrum. Unless optical filters are used, the peak spectral response is in the near IR at approximately 840 nm.
Which is slower a photodetector or a phototransistor?
This leads to the general rule that for devices with the same active area, the higher the gain of the photodetector, the slower will be its speed of response. A phototransistor takes a certain amount of time to respond to sudden changes in light intensity.