A phototransistor is a semiconductor device that is sensitive to light to operate. It functions as an electronic switch and current amplification device; the presence of light is used to activate it. Essentially it converts light to electrical energy. Two words from the word phototransistor, that are the photo (light) and transistor (electronic switch and current amplification device). It is similar to a photodiode in operation except that it has higher sensitivity to light.
It’s made from a bipolar junction transistor. A semiconductor device with three regions- collector, base, and emitter. Two junctions are formed, the collector base is reverse biased, and the base-emitter is forward biased. Two terminals namely; collector and emitter are made for connection to an external circuit or power supply. The three regions of semiconductors are either PNP or NPN, the middle region is always the base. Silicon and germanium are the most common elements used, they are doped to negative (majority free electrons) or positive (majority holes). The base is the active region, large enough and the external case allows entry of light into it.
When the collector is connected to a positive voltage source and the emitter to a negative, the collector-base junction is reverse biased by the electric field forming a depletion region. The base-emitter junction is forward biased by the same electric field. The depletion region ideally has no charge carriers however at normal room temperature there are thermally generated charge carriers (electrons and holes). Remember negatively doped semiconductor has minority holes and positively charged one has minority free electrons. This allows minority charge carriers in the collector, junction, and base to be forward-biased hence reverse current flows.
When light strikes the base region and the reverse-biased junction, the light photon excites electrons bonded to atoms to higher energy levels. The electrons become free and leave behind holes both being charge carriers. Free electrons flow from base to collector and holes flow to the emitter. The result is that the base current increases and the base-collector junction become conductive. The base current is amplified by a base-emitter junction that is already forward biased from 100 up to about 1500 times. The light activates the reverse-biased junction hence acting as a switch, the base-emitter junction acts as an amplifier of current hence raising the sensitivity of the device.
- The wavelength of incident light- the energy in light depends on the wavelength. The energy in the light must be equal to or more than required to dislodge a covalent electron from an atom to create a free electron and hole. Visible light and infrared are mostly used.
- Luminous intensity- the more the intensity the more the generated charge carriers and the faster the device response.
- Sensitivity- this is the time period between application of light and device response to it. They have a reasonable response speed but slower than a photodiode.
- Voltage and current rating- each device has specified allowable working voltage and current that it can carry. Do not exceed these limits.
There are numerous uses of a phototransistor, we will just consider a few here.
- Optocouplers- a combination of a light emitting diode and a phototransistor are used to offer electrical connection to two physically separate circuits. Light is used as a linking medium.
- Encoder- the speed and direction of rotary machines is determined by the frequency of activation of the phototransistor. Whenever a rotating hole is in line with the light source and the phototransistor, it’s activated.
- Infrared detectors- used as a receiver module in equipment remotely controlled by handheld infrared emitters.
- Security system- an alarm is made to ring whenever an object obstructs the light from the source reaching the phototransistor.
- Other applications include electronic card readers, light control devices and optoisolators.
Semiconductors are interesting materials owing to their myriad properties. Automation and control engineering uses sensors and actuators to operate devices. A phototransistor senses presence of light to produce electrical signal that can be used to trigger electrical actuated device. You may be interested to learn more on this device. There are many books and journals to help you. Enjoy your exploration.