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Difference between Photodiode and Phototransistor
Photodiodes and Phototransistors are both semiconductor electronic devices that convert optical energy into electrical energy. Therefore, due to their similar function, one may get confused between these two devices. Both photodiode and phototransistor sense the light from a source and generate an electric signal having the strength corresponding to intensity of the light. However, these two devices are quite different from each other in many aspects.
In this article, we will highlight all the significant differences between photodiodes and phototransistors. Let's start with some basics so that it becomes easy to understand the differences between them.
The one most significant difference between photodiode and phototransistor is that the photodiode is a PN junction semiconductor diode which transform the energy of light into electrical energy, while the phototransistor is a bipolar junction transistor (usually NPN transistor) which can convert energy of light into electrical energy. All other noticeable differences are listed in the table given below.
What is a Photodiode?
A type of PN junction semiconductor diode which converts light energy into electrical energy is called a photodiode. The operation of a photodiode is based on the principle of photoconductance. According to the photoconductance, when a beam of light having high energy exposes the PN junction of the photodiode, it produces charge carries (electrons and holes) at the junction. These charges carriers receives energy from the light and move. Thus, the movement of charge carriers in the photodiode results in the flow of current through the diode.
The photodiodes are designed to operate in reverse biased mode. The photodiode has two terminals namely anode and cathode. Where, in case of the photodiode, the anode is connected to negative terminal and the cathode is connected to the positive terminal of the battery to be charged.
Once the PN junction of the photodiode is supplied with the light of high energy, it rises the temperature of the junction and causes production of electrons and holes in the depilation region. These electrons and holes receive energy from the light and move in the opposite direction. Hence, the movement of electrons and holes generates electric current. The electric current produced by the photodiode is directly proportional to intensity of light, i.e. the light of high intensity produces large number electron-hole pairs and hence the large current.
The photodiode are used in several practical applications such as solar panels, safety equipment, automatic shutter control, fiber optic links, X-ray detection, etc.
What is a Phototransistor?
A phototransistor is a semiconductor bipolar junction transistor which can convert energy of light into electrical energy. It has three semiconductor regions namely emitter, base and collector. The emitter and collector are attached with metallic contacts to form the emitter and collector terminals whereas the base region is made transparent to expose it with light. Therefore, the only difference between an ordinary transistor and phototransistor is that the phototransistor has a light sensitive base region.
When a light beam of specific intensity is incident at the base region of the phototransistor, it turns on the transistor and makes it conducting. The amplitude of output current depends on the intensity of the incident light.
The structure of a typical phototransistor is covered with an opaque enclosure so that the photons of light do not scatter. When the base of the phototransistor absorbs the energy of light photons, it generates electron-hole pairs. These electron-hole pairs decrease the width of depletion region at the junctions. As a result, the majority charge carriers start moving from emitter region to the collector region.
In a phototransistor, a small amount of light energy can produce a large collector current. Therefore, the phototransistor can be used for switching as well as amplification of electric signals with the help of light energy. One important characteristics of a phototransistor is that it can be forward biased only.
The phototransistors are extensively used in punch card readers, electric signal controls, security systems, light switching controls, signal amplification, etc.
Difference between Photodiode and Phototransistor
Both photodiodes and phototransistors are photosensitive semiconductor devices having similar function. However, there are several differences between them that are given in the following table:
| Basis of Difference | Photodiode | Phototransistor |
|---|---|---|
| Definition | A photodiode is a semiconductor PN junction diode which converts the light energy into electrical energy. | A phototransistor is two terminal bipolar junction transistor which converts light energy into electrical energy. |
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| Terminals | A photodiode has two terminals namely anode and cathode. | A phototransistor may have two or three terminals depending on the design which are emitter, base and collector. |
| Semiconductor regions | A photodiode has two semiconductor regions, i.e. a P-type anode region and N-type cathode region. | A phototransistor has three semiconductor regions namely emitter, base and collector. |
| Number of PN junctions | There is only one PN junction in the photodiode. | A phototransistor consists of two PN junctions. |
| Types | There is no further classification of photodiode. | There are two types of phototransistors namely NPN and PNP. |
| Biasing | Photodiode is always used in reverse biased mode. | Phototransistor is always used in forward biased mode. |
| Primary functions | The main function of a photodiode is to convert the light energy into electrical energy. | The phototransistor performs two functions viz. switching and amplification. |
| Sensitivity | The sensitivity of a photodiode is comparatively less. | Phototransistor has high sensitivity. It is because of the amplification factor of the transistor. |
| Response time | The response time of a photodiode is less. Therefore, it has quick response. | The response time of a phototransistor is more. As a result, its response is slower. |
| Need of external power source | A photodiode does not require external power source to operate. | A phototransistor needs an external power source to operate. |
| Cost | The cost of photodiode is less. | The cost of transistor is comparatively high. |
| Applications | Photodiodes are extensively used in solar panels to generate electricity. These also find applications in fiber optic communication, object counter, smoke detectors, safety equipment, automatic shutter control, etc. | Phototransistors are used in light detection, in printers, remote controls, IR detectors, punch card readers, security systems, highways lighting controls, relays, etc. |
Conclusion
From the above discussion and comparison, it is clear that both Photodiodes and Phototransistors are optically-operated semiconductor devices that are used to convert light energy into electrical energy. Therefore, both are used as the light-operated solid state switching devices.
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