Because of their similar functions, photodiode and phototransistor are frequently misunderstood. Both are semiconductor-based components that detect light intensity and convert it to an electrical signal, but they are not the same.
Let’s go through the basics of photodiode and phototransistor before we get into the distinctions between these tools.
What is a Photodiode?
A photodiode is a light-sensitive diode that transforms light into electricity. Silicon or germanium are used to make it. It is a single PN junction device that operates on the photoelectric effect concept.
The construction of a photodiode is identical to that of a regular PN junction diode, with the exception that the junction is exposed to light. The light is focused on the connection using a lens. When light strikes the junction, an electron-hole pair forms, which travels in the opposite direction towards the anode and cathode. As a result, the linked circuit receives a current known as photoelectric current.
A photodiode is made to work in reverse bias mode.
Features of Photodiode
In a practical circuit, a photodiode can be used in photovoltaic mode (when in forward bias) or photodiode mode (when in reverse bias) (when run in reverse bias). Photodiodes are operated in reverse bias because this produces a linear response with a wide responsivity range. The output current can either be delivered straight to a load or through an amplifier circuit. Simply feed the amplifier output to a comparator to convert the input back to a stream of square pulses.
Avalanche photodiodes are likewise available, and they’re designed to work with a bias that’s quite close to the reverse breakdown voltage. As the device runs beyond the breakdown voltage and light is incident on it, the quantity of photo-generated carriers is doubled by the external bias. During lighting, this results in gain. These photodiodes are used to detect weak optical signals and are designed to function in breakdown mode.
To receive digital data encoded in amplitude-modulated or PWM optical pulses, a photodiode, amplifier, and analog-to-digital converter (ADC) can be utilized. You must consider for the bandwidth of your photodiode and amplifier while using PWM, as this restricts the maximum data rate. The reaction time of a photodiode is proportional to its terminal capacitance. The knee frequency for a digital pulse with a certain rise time, which is equal to 0.35/s, is commonly used as the maximum response frequency (response time).
Applications of Photodiode
In optical fibre communication, photodiodes are commonly employed. These are commonly utilized in digital and logic circuits for high-speed operation. Photodiode circuits are used in alarm and counter systems. Photodiodes are used in high-performance computer equipment.
What is a Phototransistor?
A phototransistor is a light-sensitive transistor that amplifies the photoelectric current produced when light energy is converted. Depending on the design, it is a two- or three-terminal device. It might be a three-layer BJT (bipolar junction transistor) or a FET (Field Effect Transistor).
It is divided into three sections: emitter, collector, and base. In comparison to a regular BJT, the collector region has a big size. The light reaches the base region. The lens, which also concentrates the light, allows light to penetrate the base region. Due to the photoelectric effect, photon particles collide with the junction and release an electron-hole pair. The electron-hole pair creates a base current, which the transistor amplifies.
The collector current is determined by the base current, whose magnitude is determined by the light intensity. As a result, the collector current is proportional to the amount of light that strikes the transistor.
It is similar to a BJT transistor in that it has two PN junctions.
Features of Phototransistor
To extract current, an Adalm2000 phototransistor can be placed in a common collector, common emitter, or any typical transistor arrangement. When there is no light shining on the device, it functions exactly like any other transistor (as a 3-terminal device). The light that strikes the gadget is absorbed in the base. This is the same as boosting the device’s basic current. As a result, a phototransistor can be used as a two-terminal device (i.e., with the base connection floating). The output current of a three-terminal device can be modulated by changing the base voltage (for NPN or PNP devices) or the gate voltage (for FET devices).
The output current visible at a load may be varied when run as a 3-terminal device by altering the input base current. The gadget functions as a switch with a built-in threshold. The base-emitter voltage changes when incident light is bright enough and current provided from a source into the base is large enough, and a current may readily travel through the device. However, by decreasing the total base current, which necessitates changing the external bias at the base, this may be avoided. Phototransistors are helpful in a variety of applications that require sensing an ON or OFF state rather than a particular intensity measurement due to their switching characteristic.
Applications of Photransistor
A phototransistor is commonly used as a light detector due to its great efficiency, and may thus be found in printers, remote controls, and other electronic devices. Phototransistor is also used in the circuits of relays and punch cards.
Because the action of light regulates the operation of such devices, they are commonly utilized in counting systems, where power supply is not a concern. Phototransistor is commonly used in encoder circuits in communication systems.
Photodiode vs. Phototransistor
When exposed to light, a photodiode is made up of a semiconductor diode that creates electricity. A phototransistor, on the other hand, is a junction transistor that creates current when exposed to light radiation.
When compared to a photodiode, a phototransistor is more efficient. When we talk about noise immunity, it’s important to remember that a photodiode is not immune to noise interference. Phototransistors, on the other hand, are unaffected by such interference. This might be a drawback of the technology at times.
The phototransistor fails to convert light energy into electrical energy at high frequencies. In contrast, this disadvantage is not present in the photodiode, which delivers a better response than the phototransistor.
In comparison to any other light sensitive device, a phototransistor is a less costly gadget. As a result, it is frequently employed. When compared to phototransistors, photodiodes can create an output at a significantly quicker pace when light falls on their surface.
A photodiode is a semiconductor that transforms the energy of light into an electrical current. The phototransistor, on the other hand, employs a transistor to convert light energy into an electrical current. While the phototransistor creates current, the photodiode generates both voltage and current. The photodiode responds significantly faster than the phototransistor.
Because the phototransistor provides a significant output current, the photodiode is less sensitive than the phototransistor. The photodiode may be biased forward or reversed, but the phototransistor can only be biased forward. In comparison to the collector area, the phototransistor’s emitter is negative.
Conclusion
In conclusion, both the photodiode and the photodiode turn light into electrical energy. However, because of the transistor, the phototransistor is more sensitive than the photodiode. The transistor amplifies the base current, which is caused by light absorption, and thus the collector terminal obtains a large output current. The photodiode’s temporal response is significantly faster than that of the phototransistor, thus it’s utilized in circuits where there’s a lot of fluctuation.
A photodiode is a device that efficiently executes high-speed activities. When compared to a photodiode, phototransistors are more sensitive due to the amplifying effect of transistors, and so deliver a greater current at the output. In comparison to a phototransistor, a photodiode offers the response at a faster pace.
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