Photodiode vs Phototransistor: Difference and Comparison

Key Takeaways

1. Photodiodes convert light into current, phototransistors convert light into voltage.
2. Photodiodes have greater speed and accuracy, phototransistors are more sensitive and amplify signals.
3. Photodiodes work well for digital circuits, phototransistors for analog circuits.

What is Photodiode?

A photodiode is a semiconductor device that helps in generating current in response to light. The primary application of this semiconductor device is used in optical communication, light detection, light sensing in electronic devices, etc. The design of a photodiode is a combination of a p-n junction. 

In a photodiode, when light strikes the semiconductor material of the diode, it generates electron-hole pairs in the junction region. This complete process is known as the photovoltaic effect. 

The key feature of the photodiode is its ability to show high sensitivity to light. They are best known for their fast response time and good noise susceptibility. 

What is Phototransistor?

A phototransistor is another type of semiconductor that helps control the current in response to light. This semiconductor device’s primary function is optical switches, light sensing, optical communication, etc. The design of the phototransistor is made up of an integrated transistor and photodiode.

A phototransistor includes a photodiode, a light-sensitive region, and is integrated into a base-collector junction. So, electron-hole pairs are generated when the light strikes the semiconductor, which modulates the base current. The base current controls result in an amplified signal.

The phototransistor’s key feature is to amplify light-generated electrical signals. The response time of phototransistors is slow. 

Difference Between Photodiode and Phototransistor

1. A photodiode is a device that helps generate current in response to light. In contrast, on the other side, a phototransistor is defined as a device that controls the current in response to light.
2. The structure of a photodiode is the combination of a p-n junction, whereas, on the other hand, the structure of a phototransistor consists of an integrated transistor and photodiode.
3. The primary function of a photodiode is to convert the light energy straightaway to an electric current. At the same time, the primary function of a phototransistor is to convert the light energy straightaway to electrical current but with amplification.
4. The sensitivity of the photodiode toward light is relatively high, while the sensitivity of the phototransistor toward light is moderate.
5. Photodiode does not amplify current, but phototransistors do amplify the current.
6. The response time of a photodiode is fast, and for a phototransistor, the response time is slow.
7. Considering the complexity of a circuit photodiode has a simple circuit, while a phototransistor has a complex circuit.
8. A photodiode is primarily used in optical communication, light detection, light sensing in electronic devices, etc. On the other hand, the phototransistor is primarily used in optical switches, light sensing, optical communication, etc.
9. The power consumption of a photodiode is low, while the power consumption of a phototransistor is high.
10. The output signal of a photodiode is that the current is proportional to incident light intensity. In contrast, this phototransistor gives an output signal that the current is proportional to incident light intensity in addition to amplification.

Comparison Between Photodiode and Phototransistor

Parameter of Comparison	Photodiode	Phototransistor
Definition	A device that helps in generating current in response to light	A device that controls current in response to light
Structure	p-n junction	Integrated transistor and photodiode
Operation	It helps in converting the light energy straightaway to electrical current	It helps in converting the light energy straightaway to electrical current but with amplification
Sensitivity towards light	High	Moderate
Amplification	Do not	Do amplify the current
Response Time	Fast	Slow
Circuit Complexity	Simple	Complex
Application	Optical communication, light detection, light sensing in electronic devices, etc	Optical switches, light sensing, optical communication, etc
Power Consumption	Low	High
Output Signal	The current is proportional to incident light intensity	The current is proportional to incident light intensity in addition to amplification

References

1. https://pubs.acs.org/doi/abs/10.1021/acsnano.6b06468
2. https://aapm.onlinelibrary.wiley.com/doi/abs/10.1002/mp.14921