How does the currentvoltage IV curve of a photodiode differ from that of a phototransistor?

Sure, here's the difference between the current-voltage (IV) curve of a photodiode and a phototransistor:

Photodiode:

• The IV curve of a photodiode exhibits a reverse-biased knee at a low reverse bias voltage.
• The reverse-biased knee is caused by the built-in junction capacitance of the depletion region.
• When a reverse voltage is applied, the depletion region becomes heavily doped, effectively blocking the flow of holes from the p-type region to the n-type region.
• This results in a significant drop in reverse current, even though the forward voltage is higher than the reverse bias voltage.

Phototransistor:

• The IV curve of a phototransistor is similar to that of a bipolar junction transistor (BJT).
• However, the knee in the phototransistor IV curve is much more pronounced.
• This is because the phototransistor has a much higher gain than a BJT, which means that it can amplify the current flowing through it.
• The phototransistor IV curve also exhibits a linear region in the forward bias region, due to the intrinsic region of the device.

Key differences:

| Feature | Photodiode | Phototransistor | |---|---|---| | Reverse-biased knee | Yes | No | | Knee location | Low reverse bias voltage | High reverse bias voltage | | Forward bias region | Reverse-biased | Linear | | Gain | Lower | Higher |

Conclusion:

The IV curve of a photodiode is different from that of a phototransistor due to the built-in junction capacitance of the depletion region in the photodiode. This results in a reverse-biased knee at low reverse bias voltage, while the phototransistor IV curve has a pronounced knee due to its higher gain.