Analysis of photovoltaic and photoconductivity modes of photodiode operation
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Analysis of photovoltaic and photoconductivity modes of photodiode operation

Posted Date: 2024-02-02

Photocurrent

The basic output of a photodiode is the current flowing through the device from the cathode to the anode, which is roughly linearly proportional to the illumination. (Keep in mind, though, that the magnitude of the photocurrent is also affected by the wavelength of the incident light - more on this in the next post.) The photocurrent is converted to a voltage for further signal processing via a series resistor or current - Voltage amplifier.

The details of the photodiode's photocurrent relationship will vary depending on the diode's bias conditions. This is the essence of the difference between photovoltaic mode and photoconductive mode: In a photovoltaic implementation, the circuitry around the photodiode keeps the anode and cathode at the same potential; in other words, the diode is zero-biased. In a photoconductive implementation, the circuit surrounding the photodiode is reverse biased, meaning the cathode is at a higher potential than the anode.

dark current

The main non-ideal condition affecting photodiode systems is called dark current, because current flows through the photodiode even when there is no illumination. The total current flowing through the diode is the sum of dark current and photocurrent. If these intensities produce photocurrents of similar magnitude to the dark current, the dark current will limit the system's ability to accurately measure low light intensities.

The harmful effects of dark current can be mitigated through techniques that subtract the expected dark current from the diode current. However, dark current is accompanied by dark noise, a form of shot noise observed as random variations in the amplitude of dark current. The system cannot measure light intensity whose associated photocurrent is so small that it is lost in dark noise.

Photovoltaic Mode in Photodiode Circuits

The image below is an example of a photovoltaic implementation.

This type of op amp circuit is called a transimpedance amplifier (TIA). It is specifically used to convert current signals into voltage signals, and the current-to-voltage ratio is determined by the value of the feedback resistor RF. The non-inverting input of the op amp is connected to ground, and if we apply the virtual short circuit assumption, we know that the inverting input will always be at approximately 0 V. Therefore, both the cathode and the anode of the photodiode are held at 0 V.

I don't believe "photovoltaic" is an entirely accurate name for this op amp based implementation. I don't think photodiodes function like solar cells which generate voltage via the photovoltaic effect. But "photovoltaic" is the accepted term, whether I like it or not. "Zero bias mode" is better, I think, because we can use the same TIA and photodiode in photovoltaic or photoconductive mode, so no reverse bias voltage is a significant differentiating factor.

When to use photovoltaic mode

The advantage of photovoltaic mode is the reduction of dark current. In a normal diode, applying a reverse bias voltage increases the reverse current because reverse bias reduces the diffusion current but not the drift current and also because of leakage.

The same thing happens in a photodiode, but the reverse current is called dark current. Higher reverse bias voltage results in more dark current, so by using an op amp to keep the photodiode at approximately zero bias, we actually eliminate dark current. Therefore, the photovoltaic mode is suitable for applications requiring optimized low-light performance.

Photoconductivity Mode in Photodiode Circuits

To switch the above detector circuit to photoconductive mode, we connect the anode of the photodiode to a negative voltage supply instead of ground. The cathode is still at 0 V, but the anode voltage is below 0 V; therefore, the photodiode is reverse biased.

When to use photoconductive mode

Applying a reverse bias voltage to the pn junction causes the depletion region to widen. This has two beneficial effects in the context of photodiode applications. First, as mentioned in the previous article, a wider depletion region makes the photodiode more sensitive. Therefore, photoconductive mode is a good choice when you want to generate more output signals related to illumination.

Second, a wider depletion region reduces the junction capacitance of the photodiode. In the circuit shown above, the presence of the feedback resistor and junction capacitance (as well as other sources of capacitance) limits the closed-loop bandwidth of the system. As with the basic RC low-pass filter, reducing the capacitance increases the cutoff frequency. Therefore, the photoconductive mode allows a wider bandwidth and is preferable when you need to optimize the detector's ability to respond to rapid changes in illumination.

, reverse bias also extends the linear operating range of the photodiode. If you are concerned about maintaining measurements under high illumination, you can use photoconductive mode and then select the reverse bias voltage based on your system requirements. But keep in mind that more reverse bias will also increase dark current.

Hamamatsu is a leading manufacturer of photodetectors. This graph is taken from their Silicon Photodiode Handbook and gives you an idea of ​​how much you can extend the linear response area of ​​a photodiode by increasing the reverse bias voltage.


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