Understanding the Photodiode Equivalent Circuit - Technical Articles

The equivalent circuit helps us understand and predict the actual function of electronic components. For photodiodes, the equivalent circuit model is an indispensable analysis tool, because just inserting the photodiode symbol into the schematic does not tell you much about the signal that will be generated and how the photodiode interacts with the amplifier circuit.

This article is the fifth in our series on photodiodes. Continue learning the rest to learn about the following:

Not all photodiode models are the same, but four elements appear consistently: current source, shunt capacitor, shunt resistor and series resistor, and the ordinary pn junction represented by the diode symbol.

Ideal current source (I

) Represents the photocurrent, that is, the current generated by the diode in response to incident light. Note that the direction of the photocurrent corresponds to the current flowing from the cathode of the diode to the anode of the diode-this is a good reminder that photodiodes use zero bias or reverse bias, and the direction of current they generate is opposite to the reverse. We expect to use ordinary forward-biased diodes.

As mentioned in the previous article, we use responsivity to quantify the relationship between incident light power and photocurrent. The responsivity of a typical silicon photodiode ranges from about 0.08 A/W (A/W) for 400 nm EMR to 0.48 A/W for 700 nm EMR.

Parallel capacitor (C

) Represents the junction capacitance of the diode, that is, the capacitance associated with the depletion region of the pn junction. The junction capacitance is an important parameter because it will seriously affect the frequency response of the photodiode. Lower junction capacitance can achieve excellent high frequency operation.

You may notice the photodiode model of C

It is a variable capacitor. Although this method of representation seems unusual, it is by no means a bad idea, because it reminds us that the junction capacitance depends on the bias voltage. We can deliberately design a higher bandwidth photodiode system by increasing the reverse bias voltage.

The resistance in parallel with the photodiode is called the parallel resistance (R

). Usually, like the current source, when R

Is infinite. With infinite shunt resistance (or extremely high shunt resistance in real life), the current source delivers all of its current to the load, and the current-to-voltage ratio depends entirely on the load resistance. As the shunt resistance approaches the value of the load resistance, it starts to affect the current-to-voltage ratio more significantly.

For many photodiodes, the shunt resistance is so high that it does not severely affect the overall performance in typical applications. For silicon photodiodes, R

The resistance is tens, hundreds or even thousands of megaohms, and indium gallium arsenide can also have extremely high shunt resistance. However, with germanium, you need to be more careful because R

It is usually in the kiloohm range, and may even be in the low kiloohm range.

Shunt resistance also affects noise performance. As R

Decrease, the Johnson noise of the photodiode increases.

Photodiodes have contacts, wire bonding and contribute to series resistance (R

). The resistance tends to be quite low, for example a few ohms or tens of ohms, although higher values ​​are possible.

As far as I know, series resistance is usually not a major issue in photodiode system design. However, too much series resistance will reduce linearity: the photocurrent through R

A voltage drop is generated, which begins to forward bias the photodiode operating in a zero-bias configuration (see figure below). A forward-biased diode has an exponential current-voltage relationship. Therefore, the voltage across R increases

Reduce the photocurrent reaching the load because it will cause some photocurrent to be transferred to ground through the diode itself, and this transfer of current occurs in a non-linear manner.

When designing or analyzing photodiode-based detection circuits, we use equivalent circuits to help us understand the various electrical parameters involved in the photodiode function. The basic components of the photodiode equivalent circuit are the current source of the photocurrent, the diode symbol representing the pn junction, the capacitor in parallel with the current source, the resistor in parallel with the current source and the resistor in series with the output.

At last!

A person who explains the operation of a photodiode in an easy to understand form.

When the pn junction has been modeled with junction capacitance, current source, and parasitic resistance, why model the pn junction itself (using ideal diodes)?

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