Avalanche Photodiode Bias Design

An avalanche photodiode multiplies the photocurrent internally by operating close to its breakdown voltage, which gives it a sensitivity that a plain photodiode cannot match. The multiplication factor changes steeply with the bias, so the supply and its stability are the whole design.

How the Multiplication Works

An absorbed photon creates an electron hole pair, and the high field in the multiplication region accelerates the carriers until they create further pairs by impact ionisation. Each absorbed photon therefore produces a pulse of many carriers.

The gain is a strong function of the bias voltage, rising steeply as the bias approaches the breakdown value. A change of a fraction of a volt near breakdown can change the gain by a large factor, which is why the supply has to be precise.

The multiplication is a statistical process, so it adds excess noise beyond the simple amplification. The excess noise factor rises with the gain, and there is an optimum gain above which the signal to noise ratio stops improving.

breakdown voltage and Its Variation

breakdown voltage is the value at which the device conducts with no light at all. It varies between devices from the same wafer and rises with temperature by roughly a tenth of a volt per degree.

The bias is set as a fixed voltage below the breakdown, for example twenty volts below a breakdown of two hundred. The gain then depends on the difference, and the temperature coefficient of the breakdown voltage becomes a scale error on the gain.

Measuring the breakdown voltage of each device is part of production. A device with a breakdown outside the expected band is rejected, and the working bias is set relative to the measured value rather than to a nominal one.

Generating the bias voltage

The bias voltage is high, between fifty and four hundred volts, and it has to be quiet. A boost converter followed by a linear regulator or a filter is the usual arrangement, and the switching noise of the converter has to be removed because it appears directly in the photocurrent.

A filtered supply with an RC or LC network at the photodiode is common. The series resistor also limits the current if the photodiode breaks down, which protects both the device and the supply.

The current drawn is small, so the supply can be built around a low power converter. What matters is the noise and the stability, not the current capability, and a supply with a large output capacitor is not necessarily the quietest.

temperature compensation in Practice

The bias has to track the breakdown voltage, which rises with temperature. A temperature sensor near the photodiode and a compensation term in the firmware, or an analog network with a matched temperature coefficient, provides the tracking.

An analog implementation uses a resistor with a positive temperature coefficient in the feedback network of the supply, arranged so that the output rises by the right amount. It is fast and does not need a processor, which suits a simple instrument.

A digital implementation measures the temperature and adjusts the set point of the supply. It is slower and more flexible, and it allows the compensation curve to be characterised for the individual device.

Operating Modes and quenching

There are two ways to use the device. In linear mode the bias is held below the breakdown and the output current is proportional to the light, which is what a measurement instrument needs.

In Geiger mode the bias is held above the breakdown, and a single photon triggers a self sustaining avalanche that has to be stopped. quenching is the process that stops it, by reducing the bias below the breakdown for a short time.

A passive quench uses a large series resistor that lets the current fall until the avalanche stops, and the recovery time is set by the time constant of the circuit. An active quench uses a transistor to detect the pulse and reset the bias much faster.

Protecting the Photodiode

The device is damaged by excessive current rather than by excessive voltage, and the current in the avalanche is limited by the series resistor. Its value is a compromise between the response time and the protection it gives.

optical overload is the other hazard. A bright flash produces a current that heats the junction, and the resulting damage is permanent. A mechanical shutter or an optical attenuator is part of the protection of a sensitive instrument.

The input of the following amplifier needs its own protection. The voltage at the photodiode node moves when the avalanche occurs, and the amplifier input has to survive that swing without conducting through its own protection diodes.

Noise, Bandwidth and the Amplifier

The gain bandwidth product of the device is finite, so a high gain and a wide bandwidth cannot both be obtained. The junction capacitance and the load resistance set the bandwidth, and the capacitance is fixed by the physics of the device.

The following amplifier is a transimpedance stage, and its noise contribution has to be small compared with the excess noise of the multiplication. The feedback resistor is the dominant noise source at low gain, and the input capacitance sets the stability.

Filtering after the amplifier reduces the bandwidth to what the measurement needs, which improves the signal to noise ratio. A measurement that only needs a kilohertz of bandwidth should not be made at a megahertz, because the extra noise is then added for nothing.

Layout and Shielding

The bias node is a high voltage node with a small current, and it should be routed away from the low level signals. A leakage path from the bias to the amplifier input produces a current that is indistinguishable from light.

A guard ring around the photodiode input, connected to a low impedance point at the same potential, intercepts surface leakage. Cleanliness and a conformal coating matter more here than in most circuits because the currents are so small.

The high voltage section should be separated from the digital section and clearly marked on the assembly drawing. A design where the bias can be touched during adjustment is a design that will eventually hurt somebody.

Verification and Faults

Verify the bias with a high impedance meter and a divider that does not load the supply. The measurement of the breakdown voltage itself is made by raising the bias slowly and watching the current, with the device in the dark.

Check the gain against a known optical input at two temperatures. A gain that changes more than the compensation predicts indicates a problem with the temperature sensing or with the placement of the sensor.

A photodiode that becomes noisy over months usually has an increase in leakage, often from moisture or contamination on the board. The practices that keep such a surface clean are described in our guide to conformal coating board protection, the release checks in our PCB design release checklist, and the assembly points in judging PCB quality.

FAQ

How far below breakdown should I set the bias? It depends on the gain required, and it is set from the measured breakdown voltage of the individual device rather than from a nominal value.

Why does the gain change with temperature? The breakdown voltage rises with temperature, so a fixed bias moves further below breakdown and the gain falls. Compensate the bias.

What limits how much gain I can use? Excess noise. Beyond an optimum gain the signal to noise ratio stops improving, and the bandwidth falls as well.