Your next optical system for precision intensity measurements will need some type of detector. Photodiodes and phototransistors are common choices for these applications, although they provide different types of responses to incoming light.
If you need to gather intensity measurements, a phototransistor or photodiode is an excellent choice for an optical detector. Choosing between a phototransistor vs. photodiode depends on the particular application, but they are interchangeable in some ways. Here’s what you need to know about choosing a photodiode or phototransistor as a detector for your next optical product.
All phototransistors and photodiodes perform the same function: they receive incoming light and convert it to electricity. This occurs through the same phenomenon that occurs in photovoltaic cells: incoming photons excite charge carriers into a higher energy level, and the charge carriers can be extracted into a load component/circuit. Phototransistors and photodiodes are analogs of regular transistors and diodes.
The structures of these devices are similar to their electrical analog in terms of doping. Photodiodes have similar structure as a regular diode, where a p-n, p-i-n, or similar doping profile is used in the device. A phototransistor is generally built as an NPN or PNP bipolar transistor, or as an FET transistor. The built-in voltage in the material is used to extract charge carriers, just as in a regular diode or transistor.
Both circuit elements are designed to operate within a range of wavelengths, and this range of possible operating wavelengths can be quite broad in common semiconductor materials. A phototransistor or photodiode sensor will have responsivity spectrum that depends on the absorption spectrum of the materials used to construct the device. The absorption spectrum of these materials are normally modified through standard doping processes. Some common materials and their useful wavelengths are:
These two circuit elements are brought into a real circuit in different ways. They can also be integrated into array detectors (e.g., CMOS detectors or CCDs), where the required circuit elements are implemented on-chip. If you are working with a customized system that uses discrete components, then you’ll need to use particular circuits to work with each type of detector.
A phototransistor can be brought into common collector, common emitter, or other standard transistor configuration to extract current. When no light is incident on the device, they operate just like any other transistor (as a 3-terminal device). Once light is incident on the device, it is absorbed in the base. This is equivalent to increasing the base current into the device. Because of this, a phototransistor can be operated as a 2-terminal device (i.e., with the base connection floating). When operated as a 3-terminal device, the output current can be modulated by adjusting the base voltage (for NPN or PNP devices) or the gate voltage (for FET devices).
NPN phototransistor circuits
When run as a 3-terminal device, the output current seen at a load can be modulated by adjusting the input base current. This means the device acts like a switch with a built-in threshold. When incident light is intense enough, and current sent from a source into the base is large enough, the base-emitter voltage changes and a current can easily pass through the device. However, this can be suppressed by lowering the total base current, which requires adjusting the external bias at the base. This switching behavior makes phototransistors useful in a number of applications that require measuring an ON or OFF state, rather than a specific intensity measurement.
A photodiode in a real circuit can be run in photovoltaic mode (when run in forward bias) or in photodiode mode (when run in reverse bias). Photodiodes are run in reverse bias as this provides a linear response, and the responsivity range can be quite large. The output current can be sent directly to a load, or it can be sent to an amplifier circuit. If you want to convert the input back to a stream of square pulses, just send the amplifier output to a comparator.
Avalanche photodiodes are also available and are always intended to operate with bias very near the reverse breakdown voltage. Once light is incident on the device, the number of photo-generated carriers is multiplied by the external bias as the device runs beyond the breakdown voltage. This produces gain during illumination. These photodiodes are designed to run in breakdown and are useful for detecting weak optical signals.
A photodiode alongside an amplifier and analog-to-digital converter (ADC) can also be used to receive digital data encoded in amplitude-modulated or PWM optical pulses. In the case of PWM, you will need to account for the bandwidth of your photodiode and amplifier as this limits the maximum data rate. Photodiodes have a response time, which is related to its terminal capacitance. The maximum response frequency is typically taken as the knee frequency for a digital pulse with a particular rise time, which is equal to 0.35/(response time).
Photodiode circuit with an operational amplifier.
In both types of devices, you can simulate the input light into the device by simply adding a current source to the base terminal (in a phototransistor) or to the high side of the device (for a photodiode). This allows these devices to be easily brought into circuit simulations and examined alongside your other components. You should also simulate impedance matching in your circuits, especially when run with fast data pulses. Impedance matching is critical for ensuring maximum power transfer into the receiver.
A critical point in designing a circuit for a phototransistor or photodiode is determining a load line. This tells you the range of input light intensities that will produce a linear output. Similarly, it will tell you how the linear range varies as a function of load impedance. For a phototransistor, the load line will look the same as it does for a regular transistor. For a photodiode, the load line is very different and looks like the load line one would draw for a solar cell.
Load lines for a photodiode (left) and phototransistor (right).
These graphs can be determined from simulations, or they can be measured for various load impedances and bias values. If you want your phototransistor to act like an optical switch, then it needs to be run in the saturation region. This will cause the output current to saturate once light is incident on the device. In contrast, a photodiode does not saturate, although it will exhibit a nonlinear response once the incident light intensity is high enough.
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