Using an Infrared Phototransistor in Electro-optics

Key Takeaways

• Phototransistors are one option for detecting light and converting it to an electric current. 

• These devices function like regular transistors, but the gate/base is activated by incoming photons.

• A variety of materials and device architectures can be used to build phototransistors, and these devices can be incorporated into several electro-optical systems.

Infrared phototransistors and other infrared detector components can be used in imaging systems and as detectors in a number of other systems.

Infrared electro-optical systems need sources and detectors to operate properly, and each will need to be added to a circuit in certain ways to maximize power transfer into/out of a circuit. Common photosensitive elements for use as detectors include photodiodes and phototransistors. Infrared phototransistors are largely the same as their visible-light-sensitive cousins, and they are often compared to infrared photodiodes.

These components need some supporting circuitry to be implemented in a larger system, but this allows the designer to tune the electrical output. As infrared phototransistors provide wideband absorption and quantum efficiency, they can be integrated into a compact image sensor (e.g., CMOS image sensor) over a broad range of wavelengths. 

Here’s what you need to know about designing with infrared phototransistors and how they are integrated into compact sensing elements.

What Is an Infrared Phototransistor?

Simply put, an infrared phototransistor is a type of optical switch where the device outputs a current in response to some input light. In reality, all as-constructed phototransistors built from commonly available semiconductors are infrared phototransistors. Other wide bandgap materials (metal oxides or polymers) could be used to build a phototransistor that is only sensitive to visible or UV light, but these devices and materials are not widely commercialized.

As a standard material for semiconductor devices, Si is normally used to build infrared phototransistors. The optical absorption band in Si spans from ~1100 nm (1.1 eV) to UV wavelengths in ambient conditions. In order to confine absorption to infrared or visible wavelengths, an absorptive film is normally applied to the phototransistor to filter out undesired wavelengths. This allows a general-purpose phototransistor to operate exclusively at infrared wavelengths, visible wavelengths, or a narrow wavelength range. The image below shows the structure of a typical NPN photodiode with an absorptive film.

Infrared phototransistor structure with an absorbing film.

A high-cut absorbing film is normally used to provide tuning over the optical response. When combined with the natural absorption edge of the semiconductor wafer, only wavelengths between the semiconductor absorption edge and the filter’s absorption edge will be absorbed and converted into an electric current. This characteristic, as well as many others, separate phototransistors from photodiodes.

Infrared Phototransistor vs. Photodiode

Photodiodes are often compared to phototransistors, and for good reason. They can be added to a circuit to perform similar functions, but they operate differently and provide different types of advantages. The table below shows a brief comparison of infrared phototransistors and photodiodes.

When looking through the above table, we can see that the main differences between these components are: how amplification is implemented, how the devices are tuned into different operation modes, and how each device is constructed. Aside from these points, the devices provide practically the same functionality. Different materials will provide sensitivity over different wavelength ranges. When combined with an absorptive film as shown in the device structure above, these materials allow easy tuning of sensitivity, while external circuitry provides easy tuning of the electrical response.

Load Lines

Each of these components requires some supporting circuitry to be brought into a linear response range. This allows the measurement range to be set by tuning the saturation level in the device. In order to visualize this effect, a load line needs to be constructed for a photodiode circuit or phototransistor circuit. The load line allows you to easily see when the input power level causes the device to saturate for a given input power.

The load lines for a photodiode and phototransistor are shown below. These load lines correspond to an input photocurrent for a common collector (CC) or common emitter (CE) configuration. These two configurations match their electrical transistor circuit analogs except the base region is left floating. Upon exposure to light, the device is activated and provides an output current that can be determined from the device’s load line.

Photodiode and phototransistor load lines.

Integration into Image Sensors

Just like photodiodes, a phototransistor can be integrated into a 1D or 2D array, which is useful in a number of applications. A 2D phototransistor array is basically a CMOS sensor, which uses MOS phototransistors. In order to form images, the detector needs to be coupled to some optical and mechanical components which will help direct and focus light onto the sensor. Note that a phototransistor is not the same as a CCD array; both types of sensors provide distinct advantages in different imaging applications.

If you’re building a system that will use a phototransistor, either for imaging or sensing, you’ll need to tune the device to provide the desired measurement range by adjusting the load line. This is done with DC simulations by applying some DC current to the base/gate region in the phototransistor. You can then construct a series of load lines for different load resistor values, which will allow you to tune the measurement range. For switching applications, you’ll want fast saturation (low measurement range) at low input photocurrent, while imaging and measurement applications need large measurement range without saturation.