In this article, we’ll discuss some different types of photodiode technologies and the strengths and disadvantages of the semiconductors used to create them—namely silicon
This is the fourth part of our series in photodiodes, which will prepare you for learning more about the use of photodiodes in light-sensitive circuits and their applications. If you’d like to read the rest, check out the links below.
- If you’d like to learn about the basics, start with the first article, which discusses the physics of light and how pn junctions are used to form diodes.
- This piece focuses on pn junctions that are sensitive to light.
- We cover photoconductive and photovoltaic diodes.
The Silicon Photodiode
Silicon is definitely not an exotic semiconductor material, but it makes a fine photodiode. Silicon photodiodes are an excellent choice for many visible-light applications.
This is the primary restriction to keep in mind with silicon: it is sensitive primarily to the wavelengths of visible light. In many systems, such as a light dimmer that responds to ambient light levels, this is exactly what you want. An IR-enhanced silicon photodiode will give you more sensitivity to wavelengths in the near-infrared region, if that’s important in your application.
This plot from Hamamatsu’s Silicon Photodiodes Handbook shows the spectral response for a variety of their silicon photodetector products. QE stands for quantum efficiency.
Silicon photodiodes are great general-purpose light detectors. They’re reliable and widely available, their electrical response to illuminance is highly linear, and they have good dark-current and bandwidth performance. In fact, the lowest-dark-current and highest-speed photodiodes sold by Thorlabs are both silicon devices.
Indium Antimonide (InSb)
When I think about photodiodes, the first material that comes to my mind is InSb. It’s much less common than silicon, but it was burned into my engineering consciousness because one of the most important corporate projects I ever worked on was built around an array of InSb photodiodes.
InSb is sensitive to short-wavelength and mid-wavelength infrared and offers excellent performance for applications that must detect heat signatures instead of visible light. However, to make the most of InSb, you’ll need to put in some extra effort—namely, cooling the photodiode to cryogenic temperatures. They make things called dewars that house the diode and hold liquid nitrogen. You fill up the dewar with LN2, and then your InSb detector is ready for maximum sensitivity.
Indium Gallium Arsenide (InGaAs) and Germanium (Ge)
InGaAs is widely used as a fast, high-sensitivity infrared detector material. Unlike InSb, it is commonly used at room temperature, and it has a little bit of extra responsivity at shorter wavelengths: InSb extends to about 1 µm, whereas the InGaAs range goes down to about 0.7 µm.
Germanium is similar to InGaAs with regard to spectral response, and it works at room temperature. InGaAs can achieve significantly higher signal-to-noise ratio.
Mercury Cadmium Telluride (HgCdTe)
Mercury cadmium telluride has an important role as a detector for long-wavelength IR applications. The spectral response of InGaAs and InSb taper off at 2–3 µm and 5–6 µm, respectively, whereas HgCdTe extends out to 16 µm. Long-wavelength IR (LWIR) is used for passive thermal detection and imaging.
Like InSb detectors, HgCdTe detectors are cooled to cryogenic temperatures. This is a major inconvenience, and many devices use uncooled microbolometers for LWIR imaging; microbolometers respond directly to thermal energy, in contrast to photodiodes, which respond to the incident photons in electromagnetic radiation. Microbolometers are cheaper, smaller, and more energy efficient; HgCdTe produces higher-quality imagery.
Though silicon is sensitive primarily to visible wavelengths, a silicon photodiode can be optimized for enhanced UV response. These devices are called UV-enhanced silicon photodiodes. That’s one approach to measuring UV light.
You’re probably familiar with silicon carbide (SiC). It’s an increasingly prominent semiconductor material that is associated primarily with high-power MOSFETs, but it turns out that SiC diodes are excellent as UV detectors.
Silicon carbide photodiodes are rugged devices that are naturally sensitive only to UV light in the 200 nm to 400 nm band.
This is the normalized spectral response of a silicon carbide photodiode manufactured by Electro Optical Components.
This limited spectral response means that SiC photodiodes do not require optical filtering in systems that must prevent visible or infrared light from interfering with UV measurements. UV-enhanced silicon photodiodes are just that—enhanced for UV sensitivity. They retain their sensitivity to visible light, and in fact they are much more sensitive to visible light than to UV.
The mathematical relationship between incident light power and generated photocurrent is called responsivity. SiC peak responsivity is rather low compared to the peak responsivity of silicon, but silicon’s peak responsivity isn’t relevant to UV applications because it occurs far from UV wavelengths. The responsivity of SiC is similar to the responsivity of silicon if we look only at the 200–400 nm portion of the spectrum.
Silicon photodiodes provide convenient, high-performance measurement of illuminance in the visible spectrum. Standard materials for infrared detection are indium antimonide (InSb), indium gallium arsenide (InGaAs), germanium (Ge), and mercury cadmium telluride (HgCdTe). For UV applications, UV-enhanced silicon is an option, and silicon carbide is worthy of consideration if you need reliable high-temperature operation or if your detector must ignore visible and infrared light.
Next article in Introduction to Photodiodes: Understanding the Photodiode Equivalent Circuit