Using an LED as a Photodiode
by Anton Kruger

Question:

Can you use an ordinary diode, or for that matter an LED, as a photodiode?

Answer: The answer is yes, but only with the proper "light" and diode-semiconductor material combination. Recall that one may view light as consisting of energy packets or photons, and the energy of each packet is a function of the wavelength of light. A handy equation is E=1,240 /l where the wavelength of light in nanometers (10-9 meters) and E is the photon energy. When a photon is absorbed in a pn junction and it has more energy than the bandgap energy of the semiconductor material, it generates an electron. The bandgap energy of Germanium is 0.66 eV, making these diodes sensitive to visible light and infrared light up to 1,880 nm Many diodes are made of silicon (1.122 eV) and these diodes are sensitive to visible and infrared light up to 1,106 nm GaAs (1.424 eV) diodes are sensitive up to 900 nm. Table 1 summarizes the characteristics of common materials used in the manufacture of LEDs, and as expected, the wavelength decreases as the bandgap energy increases. If one illuminates a red LED with blue light, then the red LED will behave as a photodiode. However, if we turn things around, and illuminate the blue LED with the red light, it will not generate current—the energy of the red photons are too small to generate electrons in the SiC material. Color Material Bandgap Blue SiC 2.64 eV Green Gap 2.19 eV Yellow GaP85As15 2.11 eV Orange GaP65As35 2.03 eV Red GaP4As6 1.91 eV Table 1 - Characteristics of Common Materials Used for LEDs Thus, under the proper conditions can sometimes use ordinary diodes and LEDs as photodiodes, but practically, they perform poorly. A small-signal diode, even if it enclosed in glass, is not optimized for light-current conversion—the doping profile, exposure to light, reflections, etc. is simply wrong. LEDs may seem be a better option, but are normally enclosed in colored plastic that filters out much of the light.

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