Temperature and Bias Effects

  1. How does responsivity change with temperature?
  2. How does dark current change (shunt resistance) change with temperature?
  3. What about the other characteristics of the photodiodes?


1. How does responsivity change with temperature?
In silicon, the absorption coefficient increases with the temperature. If the temperature goes up, the absorption depth of light decreases. As a consequence, the responsivity for short wavelengths has a negative temperature coefficient, since a significant amount of carriers could recombine and get lost in the heavily doped p+ layer. For the longer wavelengths, responsivity has a positive temperature coeffcient, since more carriers will be generated in or near the depletion region of th pn junction, and participate in the photocurrent by increasing it.

2. How does dark current (shunt resistance) change with temperature?
Drift current is the dominating current (dark current) in Photoconductive (PC) mode and varies directly with the temperature. In Photovoltaic (PV) mode, the diffusion current is the dominating current, which determines the Shunt resistance. It varies as square of the temperature. Thus the change in temperature affects the photodetector more in photovoltaic mode than in photoconductive mode of operation.

In general, in PC mode of operation, dark current may approximately double for every 10°C increase in temperature. In PV mode of operation, however, shunt resistance may approximately double for every 6°C decrease in temperature. The exact change is dependent on additional parameters such as the applied reverse voltage, the bulk resistivity as well as the thickness of the bulk substrate.

3. What about the other characteristics of the photodiodes?
Since noise currents are generated as result of dark current (shunt resistance), the higher the temperature, the higher the noise in the detector. As a consequence, VB (breakdown voltage) decreases as the temperature increases.

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