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Three Key Parameters Influencing Photodiode Switching Speed

The switching speed of a photodiode is often a key parameter that defines the response time of an overall system. You will always find the switching speed or rise and fall times of photodiodes in Vishay’s datasheets. However, this parameter is always specified under certain test conditions. For example, the rise and fall times of the VEMD5010X01 are 100 ns given a reverse voltage of 10 V, a load resistance of 1 kΩ, and a wavelength of 820 nm.

Basic characteristics (Tamb = 25°C, unless otherwise specified)
Parameter Test condition Symbol Min. Typ. Max. Unit
Rise time VR = 10 V, RL = 1 kΩ,
λ = 820 nm
tr 100 ns
Fall time VR = 10 V, RL = 1 kΩ,
λ = 820 nm
tf 100 ns

To assess the real behavior in a designer’s application, one needs to consider the three key parameters that influence the switching speed of a photodiode. These parameters are: wavelength of the detected light, reverse voltage, and load resistance.

The basic dependencies are:

  • The longer the wavelength, the slower the speed
  • The higher the reverse voltage, the faster the diode
  • The lower the load resistance, the faster the switching

A more technical way of putting it is that switching times are limited by carrier lifetime. Due to the absorption properties of silicon photodiodes, most of the incident light at longer wavelengths is absorbed outside the space charge region. Therefore, rise and fall times increase for wavelengths greater than 850 nm (  ). On the other hand, a higher reverse voltage widens the space charge region, which generates more charge carriers and the switching gets faster (  ). Finally, the load resistance can be used to adjust the switching speed. A lower resistance simply leads to a smaller RC time constant (τ). With τ = RC, it becomes clear that a low resistance facilitates a fast response (  ).

Switching Times vs. Wavelength for
Photodiode BPW20RF
VEMD5010X01 Bandwidth vs. Reverse Bias Voltage, Parameter: Load Resistance,
λ = 940 nm

In terms of Vishay’s photodiodes, our VEMDxx60X01 product series offers the fastest switching times, as these are trimmed to serve high speed applications.

Vishay manufactures PIN photodiodes in different packages, sizes, wavelength sensitivities, and technologies.

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This entry was posted on July 31, 2018 by in Articles.
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