Basic PIN Photodiode Characteristics

  1. What is the operating life of silicon photodiodes?
  2. Does responsivity change over time?
  3. Does dark current change over time?
  4. What is the output signal of a photodiode?
  5. How linear is the output photocurrent in the current source mode?
  6. Then what is the dynamic range of silicon photodiodes?
  7. What's the difference between a MOS linear CCD array and a Photodiode linear array?
  8. What is the responsivity matching from device to device?
  9. Can OSI Optoelectronics provide responsivity-matched photodiodes?
  10. Is the outside of the active area completely insensitive to light?
  11. How can the light absorption in the non-active area be reduced?
  12. What is the difference between the Photoconductive (PC) and Photovoltaic (PV) modes?
  13. Which mode (PV or PC) should I use for my application?
  14. How do you reverse bias a photodiode?
  15. How much reverse bias should I apply?
  16. But how do I know how much bias is sufficient enough to operate in a certain bandwidth?
  17. What happens if the photodiode is biased with a voltage larger than the specified maximum reverse bias?
  18. Does this permanently damage the photodiode?
  19. What happens if the photodiode is forward biased by mistake?
  20. Is there a specific type of power supply used to bias the photodiode?
  21. Why are some devices specified with dark current and some with shunt resistance?


1. What is the operating life of silicon photodiodes?
Photodetectors last for an indefinite period of time when used properly and within the specified specifications. However, certain applications may put the photodiodes through optical, electrical, mechanical and/or thermal stresses beyond the specified ranges, and therefore limit their useful life.

2. Does responsivity change over time?
In hermetically sealed detectors, such as TO metal packages, the responsivity is not expected to change over time. In non-hermetically sealed devices, however, the atmospheric contaminants as well as humidity may be diffused into the active area and result in trapping centers causing a short in the junction.

3. Does dark current change over time?
The surface dark current component can change over time due to ambient moisture. It is also susceptible to surface cleanness, surface contamination, i.e. sodium from hand grease could increase the dark current significantly. The bulk dark current should not increase over time.

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4. What is the output signal of a photodiode?
Photodiode operates as a voltage source as well a current source in response to the incident light in the wavelength range of 200 nm to 1100 nm. The current measurement is preferred since the output current changes linearly with incident light power. The voltage output, however, changes logarithmically with incident light power.

5. How linear is the output photocurrent in the current source mode?
Typically it is linear from a few pico-amps up to few milli-amps.

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6. What is the dynamic range of a typical silicon photodiode?
The dynamic range is the range of incident light power over which the current output from the photodiode is linearly related to the input power and is sometimes expressed in decibels:

This range for a typical device is from 1 picowatt to 10 milliwatts, or 100 dB

7. What is the difference between a MOS linear CCD array and a Photodiode linear array?
The main difference between the two is the readout scheme of the output signal from each element in the linear array. In a Charge Coupled Device (CCD), the signal (charge) is transferred from one element to the next one down the row until it reaches the end, where it is read in sequence in a time multiplexed fashion. In a photodiode array (PDA), the signal (current) is outputted at each element's unique anode and cathode. Therefore, the signal for the PDA can be read simultaneously, rather than sequentially and multiplexed.

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8. What is the responsivity matching from device to device?
responsivity matching from device to device for our catalog items is specified with 10%.

9. Can OSI Optoelectronics provide responsivity-matched photodiodes ?
OSI Optoelectronics, for an additional fee, can provide responsivity matching photodiodes within a specified tolerance at a particular wavelength. Contact the Applications Group for your specific requirements.

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10. Is the outside of the active area completely insensitive to light?
No. Irradiated light on the non-active area adjacent to an active area may generate a small photocurrent in the detector. The magnitude of this signal is dependent of many parameters such as the wavelength of the radiation, the applied bias, and the amount of incident light on the active area as well as distance from the active area.

11. How can the light absorption in the non-active area be reduced?
The silicon non-active area also absorbs light and contributes to the total photocurrent. If this contribution is not desired, a metal shield and/or a black polyamide layer can be put on the non-active area as a part of the semiconductor wafer process.

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12. What is the difference between the Photoconductive (PC) and photovoltaic (PV) modes?
In the photovoltaic mode, the photodiode is neither operated with nor biasing. It is simply acting like a solar call, which converts light into electricity. In the Photoconductive mode, however, the photodiode can be Reversed Biased by voltages up to the specified maximum reverse voltage.

13. Which mode (PV or PC) should I use for my application ?
Applying a reverse bias in the PC mode introduces additional noise current to the generated photocurrent, therefore, reducing the signal to noise ratio. Hence, consider using operating the photodiode in PC mode for high speed applications (greater than 350 kHz) and/or in applications that require a wide dynamic range. For weak signal detection the PV mode is the preferred mode of operation.

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14. How do you reverse bias a photodiode?
Reverse biasing a photodiode is accomplished by setting the cathode of the detector at a higher electric potential than the anode's. In another words, applying a negative voltage to anode.

15. How much reverse bias should I apply ?
Apply a sufficiently high reverse bias only to get the bandwidth you but low enough to avoid the risk of reaching the reverse breakdown voltage. Increasing the reverse voltage increases the response speed of the photodiode, by reducing the junction capacitance.

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16. But how do I know how much bias is sufficient enough to operate in a certain bandwidth?
As a rule of thumb, the needed bias for a certain rise time (or bandwidth) can be calculated from rise time specified at a different voltage by the following relationship:
For example if the rise of a particular photodiode at -5V is 100 ns and the application requires 45 ns, the required bias is about -25V. Use -0.3V when operating in photovoltaic mode.

17. What happens if the photodiode is biased with a voltage larger than the specified maximum reverse bias?
A device may experience reverse bias breakdown if biased over the maximum value we specify, a high current will flow through the device which could lead to the destruction of the photodiode. We do not recommend that you operate this device in this manner.

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18. Does this permanently damage the photodiode ?
Yes.

19. What happens if the photodiode is forward biased by mistake?
Photodiodes when forward biased (positive voltage on Anode) with biases over 0.7V, they will conduct a substantial amount of current. If the current exceeds a specific threshold level or if the conduction time exceeds a specified amount, the device may be permanently damaged. In the assembled form, the Aluminum bonding wire will get burnt-off if the forward current reaches over 100 mA.

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20. Is there a specific type of power supply used to bias the photodiode?
A stable DC voltage source is all that is required to reverse bias a photodiode. The supply's current requirements are minimal, since the device produces its own current.

21. Why are some devices specified with dark current and some with shunt resistance ?
Shunt resistance is basically the dark current when measured with a -10 mV applied reverse bias. This is the slope of the photodiode I-V curve at -10 mV. The -10 mV is chosen since theoretically there should be no dark current at 0V. Since there is no bias applied to the photodiode in PV mode, dark current is specified in the form of Shunt Resistance. As a bias is applied, PC mode, the term dark current is used.

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