PIN Photodiode, High Voc, Low Isc

In summary, the conversation discusses measurements being conducted on a silicon PIN-type photodiode in photovoltaic mode. The open-circuit voltage and short-circuit current are being measured for varying intensities of incident light, with the multimeter being used for precision measurements. The open-circuit voltage follows a logarithmic increase, while the short-circuit current increases linearly. However, the current is quite small due to the low value of I_0 in the equation. The conversation also discusses the internal resistance of the multimeter and the temperature effects on voltage measurements. The conversation ends with a question about why the "barrier potential" cannot be read by a voltmeter placed in parallel around the junction.
  • #1
Qianlong
2
0
I am working with a silicon PIN-type photodiode in photovoltaic mode, conducting measurements of open-circuit voltage and short-circuit current for varying intensities of incident light. The light is being measured as a proxy for another quantity, and so I must apologise for I can give no quantitative values of optical power.

The photodiode has a claimed max “Operating Current” of 10mA, and an active area of 20mm^2.

The measuring apparatus is a fluke 187 digital multimeter, which can measure to precisions of tens of microvolts and tens of nanoamps, though in the setup there is some background noise (probably electrical) that renders the last digit unreliable. Attempts have also been made to measure current with a moving coil microammeter, which has a resistance of 150Ω.

As I understand it, with increasing optical power incident on the detector, the short-circuit current should increase linearly and be equal to the photocurrent. The open-circuit voltage should increase logarithmically, but for low optical powers, the increase is approximately linear*.

*The open circuit voltage is given by the equation V = KT/q ln[(I/Io +1)], which for values of I/Io<<1 the Mercator series can be used to yield V=KT/q I/Io

Open-circuit voltages have been detected at both at ultra-low light levels and in ambient environmental lighting. The open-circuit voltage s in these scenarios has ranged from 40μV to 350mV. Linearity is confirmed up until about 20mV. However, when the short-circuit current is measured, nothing is seen until the open-circuit voltage for the same light intensity exceeds 250mV. At this light intensity, the current is 1.4μA.

My question: is the current just that small? Other measurements (albeit for p-n junctions) I have seen reported in the literature record that light intensities that lead to 1mV open-circuit voltages lead to at least 1μA short-circuit currents. Certainly, one would have thought that a current much closer to 10mA would have been seen in ambient environmental lighting.

There is a second question I wished to ask, and it relates more to the theory. I understand that when p-type semiconductor is connected with n-type semiconductor, the majority carrier in each type diffuses along the concentration gradient. This leave behind ions, trapped in the crystal lattice, which generate an electrical field that opposes further diffusion. Forgive my ignorance, but why can this “barrier potential” not be read by a voltmeter placed in parallel around the junction?

Greatly obliged for your assistance.
 
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  • #2
The fluke multimeter has some internal resistance. I don't know how much (and it depends on the measurement range: smaller ranges correspond to larger internal resistances), but that could reduce the current significantly.

Unrelated to that, at the level of millivolts you can have temperature effects that lead to additional voltage differences.

As far as I know, 1.4µA is not at the lower end of the fluke precision - do you see a sudden jump in the current?

Forgive my ignorance, but why can this “barrier potential” not be read by a voltmeter placed in parallel around the junction?
Current flows through the voltmeter (and the timescale is way too short to see that) until the potential is equal at both sides. That is true for every setup without a power supply.
 
  • #3
Qianlong said:
the open-circuit voltage for the same light intensity exceeds 250mV. At this light intensity, the current is 1.4μA. My question: is the current just that small?

Yes, that actually sounds quite reasonable.

The key to understanding this is to realize just how small [itex]I_0[/itex] is. It's value is very temperature dependent (due to its dependence on [itex]N_i^2[/itex]), but even at I=1.4uA the ratio [itex]I/I_0[/itex] is likely to be order of around magnitude [itex]10^5[/itex] or so.

Put this value into the equation for voltage and you'll find that it does indeed correspond to somewhere around 250 mV.
 
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  • #4
mfb said:
The fluke multimeter has some internal resistance. I don't know how much (and it depends on the measurement range: smaller ranges correspond to larger internal resistances), but that could reduce the current significantly.
Update: I tested that with a Fluke289, and at 1µA, the voltage is well below 1mV (measured: 0.15mV, but that is not reliable*). That is not an issue if the multimeters are not completely different.

*Edit: A more careful measurement showed 0.10mV.
 
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  • #5


I am interested in your work with the silicon PIN-type photodiode and your observations on the open-circuit voltage and short-circuit current for varying intensities of incident light. It is important to accurately measure these parameters in order to understand the performance of the photodiode in photovoltaic mode.

Firstly, the high Voc (open-circuit voltage) and low Isc (short-circuit current) values you have observed can be attributed to the unique properties of a PIN-type photodiode. Unlike traditional p-n junction photodiodes, PIN photodiodes have a wider depletion region and a lower doping concentration, resulting in a higher barrier potential and lower leakage current. This leads to a higher open-circuit voltage and lower short-circuit current.

Regarding your question about the low short-circuit current measured, it is possible that the current is indeed that small due to the low doping concentration in the photodiode. However, it is also important to consider the effects of background noise and the limitations of your measuring apparatus. I would suggest trying to minimize the background noise and using a more sensitive measuring instrument to accurately measure the short-circuit current.

To address your second question, the barrier potential generated at the p-n junction is indeed measurable by a voltmeter placed in parallel around the junction. However, in a photovoltaic mode, the photodiode is connected in reverse bias, which results in a higher barrier potential and a lower current flow. This is why the open-circuit voltage is typically higher than the short-circuit current in photovoltaic mode.

In conclusion, your work with the silicon PIN-type photodiode is an important contribution to the understanding of its performance in photovoltaic mode. I would recommend further experimentation and refining your measurement techniques to accurately measure the open-circuit voltage and short-circuit current for a better understanding of the photodiode's behavior. Thank you for sharing your work and seeking clarification on the theory behind it.
 

1. What is a PIN Photodiode?

A PIN photodiode is a type of semiconductor device that can convert light energy into electrical energy. It consists of three layers - P-type, Intrinsic, and N-type - hence the name PIN. The intrinsic layer is the key component that allows for efficient absorption of light.

2. What is the significance of a high Voc in a PIN Photodiode?

Voc, or open-circuit voltage, is a measure of the maximum voltage that a photodiode can produce when no current is flowing through it. A high Voc in a PIN photodiode indicates a high potential for converting light energy into electrical energy, making it a more efficient device.

3. Why is a low Isc desirable in a PIN Photodiode?

Isc, or short-circuit current, is a measure of the maximum current that a photodiode can produce when its terminals are shorted together. For a PIN photodiode, a low Isc is desirable because it indicates minimal leakage current, which can affect the accuracy and sensitivity of the device.

4. What are the applications of a PIN Photodiode?

PIN photodiodes are commonly used in various industries for light detection, ranging from simple light sensors to more advanced applications such as solar cells and optical communication devices. They are also used in medical and scientific instruments for their high sensitivity and fast response time.

5. How does the structure of a PIN Photodiode contribute to its performance?

The P-type, intrinsic, and N-type layers in a PIN photodiode are carefully designed to optimize its light absorption and electrical properties. The intrinsic layer, in particular, is crucial as it allows for a larger depletion region, resulting in a higher probability of light absorption and a lower dark current, ultimately leading to a higher efficiency and sensitivity of the device.

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