# What bias voltage is applied to a PIN photodiode when connected only to multimeter

If I take just a PIN photodiode (e.g. BPW34 FAS ) and connect a multimeter to its terminals to measure current, for example: expose the photodiode to the sunlight and read current of ~1 mA on the multimeter. In what region do I operate the photodiode with respect to bias voltage, that is do I have zero, forward or reverse bias voltage ? What is the bias voltage (or how it can be found) if not zero ?

• You're operating it in photovoltaic mode, it sounds like. This could be considered a form of forward bias, but the terminology isn't really all that useful for this particular case. – Hearth May 7 '19 at 21:28

expose the photodiode to the sunlight and read current of ~1 mA on the multimeter.

I will assume this means you're using the multimeter in ammeter mode.

In what region do I operate the photodiode with respect to bias voltage, that is do I have zero, forward or reverse bias voltage ?

The meter should not be applying any bias in ammeter mode.

It will have a small "burden resistance", meaning as the photodiode starts top push current through the meter, the voltage across the meter will increase slightly.

The result is you are essentially using the photodiode in photovoltaic mode, but with a very low load resistance (which isn't how we typically use photovoltaic mode).

What is the bias voltage (or how it can be found) if not zero ?

You could put a second meter across the photodiode and put it in voltmeter mode.

• Chances are that the voltage developed across the meter will be a few tens of millivolts, which will make practically no difference vs. no bias voltage at all. But yes, a second meter would do the trick. – TimWescott May 7 '19 at 22:20

You were operating in PV mode with an internal shunt R to measure almost short cct. current. assuming 1mA in current mode on DMM.

# Photovoltaic (PV) - -- - - Photoconductive (PC)

Solar cell mode
Zero bias applied               Reverse biased
No "dark" current               Has "dark" current
Linear                          Nonlinear
Low noise (i.e. Johnson)        Higher noise (i.e. Johnson, Shot)
max. junction capacitance       low capacitance reduces with higher Vr
Used in precision applications  Used in high-speed applications


Ref

BPW 34 FAS Datasheet:

- Isc = 23 µA typ.
- Voc = 250 mV min. 320 mV typ.
- 72 pF @ 0V
- 0.65 mA / mW @ λ = 870 nm
- Radiant sensitive area A = 7.02 mm² typ.


Sunlight ≈ 1 kW/m² = 100mW/mm²