diodesphotodiode
How does voltage bias affect a photovoltaic cell? I'm receiving conflicting opinions online, with some saying that photovoltaic mode is entered only with forward bias, some saying reverse bias, and some saying no bias. Furthermore, is there a breakdown voltage for photovoltaic cells? The Shockley equation as I understand it (\$I = I_0(e^{(eV/k_BT)} – 1) – I_p\$) physically cannot tend to negative infinity as it's an exponential equation.
Thanks!
I'll edit this answer severely as a general comment:
Photodiodes may be operated either forward or reverse biased.
Forward biased gives most output.
Reverse biased gives most speed. and is noisier.
In this mode Vsupply needs to be < Vreversebreakdown - hopefully in the data sheet.
Reverse biased mode is most usually used.
The Sharp BS120 is optimised for 560 nM operation. It specifies Vreverse_abs_max as 10 volts and has curves all the way up (or down) to -10V.
In "forward biased" mode the diode is usually operated with NO added bias and is used as a voltage source.
For a silicon diode with Vforwards = Vf = 0.6V . Say you want 5 mA current and are using a 5V supply then
I = V / R = = (Vsupply - Vdiode) / R = (5-0.6) / 0.005 = 880 ohms Say 1000 Ohms (which gives a notional 4.4 mA)
For 1 mA you'd use (5-0.6)/0.001 = 4400 Ohms
It depends on how you are sensing the current. If you are just using a resistor to ground, then the photodiode will become forward biased and you get a maximum voltage of about 0.5V. (If you use a TIA opamp circuit that won't happen.) If you reverse bias it then you can get output voltages up to the bias supply. With a TIA opamp circuit the only advantage (as HH plasma said) is a higher frequency response.
Best Answer
If you're asking whether light is converted into photocurrent only under specific photodiode bias voltages, the answer is that photovoltaic action is (mostly) independent of the PD bias. Notice that the photocurrent, Ip, is independent of the diode voltage, V, in your equation.
In communications circuits, photodiodes are operated with negative bias or zero bias. PD's are not operated with forward bias, because it saturates the device(somewhat defeating the purpose of having a photodiode), as well as increasing junction capacitance (undesirable in high speed optical communications circuits). Some sources classify "photovoltaic" mode as the mode under negative bias, and "photoconductive" mode as the mode with zero bias.
Yes, PD's have a reverse breakdown voltage, as does every diode. In high speed PD's used in optical communications, this reverse breakdown can be quite low, about 4-5 V. For PD's used in power generation, the reverse breakdown voltages are much higher.
Typical PIN diode characteristics:
Some photodiodes have much higher reverse voltage (up to 200 V) and are used very close to their avalanche point. This gives these diodes much higher effective optical gain (100-100*) than a low voltage PD.