Reverse bias induces only little current (known as saturation or back current) along its direction. But a more important effect of reverse bias is widening of the depletion layer (therefore expanding the reaction volume) and strengthening the photocurrent. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect and also tend to have lower capacitance, which improves the speed of their time response. On the other hand, the photovoltaic mode tends to exhibit less electronic noise.
Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias (photoconductive mode)
The
current through a photodiode is directly proportional to the light
intensity
The photodiode and phototransistor can be both photovoltaic (generators
of potential difference) and photoconductive (modifiers of an electric
current), depending on the application.
A
reverse-biased photodiode operates in what is called photoconductive mode,
since the conduction of the semiconductor junction varies with the illuminating
light intensity. If the reverse-biased
voltage is relatively large (i.e. several volts) the reverse-biased photodiode
will have a very fast response time (much faster than an LDR) and is suitable
for detecting light signals that vary down to a time scale of a fraction of a
microsecond.
When
light shines on the LDR, it has low resistance and allows current to flow. When light does not shine on it, the LDR has
a very high resistance, and a much smaller current will not flow through it.