Si Biased Detector
Page 5
Rev F, December 8, 2014
Chapter 4 Operation
4.1. Theory of Operation
A junction photodiode is an intrinsic device which behaves similarly to an
ordinary signal diode, but it generates a photocurrent when light is absorbed in
the depleted region of the junction semiconductor. A photodiode is a fast, highly
linear device that exhibits high quantum efficiency based upon the application
and may be used in a variety of different applications.
It is necessary to be able to correctly determine the level of the output current to
expect and the responsivity based upon the incident light. Depicted in Figure 1 is
a junction photodiode model with basic discrete components to help visualize the
main characteristics and gain a better understanding of the operation of Thorlabs'
photodiodes.
=
+
Figure 1 Photodiode Model
4.2. Responsivity
The responsivity of a photodiode can be defined as a ratio of generated
photocurrent (I
PD
) to the incident light power (P) at a given wavelength:
( ) =
4.3. Modes of Operation
A photodiode can be operated in one of two modes: photoconductive (reverse
bias) or photovoltaic (zero-bias). Mode selection depends upon the applications
speed requirements and the amount of tolerable dark current (leakage current).
Photodetector
Diode
Junction
Capacitance
Shunt
Resistance
Series
Resistance
External
Load
Resistance
I
PD
I
D
I
OUT
