DET08CFC(/M)
Page 5
Rev B, April 29, 2015
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, linear
device that exhibits high quantum efficiency based upon the application used in a
variety of different applications.
It is necessary to be able to determine correctly the expected level of the output
current 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 definition of photodiode responsivity is the ratio of generated photocurrent
(I
PD
) to the incident light power (P) at a given wavelength:
(
=
4.3. Modes of Operation
The photodiode can operate in one of two modes: photoconductive (reverse bias)
or photovoltaic (zero-bias). Mode selection depends upon the speed
requirements of, and the amount of tolerable dark current (leakage current)
within, each individual application.
Photodetector
Diode
Junction
Capacitance
Shunt
Resistance
Series
Resistance
External
Load
Resistance
I
PD
I
D
I
OUT
