22.3 Photodiodes
599
22.3 Photodiodes
22.3.1 Introduction
The principle of the photodiode is the interband absorption of light in the depletion layer of a diode
(or the i-zone of a pin-diode) and the subsequent separation of electrons and holes by the electric
field. There are opposite requirements for fast detectors (thin depletion layer) and efficient detectors
(complete light absorption, sufficiently thick depletion layer). For this reason generally semiconductors
with high absorption coefficient are most suited (Fig. 22.13). In Fig. 22.14 the quantum efficiency and
detectivity D
∗ of various semiconductor detectors are compared.
A diode can be operated without bias (photovoltaic mode) using the built-in field. An improvement
in the speed of a pn-diode is achieved with a reverse bias since it increases the field strength in the
depletion layer. However, the reverse bias is below the breakdown voltage. Operation near breakdown
is exploited in the avalanche photodiode (APD). In the following we will discuss pn-, pin-, MS(Schottky-), MSM- and heterostructure-diodes and APDs.
(a)
(b)
Fig. 22.13 Optical absorption coefficient of various semiconductor materials (as labeled) used for photodetectors a in the
UV, visible and near-infrared range (at room temperature) and b in the mid-infrared spectral range at room temperature
(solid lines) and at 77 K (dashed lines). Based on [1837]
599
22.3 Photodiodes
22.3.1 Introduction
The principle of the photodiode is the interband absorption of light in the depletion layer of a diode
(or the i-zone of a pin-diode) and the subsequent separation of electrons and holes by the electric
field. There are opposite requirements for fast detectors (thin depletion layer) and efficient detectors
(complete light absorption, sufficiently thick depletion layer). For this reason generally semiconductors
with high absorption coefficient are most suited (Fig. 22.13). In Fig. 22.14 the quantum efficiency and
detectivity D
∗ of various semiconductor detectors are compared.
A diode can be operated without bias (photovoltaic mode) using the built-in field. An improvement
in the speed of a pn-diode is achieved with a reverse bias since it increases the field strength in the
depletion layer. However, the reverse bias is below the breakdown voltage. Operation near breakdown
is exploited in the avalanche photodiode (APD). In the following we will discuss pn-, pin-, MS(Schottky-), MSM- and heterostructure-diodes and APDs.
(a)
(b)
Fig. 22.13 Optical absorption coefficient of various semiconductor materials (as labeled) used for photodetectors a in the
UV, visible and near-infrared range (at room temperature) and b in the mid-infrared spectral range at room temperature
(solid lines) and at 77 K (dashed lines). Based on [1837]