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SEMICONDUCTOR MATERIAL AND DEVICE PROPERTIES
The mechanism by which optical detectors convert optical power into electrical current
requires knowledge of semiconductor material and device properties. As stated in chapter
6, providing a complete description of these properties is beyond the scope of this
manual. In this chapter we only discuss the general properties of semiconductor PINs and
APDs.
Semiconductor detectors are designed so that optical energy (photons) incident on the
detector active area produces a current. This current is called a photocurrent. The
particular properties of the semiconductor are determined by the materials used and the
layering of the materials within the device. Silicon (Si), gallium arsenide (GaAs),
germanium (Ge), and indium phosphide (InP) are the most common semiconductor materials
used in optical detectors. In some cases aluminum (Al) and indium (In) are used as dopants
in the base semiconductor material.
Responsivity
Responsivity is the ratio of the optical detector's output photocurrent in amperes
to the incident optical power in watts. The responsivity of a detector is a function of
the wavelength of the incident light and the efficiency of the device in responding to
that wavelength. For a particular material, only photons of certain wavelengths will
generate a photocurrent when they are absorbed. Additionally, the detector material
absorbs some wavelengths better than others. These two properties cause the wavelength
dependence in the detector responsivity. Responsivity is a useful parameter for
characterizing detector performance because it relates the photocurrent generated to the
incident optical power.
Q.8 What are the four most common materials used in semiconductor detector fabrication?
Q.9 What is a photocurrent?
Q.10 Define responsivity.
PIN PHOTODIODES
A PIN photodiode is a semiconductor positive-negative (p-n) structure with an
intrinsic region sandwiched between the other two regions (see figure 7-2). It is normally
operated by applying a reverse-bias voltage. The magnitude of the reverse-bias voltage
depends on the photodiode application, but typically is less than a few volts. When no
light is incident on the photodiode, a current is still produced. This current is called
the dark current.
The dark current is the leakage current that flows when a reverse bias is applied and
no light is incident on the photodiode. Dark current is dependent on temperature. While
dark current may initially be low, it will increase as the device temperature increases.
Figure 7-2. - The basic structure of a PIN photodiode.
Q.11 How are PIN photodiodes usually biased?
Q.12 What is the dark current?
Q.13 Will dark current increase or decrease as the temperature of the photodiode
increases?
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