Biomedical Engineering Reference
In-Depth Information
1.2.5.1
Single-Channel Detectors
PIN photodiode, avalanche photodiode (APD), and PMT are the most common
single-channel detectors. Most PIN photodiodes are made from silicon or InGaAs.
Silicon PIN has a response curve between 200 and 1,100 nm, peaked around 900 nm.
InGaAs PIN has a response curve between 800 and 1,700 nm, peaked around
1,500 nm. PIN photodiode is very compact and can be made into photodiode arrays
(PDAs). The main advantages of photodiode are that there is no need of high voltage
and the signal is linearly dependent on the light input. However, because the overall
sensitivity is low and the photoactive area is small, PIN photodiode is often used in
telecommunications and light regulation applications, i.e., monitoring the excitation
intensity in fluorescence spectroscopy measurement.
APD has higher sensitivity than PIN detectors due to a built-in first stage of gain
through avalanche multiplication. However, it requires high reverse bias voltage.
The optimal gain for APD ranges from 40-100 for silicon APDs to 10-40 for germa-
nium or InGaAs-made APDs. Selection of APDs depends on the wavelength range,
the minimum size of the detecting area, and the electrical frequency bandwidth. It
can operate at the linear or breakdown mode. Because it has high sensitivity and fast
response time, APD can be used in low-level signal detection, such as fluorescence
spectroscopy. It has higher quantum efficiencies than PMTs at wavelengths greater
than 400 nm. Now, APD is widely used in laser scanning confocal microscopy. A
summary of APD types and their properties is listed in Table 1.2 .
PMT is a vacuum tube consists of a photoelectron converter (photocathode)
followed by an electron multiplier and an electron collector (anode). Light enters
the input window and excites photoelectrons in the photocathode. Photoelectrons
are accelerated and focused onto the first dynode where they are multiplied
by secondary electron emission. This process is repeated until the multiplied
photoelectrons hit the last dynode and collected by the anode. Because PMT has
high bandwidth and high gain, it is ideal for the detection of extremely low light or
short pulses of light. Most photon counting applications are based on PMTs. Choice
of PMT is based on the wavelength, intensity, beam size, and detection speed.
1.2.5.2
Multichannel Detectors
Silicon-based CCD has become a popular choice over single-channel PMTs and
PDAs because of its high quantum yield, low dark signal, and multichannel
capability. Three types of CCD technology are widely used in scientific imaging and
spectroscopy, including front-illuminated CCD, back-illuminated CCD, and back-
illuminated deep-depletion CCD [ 23 ]. A cross section and the quantum efficiency of
these three types of CCDs are shown in Fig. 1.8 . The CCD detectors are composed
of a polysilicon electrode, a silicon dioxide layer, a depletion layer (epitaxial
silicon), and a substrate layer (bulk silicon). A summary of the CCD types and their
applications is listed in Table 1.3 . For front-illuminated CCDs, the light enters the
detector from the front and is absorbed in the depletion layer after passing through
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