Biomedical Engineering Reference
In-Depth Information
6.2
Chapter 6.2
Optical sensors
Reinaldo Perez
There are basically two technologies that are widely used
in optical sensors, charge-coupled devices (CCDs) and
fiber optics. Both are major contributors to the de-
velopment of advanced diagnostics and therapeutic
techniques. In this chapter, we first address CCD and
then fiber optics.
repel. This region, which is devoid of positive charges, is
the depletion region. A photon with an energy greater
than the energy gap is absorbed in the depletion region
producing an electron-hole pair as shown in
Figure 6.2-1
.
The electron stays within the depletion region,
whereas the hole moves to the ground electrode. The
number of electrons collected is proportional to the ap-
plied voltage, oxide thickness, and gate electrode area.
The CCD register consists of a series of gates. The
manipulation of the gate voltage in a systematic and se-
quential manner transfers the electrons from one gate
to the next in a conveyor-belt-like fashion. For charge
transfer, the depletion region must overlap. The de-
pletion regions are gradients, and the gradients must
overlap for charge transfer to occur.
Initially, a voltage is applied to gate 1 and photode-
tectors are collected in well 1 (
Figure 6.2-2
). When
a voltage is applied to gate 2, electrons move to well 2 in
a waterfall manner (c).
The process is rapid and then the charge quickly
equilibrates in two wells (d). As the voltage is reduced on
gate 1, the well potential decreases and electrons again
flow in a waterfall manner into well 2 (e). The process is
repeated many times until the charge is transferred
through the shift register.
The CCD array is a series of column registers (
Figure
6.2-3
). The charge is kept within rows or columns by
channel stops and the depletion regions overlap in one
direction only. At the end of each column is a horizontal
register of pixels. The register accumulates a line at
a time, and later it transports the charge packets in
a serial mode to an output amplifier. The horizontal serial
register must be clocked out to be a sense node before
the next line enters the serial register. Therefore, sepa-
rate vertical and horizontal clocks are required for all
6.2.1 Charge-coupled devices
The CCD refers to a semiconductor architecture in
which a charge is transferred through storage areas. The
CCD architecture has three basis steps: (1) charge col-
lection, (2) charge transfer, and (3) the conversion of
charge into a measured voltage. The basic building block
of the CCD is the metal semiconductor capacitor (MIS),
also known as the gate. The most common MIS is the
metal oxide semiconductor.
Charge generation is often considered as the initial
goal of CCD. Silicon can create an electron-hole pair for
each absorbed photon. The electrons and holes can be
stored or transformed; charge generation occurs under an
MOS capacitor. The charge created at a pixel site is
proportional to the incident light. The aggregate effect of
all the pixels is to produce a spatially sampled repre-
sentation of the continuous picture. Pixel readout occurs
by sensing the charge transfer between the capacitor at
the pixel site.
When an absorbed photon creates an electron-hole
pair, photodetection has occurred. The generated car-
riers must be stored at a site. The absorption coefficient
is wavelength specific and decreases with increasing
wavelength. Applying a positive voltage to the CCD
causes the mobile positive holes in
p-
type silicon to mi-
grate toward the ground electrode because like charges
