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Incident
radiation JFET +V
Transparent
electrode
R L Output
R
signal
Pyroelectric
material
FIGURE 19.102 The basic components within a pyroelectric detector are indicated. An increase in radiation falling
on the pyroelectric material causes its temperature to rise and the charge on its surface to change. A transient current
11
flows through the resistor R L which is of the order of 10 Ω. The JFET reduces the output impedance to R.

output current or output voltage, in the case of photodiodes and transistors. All these photon detectors
require a minimum photon energy to create mobile electrons and consequently have a maximum wave-
length, dependent on the detector material, beyond which they do not operate. On the other hand,
photon detectors generally respond faster to changes in radiation level than thermal detectors and are
more sensitive.
Pyroelectric Detectors
Pyroelectric detectors employ a ferroelectric ceramic material (such as lead zirconate or lithium tantalate)
which has molecules with a permanent electric dipole moment [4]. Below a critical temperature, known
as the Curie temperature, the dipoles are partly aligned and give rise to a net electrical polarization for
the whole crystal. As the material is heated and its temperature rises, increased thermal agitation of the
molecules reduces the net polarization, which falls to zero at the Curie temperature. The basic detector,
shown in Fig. 19.102, consists of a thin slab of ferroelectric material fabricated so that the polarization
is normal to the large area faces on which transparent electrodes are evaporated. These are connected
11
together via a load resistor (up to 10 Ω). An increase in radiation falling on the detector makes its
temperature rise and causes the captive surface charge, which is proportional to the polarization, to
change. This causes a change in the charge induced in the electrodes and a current to flow in the load
resistance. Because of the large value of the load resistor used in pyroelectric detectors, an impedance
matching circuit, such as a JFET source following circuit, is usually built into the detector as shown in
Fig. 19.102. Pyroelectric detectors only respond to changing irradiation and typically can detect radiation
−8
powers down to about 10 W at 1 Hz. Because they respond to the heating caused by absorption of the
radiation, they have a wide spectral response. They are useful as low-cost infrared detectors, intruder
alarms, and fire detectors.
Photon Detectors
The most widely used photon detectors are made from a semiconducting material. In semiconductors,
the electrons fill the available energy levels in the material up to the top of the valence band (VB), which
is separated from the bottom of the empty conduction band (CB) by an energy gap E g , which is
characteristic of the material. These energy bands are completely full or empty, respectively, only at a
temperature of absolute zero (0 K). At a higher temperature, an equilibrium is reached between the
thermal excitation of electrons across the gap (producing free electrons in the CB and positively charged
free holes in the VB) and the recombination of pairs of free electrons and holes. The equilibrium number
of free electrons and holes increases rapidly with temperature (T) according to the Boltzmann factor
−23
exp(−E g /kT), where k is Boltzmann’s constant (1.38 × 10 J/K). This equilibrium is disturbed when
photons, with energy greater than E g , are absorbed by electrons which are excited across the gap. When
the radiation source is removed, the number of excess electrons and holes quickly falls back to zero over
a time period governed by the recombination time of the material. While excess free charge is present
there is a measurable change in the electrical conductivity and this is used in photoresistive (also called
photoconductive) detectors. Alternatively, in junction detectors, the rate of generation of photocharge is
converted to an output current, or voltage. All semiconductor photon detectors have a relatively narrow

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