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Unformatted text preview: l Fermc/eclrics. 1981. Vol. 33. pp. l937206
OOl5<0193/8l/3301—Ol93/806.50/0 "“ 198] Gordon and Breach. Science Publishers, Inc.
Primed in the United States of America A BRIEF GUIDE TO PYROELECTRIC DETECTORS S. G. PORTER l’lessey ()ptoe/ecrronics & Microwave Ltd. Towcester. Nari/lama. NN/2 7JN Eng/turd (Received Auguxl 4. I‘lt‘tt); in final/mm November IU, Islam [he principles of operation of a p)roelectric detector are summaii/ed and responsiut}. none. noise equivalent power
and detectivity are derived. The relationship between responsivily and frequency is discussed as illC the various sources of noise. There lollous .1 discussion ol the relati\e merits of the me principal pxroelectiic materials in common use. Trigly cine Sulphate. lithium lantalate. Strontium Barium Niobate. Pyriielectric (feramics and Pol_\\in_\lidene Fluoride l‘llL‘e tors infliiencmg the choice of material are outlined and summari/ed in performance ctll'\cs loi three si/es of detector. Mention is made ol a variety of detector types currently available and some of the current applications. A \cry brief summary of possible future developments is included. 1. THE PYROELECTRIC EFFECT A pyroelectric material is one which possesses an
inherent electrical polarization. the magnitude of
which is a function of temperature)“ Most p} roe—
lcctrics are also ferroelcctric. which means that
the direction of their polarization can be reversed
by the application of a suitable electric field. and
their polarization reduces to zero at some temper
ature known as the Curie temperature. T“ by
analogy with ferromagnetism. ‘ The dependence of
polari7ati0n on temperature is typically of the
form illustrated in Figure 1, The gradient of this
curve. (JP/(1T, at a particular temperature. T. is
the pyroelectric coefficient. which will be denoted
by p. The strange effects produced when the mineral
tourmaline is heated have been known for man) Polarization (Pl Temperature (T) Tc
HG L~ RE I
electric. Temperature dependence of polarization of lerroi hundreds of years. and the pyroelecti'ic properties
of Rochelle salt were studied early in the nine—
teenth century.N but it is only in the last tV'cnty or
thirty years that pyroelectric infrared detectors
have been developed The importance of the pyroelectric effect in infra
red detcction was becoming obvious about ten
years ago.“ and a widely acclaimed review of pyro—
electric detectors \\ as published in 1970 by l’utleyf
At this time the only p) roelectric material of sig
nificant value was triglycine sulphate. A considerable amount of research and devel
opment has been devoted to p_\ roelectric detectors
during recent years. covering all aspects of mate
rials. device fabrication. and applications. Putley
therefore published an update to his review article
in 1977.X 2. THE PYROELECTRIC DETECTOR A simple pyroelectric detector consists of a slice
of p_\roelectric material with metal electrodes on
opposite faces. the material being oriented such
that its polar axis is perpendicular to the elec
troded faces. Generally a ferroelcctric consists of
a large number of separate domains with differing
directions of polarization. so that the net effect
over the whole slice is zero. Before Lise. therefore.
these domains must be reoriented by the appliczr
tion of an electric field so that all become parallel
to one another for as near parallel as possible. in
the case of a ceramic). This is usually done at an ”Lunamu m“. mum.“ m... ma... NW .1... t. 194 S (j. elevated temperature so that the coercive field is
reduced. Even across a correctly poled detector. there
will generally be no observable voltage. This is
because its internal polarization is balanced by a
surface charge which accumulates via various
leakage paths between the two faces. For this rea<
son. the pyroelectric detector can only be used in
an ac. mode and at a frequency high enough for
this electrical leakage to be ineffective. In other
words. the pyroeleetrie detector can only be used
to detect changes in irradiance. When the detector is heated by incident radia—
tion. the polarization changes by an amount deter—
mined by the temperature change and the pyro—
electric coefficient of the material. This change in
polarization appears as a charge on the capacitor
formed by the pyroelectric with its two electrodes.
Typically this charge is of the order of 101" cou—
lombs on a capacitance of the order of ltlpF, 3. AMPLIFIERS In order to detect these very small charges low
noise high impedance amplifiers are necessary.
The most common arrangement is a simple JITET
source follower. as shown in Figure 2(a). If the bias resistor. R, is sufficiently high, then
the output voltage. V“. corresponds to the pyro
electric charge on the capacitance of the detector.
For this to be true at low frequencies ( ~ 1 Hz)
the value of R must be at least 10” ohms. If, however. R is small. then V” is proportional
to the current generated by the detector and flow
ing through R. This is true for modulation fre»
quencies well below the inverse of the electrical
time constant. RC. An alternative arrangement
for monitoring the detector current is shown in +VS Pyroelectric C R RL —oOV PORTER Figure 2(b). Here the signal current ﬂows through
the amplifier feedback resistor, RF. which can now
be much larger since the time constant is now
RFC divided by the open loop gain of the
amplifier. It is generally more difficult to produce current
amplifiers which give a signal to noise ratio as
good as the stmple voltage amplifier of Figure 2(a).
Stray capacitances across the feedback resistor.
Rp. also present problems in obtaining the desired
frequency response. 4. RESPONSIVITY When a pyroelectrie detector is exposed to radia~
tion which is modulated at an angular frequency
(L). the temperature of the detector \till be modu
lated at this frequency by an amount which de—
pends on the fraction of the incident radiation ab—
sorbed. r] (the emissivity of the surface). and the
heat capacity. H. of the detector. This tempera~
ture modulation will also depend on the thermal
conductance Gr. coupling the detector to its eni
capsulation. which may be considered as a heat
sink at constant temperature. The temperature difference. 6. between the de
tector and the heat sink is related to the incident
radiant power. W. by the equation: . dB ‘
ﬂatly—+070 (l)
d! If the incident radiation can be expressed in
the form W: War/W". then equation ll) has the
solution: H"
9 —‘ nh‘eW l3)
(1 1+ _/(u//
RF
0V0
Pyroelectric C
00v
(bl FIGURE 3 Altername amplifier arrangements l
l
l lows through
hich can now
stant is now
gain of the )duce current
oise ratio as
If Figure 2(a),
rack resistor.
ig the desired sed to radia
ar frequency
ill be modu
it which de
‘adiation ab
ice). and the
his tempera—
the thermal
or to its en
:d as a heat veen the de—
the incident (1) xpressed in
(I) has the GlllDt l‘O PYROLIECIRK~ DETICTORS I95 4. 1. Current i'esponsii'i'ly
The pyroelectric charge generated. q. is given by:
‘1 : FAQ (3) where p is the p_\‘roelectrie coefficient and A is the
electrode area of the detector.
The current responsivity is defined as: i
6i, =l~ (4)
 W
dt]
where i : —
(1!
Thus
)A
a, : —L—r (Si
G,“ l" (u‘ri) ”
II . .
where 71': T is the thermal time constant.
If The current responsivity as a function of fre—
quenc} is thus of the form shown in Figure 3. At frequencies which are high compared with
'/r;. equation 5 may be reduced to: E R. =
‘ .rd (6) where 5 is the heat capacity per unit volume (vol—
uriie specific heat) and d is the thickness of the
detector. 4.2. l”'0/tage respmisiw'ly If the detector is connected to a high impedance
amplifier. such as that shown in Figure 2(a). then
the observed signal is equal to the voltage pro— IOQ R1 >
log to FIGURE 3 Frequenc} dependence of current responsiui) duced by the charge. q. The detector may be rep
resented as a capacitor. a current generator. and a
shunt conductance, as shown in Figure 4. The voltage generated is therefore giVen b}:
i
6,; +ij and the voltage responsivit)’ is defined as: v
R : —— (N)
W
giving:
Au)
82. .7”... i9>
(1105(1 r air?) ‘(1 r (tr—72;) ‘
where TE : T is the electrical time constant.
It Again. this simpliﬁes at frequencies which are
high compared with '/n and I/n— to give: 8. =i— (in) setter/l (u where a. is the permittivity of free space and e, is
the relative permittivity of the pyroelectricma
terial. Equation 10 shows that. at high frequencies. the
voltage responsivit) of a pyroelectric detector is
inversely proportional to frequency. At low fre
quencies this is modified by the electrical and
thermal time constants. as in equation 9. so that
the true frequency response is of the form shown
in Figure 5. Typically 77> is within the range 0.01 seconds to
l() seconds. n. howex'er. can be anywhere between
ll)”: seconds and 100 seconds. depending on the
sizes of the detector capacitance and the shunt
resistor. 1_
GE .l.
l o<—<——>o l'lGURlE 4 Equivalent circuit of simple detector. 196 S. G log RV I
I
I
I
l

I
I
I
I _1_ ,l w
wTT wTE '09 FlGllRli 5 Frequenc} dependence of voltage responsiwty As mentioned in Section 3. at frequencies below
r, the output voltage is that produced by the p)—
roelectric current ﬂowmg through the resistor R.
This is equivalent to using a current amplifier. the
rcsponsivitv being equal to R8,. In practical detectors there is usually an ampli—
fier whose input impedance must be included in
Figure 4. [n the case of a JFET this impedance
may be considered as a capacitance. C4. with a
parallel resistance. R1,}. In practice Rs is large
compared with the shunt resistor, R. and can be
ignored. but (.1 is not always small compared
with the detector capacitance C and 7,; must be
written as: ri:——’ (11) More rigorous analvses of pvroelectric detectors
 '9 14 ' 
have been performed. taking into account the
effects of mounting techniques and black coat—
ings. The above treatment. however. is adequate
for the majority of applications. 5. NOISE The uselulncss of a detector is usuall} assessed in
terms of the minimtim detectable incident power.
This is a function of both the responsivity and the
noise generated in the detector and its amplifier.
Therefore an analysis of home sources is necessary
fora full understanding of pyroelectric detectors. There are three major noise sources in a simple
p_\'roelectric detector mm a shunt resistori as
shown In Figure (J. The following discussion re—
lates to noise in unit bandwidth. 5.]. Thermal Noise Thermal noise. AW]. in the detector occurs ac— 1» PORTER l.
l 0 HOUR}? (x Noise equIvalent circuit for simple detector \suli
shunt resistor g  l
cording to the relation: _\W, 2 onion“ I12I
Which gives a noise current;
8.4kT1(‘ 13
1., : ( Jr) (13.)
77 5. 2 Dielectric Nurse The pyroclcctric detector is a capacitance C with a dielectric loss tan 5 giving an eqtiIvalent conduc—
. I . . tance wC tan 6. " The norse voltage generated by this conductance is given by the standard expres sion for Johnson noise in a resistor:I I J
4kT l'[I : %_ (l4l (uC tan 6 giving an equivalent noise current of: In : (4szuC tan aI‘ 3 (15) 5.3 Resistor Avise In similar manner. the shunt resistor. R. gives a
current noise: ,‘R, g (1m 7 4k!‘ '1
i R 5.4 Aliip/[ﬁer Noise The noise produced h) an amplifier. such as an
FET. may he represented by two noise generators
at the amplifier input: c.,. the equivalent input volt—
age noise. and i... the equivalent input current
noised‘ HIT r H
H ill b
it Ln (TC—l ...
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