SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
4.4 Bolometric Detector Arrays
4.4.1 Principle of semiconductor bolometers
The SPIRE detectors are semiconductor bolometers. The basic principles of operation are illustrated in
Figure 4-16.
Radiant power Q
Thermal
conductance
G
Absorber
Temperature T
Heat capacity C
Heat sink: Temperature To
Figure 4-16 Principles of bolometer operation.
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Bias current, I
Thermometer
The bolometer comprises a low heat capacity absorber designed to absorb the incident submillimetre
radiation. The absorber is coupled to a heat sink at a fixed temperature To by a thermal conductance, G.
Absorbed radiant power, Q, is thermalised in the absorber resulting in an increase in temperature over the
equilibrium value in the absence of illumination. A semiconductor thermometer is attached to the absorber.
A bias current, I, is passed throughout the thermometer, and the corresponding voltage across the
thermometer is measured. The bias current dissipates electrical power P = I 2 R, which heats the bolometer to
an operating temperature T, slightly higher than To. The main performance parameters for bolometric
detectors are:
Responsivity: S = dV/dQ
Noise Equivalent Power NEP: = a(4kTo 2 G) 1/2 where k is Boltzmann's constant and a
is a constant of order 1.
Time constant: τ ~ C/G where C is the heat capacity of the
bolometer.
For a full account of the theory and practice of semiconductor bolometers, see, for example, Mather (1982),
Griffin & Holland (1988), Richards (1998).
4.4.2 SPIRE bolometer performance requirements
The combination of good sensitivity and speed of response requires low-temperature operation. The ultimate
limit to the sensitivity of the SPIRE instrument is determined by the thermal background power from the
telescope. The background power incident on a SPIRE detector is typically a few pW , almost all of which is