At the time of the ESA Call for ``Proposals for ISO instruments",
there was no array available in Europe for the long wavelength
channel. A specific development was undertaken at the Laboratoire
d'imagerie Infrarouge du CEA-LETI in Grenoble. It is a 32 x 32
Gallium doped Silicon photoconductor array hybridized by Indium
bumps to a direct voltage readout circuit. The pixel pitch is 100
m and the detectors are 500 m thick. A 25 V bias
voltage is applied to the photoconductor, providing the optimum
trade off between responsivity and stabilization overhead. With
this bias voltage, the responsivity at 15 m is 10
V/W, which corresponds to a photogain 
quantum
efficiency product of 
g
1. In first approximation, at
shorter wavelength, the responsivity (in Volt/Watt) decreases as
, and there is a long wavelength cut-off at 18 m.
To obtain a 100% filling factor, the front surface is doped to ensure
a good electrical surface conductivity and the bias voltage is applied
to an Aluminium frame on the side of the optical sensitive area. An
external guard, 3 pixels wide, has been added around the 32 x 32
sensitive pixel frame to prevent field line distortion in the detectors
at the edges. The optical crosstalk is very low, and remains below 1.5
%, even for the 12 arcesc PFOV which has the fastest lens of the
camera (numerical aperture f/1).