Introduction to radiation-resistant semiconductor devices and circuits

FIGURE 8. Detector preamplifier illustrating the use of current-mirror biasing to
maintain the operating current of the input transistor independent of MOSFET
threshold shifts during irradiation
or rendered irrelevant by AC coupling. The operating voltage and the gate voltages
of the cascode transistors Q2 and Q3 must be chosen somewhat higher than
for unirradiated operation to accommodate the threshold shifts, so overall power
dissipation will be somewhat higher. Techniques of this type can provide radiationresistant
amplifiers with radiation-soft transistors.
In general, the use of fully differential circuitry and current mirrors yields
circuitry whose operating point relies primarily on relative device matching.
(40,26,28) Changes in threshold voltages or current gain in adjacent devices tend
to track after radiation damage, so the circuit will maintain its operating point.
Circuitry should also be designed to minimized single-point failure modes. Failure
of common bias networks will cause all associated circuitry to fail. Local biasing
with highly parallel architectures reduces these problems These design principles
have been applied to 64 and 128 channel bipolar transistor integrated circuits for
the readout of strip detectors. (40,26,29) Fig. 9 shows a block diagram.
This system only records the presence of a detector signal, so each channel
comprises an amplifier, pulse shaper and threshold comparator. The input transistor
is biased through a current mirror, as just described. The gain stages must
provide sufficient gain so that the threshold voltage at the comparator is sufficiently
high to provide good channel-to-channel and chip-to-chip uniformity. the.
Since the first three stages are single-ended to reduce power consumption,
24