Introduction to radiation-resistant semiconductor devices and circuits

have been obtained on fully irradiated strip detectors read out by LHC compatible
electronics. (21)
The basic detector is insensitive to ionization effects. In the bulk, ionizing
radiation creates electrons and holes that are swept from the sensitive volume;
charge can flow freely through the external circuitry to restore equilibrium. The
problem lies in the peripheral structures, the oxide layers that are essential to controlling
leakage paths at the edge of the diode and to preserving inter-electrode
isolation in segmented detectors.
The positive space charge due to hole trapping in the oxide and at the interface
(see Fig. 2) attracts electrons in the silicon bulk to the interface. These accumulation
layers can exhibit high local electron densities and form conducting channels,
for example between the detector electrodes. This is especially critical at the
“ohmic” electrodes in double-sided detectors, where the absence of pn junctions
makes operation rely on full depletion of the silicon surface (even without radiation,
the silicon surface tends to be n-type, so the ohmic side of n type detectors is
inherently more difficult to control). (22,23)
Some detectors include integrated coupling capacitors and biasing networks.
Biasing structures such as punch-through resistors and MOSFET structures are
subject to ionization damage. Although these devices can remain functional, substantial
changes in voltage drop have been reported for punch-through and accumulation
layer devices, whereas measurements on polysilicon resistors irradiated to
4 Mrad (65 MeV p) show no effect. (24)
Radiation Damage in Transistors and Integrated Circuits
In principle, the same phenomena discussed for detectors also occur in transistors,
except that the geometries of transistors are much smaller (depletion widths
10 15 cm -3 ).
Bipolar Transistors
The most important damage mechanism in bipolar transistors is the degradation
of DC current gain at low currents. The damage mechanism is the same that causes
increased leakage current in detectors, formation of mid-gap states by displacement
damage. The difference is that the base-emitter junction is forward biased, so
the high carrier concentration in the conduction band tips the balance from generation
to recombination (see Fig. 1). The fractional carrier loss depends on the
relative concentrations of injected carriers and defects. Consequently, the
reduction of DC current gain due to radiation damage depends on current density.
For a given collector current a small device will suffer less degradation in DC
current gain than a large one.
12