positive bias, however, the electrons are swept away while the holes migrate slowly to the negative channel, and become trapped. This causes N-channel enhancement transistors to become easier to turn on, while P-channel transistors are harder to turn on adversely impacting the ROIC performance.
There are four basic categories of radiation vulnerabilities for an integrated circuit. These four effects are Neutron, Total Ionizing Dose (TID), Transient Dose, and Single Event Effect (SEE).
Neutron Effects: When neutrons strike a semiconductor chip, they displace atoms within the crystal lattice structure. The minority carrier lifetime is reduced because of the increased density of recombination centers. Silicon devices begin exhibiting changes in their electrical characteristics at levels of 1×1010 to 1×1011 neutrons/cm2. Because bipolar components are minority carrier type devices, neutron radiation affects them at lower doses than for MOS devices. In bipolar integrated circuits, the base transit time and width are the main physical parameters affected. Neutron radiation significantly reduces gain in bipolar devices. MOS devices aren’t normally affected until levels of 1×1015 neutrons/cm2 are reached.
Total Ionizing Dose Effects: Total ionizing dose is the accumulation of ionizing radiation over time, typically measured in rads. Slow, steady accumulation of ionization over the life of an integrated circuit causes performance parameters to degrade. Eventually, the device fails. The total dose creates a number of electron-hole pairs in the silicon dioxide layers of MOS devices. As these recombine, they create photocurrents and changes in the threshold voltage that make n-channel devices easier to turn on and p-channel devices more difficult to turn on. Even though some recovery and self-healing takes place in the device, the change is essentially permanent. Some holes created during ionizing pulses are trapped at defect centers near the silicon/silicon oxide interface. Charges induced in the device create a field across the gate oxide sufficiently high to cause the gate oxide to fail, or sufficient carriers are generated in the gate oxide itself to cause failure.
Transient Dose Effects: A transient dose is a high-level pulse of radiation, typical in a nuclear burst, which generates photocurrents in all semiconductor regions. This pulse creates sudden, immediate effects such as changes in logic states, corruption of a memory cell’s content, or circuit ringing. If the pulse is large enough, permanent damage may occur. Transient doses can also cause junction breakdown or trigger latch-up, destroying the device.
Single Event Effect (SEE): Single event effects typically only affect digital devices significantly. A SEE occurs when a single high-energy particle strikes a device, leaving behind an ionized track. The ionization along the path of the impinging particle collects at a circuit node. If the charge is high