Effects of Radiation on Semiconductors

There are four basic categories of radiation effects that an integrated circuit is vulnerable to. These four effects are Neutron, Total Ionizing Dose, Transient Dose, and Single Event Effect (SEE).

1. 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 recombination centers created. Silicon devices begin exhibiting changes in their electrical characteristics at levels of 1x10¹⁰ to 1x10¹¹ neutrons/cm². Because bipolar components are minority carrier type devices, neutron radiation affects them more than MOS devices. In bipolar integrated circuits, the base transit time and width are the main physical parameters affected. Therefore, neutron radiation significantly reduces gain in bipolar devices. MOS devices aren't normally affected until levels of 1x10¹⁵ neutrons/cm² are reached.

2. 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 begin to 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.

3. 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 latchup, destroying the device.

4. Single Event Effect (SEE): Single Event Effects have been studied only recently. They typically only affect digital devices significantly, but SEE's are of primary concern in today's digital age. 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 enough, it can create a soft error Single Event Upset (SEU), such as a bit flip, a change in state that causes a momentary glitch in the device output, or corruption of the data in a storage element. A SEE can possibly trigger a device latchup and burnout. Latchup occurs when sufficient current is induced in part of the device that causes the device to latch into a fixed state regardless of circuit input. Burnout occurs when the radiation induces sufficient power dissipation to cause catastrophic device failure. Burnout often occurs as a result of latchup. SEEs can wreak havoc on sattelites, spacecraft, and aircraft as well. Therefore, circuits used in aerospace controls systems must be protected from potentially disastrous SEEs.

1. Single Event Upsets (SEU): These are also known as soft errors that occur due to either the deposition of depletion of charge by a single ion at a circuit node, causing a change of state in a memory cell. In very sensitive devices, a single ion hit can also cause multiple-bit upsets (MBUs) in adjacent memory cells. This type of event causes no permanent damage and the device can be reprogrammed for correct function after such an event has occurred.

2. Single Event Latchup (SEL): This can occur in any semiconductor device which has a parasitic n-p-n-p path. A single heavy ion or high energy proton passing through either the base emitter junction of the parasitic n-p-n transistor, or the emitter-base junction of the p-n-p transistor can initiate regenerative action. This leads to excessive power supply current and loss of device functionality. The device can burnout unless the current is limited or the power to the device is reset. SEL is the most concern in bulk CMOS devices.

3. Single Event Snapback: This is also a regenerative current mechanism similar to SEL, but a device does not need to have a p-n-p structure. It can be triggered in a n-channel MOS transistor with large currents, such as IC output driver devices, by a single event hit-induced avalanche multiplication near the drain junction of the device.

4. Single Event-Induced Burnout (SEB): This event may occur in power MOSFETs when the passage of a single heavy ion forward biases the thin body region under the source of the device. If the drain-to-source voltage of the device exceeds the local breakdown voltage of the parasitic bipolar, the device can burn out due to large currents and high local power dissipation.

Figure 2. Semiconductor before and after a Single Event-Induced Burnout.

5. Single Event Gate Rupture (SEGR): This has been observed due to heavy ion hits in power MOSFETs when a large bias is applied to the gate, leading to thermal breakdown and destruction of the gate oxide. It can also occur in nonvolatile memories such as EEPROM during write or erase operations, the time when high voltage is applied to the gate.

References: See Bibliography references 4,5 and 6.
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