ELECTRONICS UNDER RADIATION
Radiation! Everyone knows the effects of radiation on human health. But is living tissue the only thing it harms?
No. Ionising radiation can weaken materials, embrittle them or cause electric breakdown. Semiconductors are particularly affected by such radiations. There are two incidents. What will happen if you use a semiconductor device in a radiation environment for a long period of time? What will happen if the device experiences a short burst of radiation energy?
In the first case, the device characteristics deteriorate. There is an increase in leakage current, change in the threshold and many such long-term effects. In the second, there may be bitflips in memory or transient pulses in logic circuitry. The first effect is termed as Total Ionisation Dose (TID) effect. The second, Single Event Effects (SEE). As these names suggest, TID is due to prolonged exposure and SEE, the result of a sudden, single energetic particle.
So, what really happens?
At the core, both effects occur because of generation of free charges. The high-energy particles hit the device and liberate additional free charges. In TID, a fraction of the generated holes get trapped in the oxide regions of the device. This happens because, electrons having a high mobility are quickly swept away leaving holes behind. This situation is somewhat like a place with a skewed sex ratio resulting in not enough partners for marriage. These holes that are left behind, affect the electrical characteristics of the device.
Ninad Chitnis T.E ETRX
A major factor affecting the degradation is of course, the total dose of radiation received. The popularly used unit for measuring it is rad where 100 rad = 1 J/kg. Another important factor is the dose rate, measured in rad/sec. Higher the dose rate, greater will be the degradation. Then, there are factors like the geometry of the device, method of fabrication, its bias conditions and temperature among others. TID however, affects at the device level. It will cause degradation in the individual device parameters.