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 an 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.
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| Change in device characteristics |
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 gets 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.
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.
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| Trapped charges, red indicating more charge |
SEE, on the other hand, affects at the circuit level. In memory cells, it may cause a bit to flip. In digital circuits, it may cause a pulse to propagate through the circuit. These, however, are not permanently damaging. Strong bursts of energetic particles can cause severe effects like generating shorts in the circuit (called latch-ups) or damage the gate oxide.
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| Single Event Effects |
But where will devices experience radiation?
It’s not as if we expose our phones to high energy radiations on a daily basis. The circuits that are actually exposed to such high doses are those meant for special purpose applications. Beyond the atmosphere, there’s always an incoming barrage of high energy particles of all sorts. So, all devices meant for space applications are at risk. Further, circuits used in high energy physics experiments such as particle accelerators also are under threat from radiation.
Is there any solution?
Yes. The process is called radiation hardening. Literally, it means making the devices ‘hard’ or resistant to radiation. One of the methods is to use Silicon-On-Insulator technology. But that brings with it, its own set of problems. Shielding is a good option. There are also various fabrication methods and layouts on the chip which are used and which give a better performance when attacked by radiation.
What if the device is damaged?
TID can be mitigated by annealing at a specific temperature. This causes the traps to escape as they gain energy from the high temperature. As for SEE, an entire reboot of the system might be helpful. But imagine the losses if an entire system on a space station needs to be rebooted!
So, the best we can do is use proper radiation hardening techniques and avoid radiation related side-effects. But of course, we can never be sure!
