In the first part of this blog, we started looking at the mechanism by which high energy particles (alpha and beta particles, ions) and photons (typically γ rays) could affect or damage semiconductor devices.
To understand what kind of damages these sources can do in the silicon structures, let's look at the different mechanisms the radiation can do inside the atom.
The mechanism of energy deposition for beta particles — fast-moving free electrons — is related to the electrical interaction of the electron that crosses the silicon material and the silicon electron cloud (see Figure 1):
The mechanism of energy deposition for protons and heavy ions is quite the same: The radiation source (a proton or a heavy ion) interacts electrically with the electrons of the silicon electron cloud (see Figure 2):
Each radiation source has a particular mechanism of energy deposition, but the final effect is the same: the creation of couples e− /p+ . The mechanism inside the material is the same for all types of radiation sources. It is important to understand the correct testing procedure of the device after it has been exposed to the ionizing radiations. You need to understand how the device is impacted by the creation of these e− /p+ couples.
To understand the effect on an IC, let's examine the effect of the radiation on a power MOSFET switch which can be considered an elementary component of an IC. Due to ionizing radiations, e− /p+ pairs are created inside the gate oxide (SiO2 ) and the electrical field moves the more mobile charges to the gate terminal. Meanwhile, the holes (shown as h+ in Figure 3) remain inside the oxide. Some of the electrons that flow from the source to the drain will recombine with the holes inside the gate oxide, resulting in some holes disappearing from the oxide.
Here are the parameters to consider with regard to irradiation:
VGS = V-thbefore_ irradiation (1) condition: irradiation performed, switch MOSFET state OFF
There is no flow of electrons from source to drain, so the only recombination is related to the electrons present at the source and the holes in the oxide. After that the irradiation of the device has been performed, resulting in:
VGS > Vthafter_ irradiation (2) condition: irradiation performed, switch MOSFET state ON)
Now, the electrons flow from the source to the drain and some electrons will recombine with the holes that are still trapped into the gate oxide and survived to the recombination with regard to condition (1); again, refer to Figure 3. An electron coming from the source and having enough energy will reach the drain, provided it does not recombine with a hole. This results in a new threshold value of the gate-source voltage. To monitor the effect of the radiation on the switch, a threshold voltage test is recommended.
In the next (and last) part of this series, we'll look at the details of this gate threshold voltage change. We will also look at the performance of bipolar junction transistors with respect to irradiation.