Well, I do not know exactly which point defects you are aiming at.
If you plot a potential energy that is the sum of an attractive coulomb potential and a uniform electric field , for symplicity in one dimension, you have qualitatively the function -1/|x| + E x, and take E>0 . Then, this function goes to + infinity for x going to + infinity, and to - infinity for x going to -infinity. For x < 0, you see some maximum of the function (it is negative there).
If you imagine switching on the electric field E, and an electron, say in a bound state for E=0, it will not be in a bound state for E>0: There is a finite, nonzero probability that it tunnels through the potential wall, even if it deeply bound in comparison to the aforementioned maximum for some nonzero E. Thus, a bound state becomes a quasibound state or a resonance for nonzero E. In the onedimensional case, you can calculate the tunneling probability to a good approximation by a one-dimensional integral. Just remember, how you would calculate the tunneling probability through a rectangular barrier, and approximate the potential wall as a sum of rectangular barriers.
If you consider the same problem in three dimensions with an electric field in z direction, you have to consider essentially -1/|r| + E z. Again you will find a potential wall that now, however, is a bit more complicated to visualize. And the tunneling probability can be calculated by a somewhat more complicated integral.
This is, how you can in principle obtain the ionization probabilty, that is nothing else than the tunneling probability.
You should be able to find such expressions if you search for field emission, and the author name Kingham.
Good luck
Herbert
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