Background Ionization
In most situations, background ionization occurs at a low level due to natural terrestrial radioactivity and cosmic ray, with a rate of about 10 electron-ion pairs per cubic centimeter per second (
Ref. 7
,
Ref. 8
).
From the physics standpoint, this background ionization serves as a crucial source of seed electrons. In numerical simulations that use a logarithmic formulation for charge carriers, it also contributes to improved numerical stability. However, since the background ionization rate is several orders of magnitude lower than that of dominant source terms — such as impact ionization — it has a negligible effect in models where those stronger processes prevail.
For the above reasons, background ionization is included by default in the charge transport models for the gas, liquid, and solid domains as described earlier.