Under the influence of the high electric field present in short-channel
MOSFETs, a fraction of the carriers gains enough energy to surmount
the energy gap or the Si/SiO
interface barrier generating an
electron-hole pair in a process referred to as impact ionization. The
generated carriers flow as gate and substrate currents. These
currents are the main cause of device damage and performance degradation.
Thus a good indicator of MOSFET device reliability is the magnitude of these
currents. Due to difficulties in measuring gate currents, it is common
practice to use the substrate current (
) as the sole indicator.
As is reduced,
the device lifetime is expected to increase. It is therefore important to
be able to accurately predict as a means to evaluate different
design alternatives.
Both theoretical and experimental investigations on modeling the impact
ionization rate have resulted in non-unique modeling equations and
parameter values [91]. A semi-empirical model that takes into
account the depth dependence of the critical electrical field
has been shown capable of achieving good agreement with experimental
data [90]. It is described in the next section.