  Sub-band-gap (SBG) impact ionization (II) enables steep subthreshold slope
that enables devices to overcome the thermal limit of 60mV/decade. This
phenomenon at low voltage enables various applications in logic, memory and
neuromorphic engineering. Recently, we have demonstrated sub-0.2V II in NIPIN
diode experimentally primarily based on the steady-state analysis. In this
paper, we present the detailed experimental transient behavior of SBG-II in
NIPIN. The SBG-II generated holes are stored in the p-well. First, we extract
the leakage mechanism from the p-well to show two mechanisms (i)
recombination-generation (RG) and (ii) over the barrier (OTB) where OTB
dominates when barrier height $ phy_b<0.59eV $. Second, we analytically extract
the SBG II current (Iii) at 300K from experimental results. The drain current
(Id), the electric field (E-field), and Iii are plotted in time. We observe
that Iii increase as E-field reduces which indicates that E-field does not
primarily contribute to Iii. Further, the Id shows two distinct behaviors (i)
Iii (Id) is constant at the beginning and (ii) eventually universal Iii (Id) is
linear, i.e. Iii=k X Id where $ k=10^-3$; We also show that the electrons
primarily contributing to Id are directly incapable of II due to insufficient
energy $(<Eg)$. Fischetti's model showed that SBG-II is primarily caused by hot
electrons that accept energy in an Auger-like process from cold drain electrons
to enable SBG-II. We speculate that if the Id electrons heat-up the cold drain
electrons, which would further energize the hot electrons to produce the
observed Iii (Id) universal dependence.
