Transient Phenomena in Sub-Bandgap Impact Ionization in Si n-i-p-i-n Diode

Sub-bandgap (SBG) impact ionization (II) enables steep subthreshold slope that enable devices to overcome the thermal limit of 60 mV/decade. This phenomenon at low voltage enables various applications in logic, memory, and neuromorphic engineering. Recently, we have demonstrated sub-0.2-V II in n-i-p-i-n diode experimentally primarily based on steady-state analysis. In this paper, we present the detailed experimental transient behavior of SBG-II in n-i-p-i-n. 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: 1) recombination- generation and 2) over the barrier (OTB), where OTB dominates when barrier height φ
_b
<; 0.59 eV. Second, we analytically extract the SBG-II current (I
_II
) at 300 K from the experimental results. The drain current (I
_D
), electric field (E-field), and I
_II
are plotted in time. We observe that III increase as E-field reduces which indicates that E-field does not primarily contribute to III. Furthermore, I
_D
shows two distinct behaviors: 1) I
_II
(I
_D
) is constant at the beginning and 2) eventually “universal” I
_II
(I
_D
) is linear, i.e., I
_II
= k x I
_D
where k = 10
^-3
; we also show that the electrons primarily contributing to I
_D
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 I
_D
electrons “heat-up” the cold drain electrons, which would further energize the hot electrons to produce the observed I
_II
(I
_D
) universal dependence.