Enhanced performance analysis of vertical strained-sigeimpact Ionization MOSFET (VESIMOS)

The Vertical Strained Silicon Germanium (SiGe) Impact Ionization MOSFET (VESIMOS) has been successfully developed and analyzed in this paper. VESIMOS device integrates vertical structure concept of Impact Ionization MOSFET (IMOS) and strained technology. The transfer characteristics of VESIMOS revealed an inverse proportionality of supply voltage, V
D
and sub-threshold, S due to lower breakdown strength of Ge content. However, the Sis in direct proportion to the leakage current. The S=10mV/dec was successfully obtained at threshold voltage, V
T
=0.9V, with V
D
=1.75V. This V
T
is 40% lower than V
T
for Si-vertical IMOS. The output characteristics goes into saturation for V
D
more than 2.5V, attributed to the presence of Ge that has high and symmetric impact ionization rates. Electron mobility wasimproved by 40% compared to Si-vertical IMOS and an increase in strain will also increase mobility and reduce further the V
T
. However, the increase in strain layer thickness, T
SiGe
, resulted in an increase of V
T
and lowered the mobility. This is due to the strain relaxation in the SiGe layer. Finally, at high source-drain doping concentration, S/D=2×10
¹⁸
/cm
³
, the V
T
dropped to 0.88V, with V
D
of 1.75V. This is due to high electric field effect in the channel at high doping concentration, which is contrary to the doping effects of conventional MOSFET.