50-Å gate-Oxide MOSFET's at 77 K

While hot-carrier-induced degradation is aggravated at cryogenic temperature, a very thin gate-oxide (52-Å) device can still tolerate a 3-V power-supply voltage at 77 K. Hot-carrier-induced degradation may not be the limiting factor in choosing the power-supply voltage and special drain structures may be necessary for very thin gate MOSFET's even at 77 K. However, mobility reduction at high V
G
is more severe both at lower temperatures and for thinner oxides. Electron mobility appears to be oxide-thickness-dependent at 77 K. The dependence of the electron mobility on the normal field is so strong that it results in unusual I-V characteristics such as negative transconductance at 77 K for an oxide field above 3 MV/cm. The I--V characteristics have been modeled with a mobility dependence on V
GS
of the form µn ∞ (1 + η(V
GS
- V
t
/T
ox
)
²
+ (E/E
c
))
^-1
for 52-Å devices.