Transistors used in digital applications should be able to switch between ON-state and OFF-state as quickly as possible. At cryogenic temperatures, the switching between ON- and OFF-state is superior compared to
room-temperature, because of the increased ON-state current and the increased transconductance. However, at scaled devices resonance phenomena have been observed across various technologies including 40 nm bulk
[223], 28 nm bulk CMOS [200, 224], 40 nm SOI [225], 22 nm FDSOI [226] and 16 nm FinFET [227], which can lead to large humps in the transition from the OFF- to ON-state. Such humps have been
measured on Tech. A on nMOS SmartArray A (Fig. 6.9 (left)) with dimensions at 4.2 K with
[MJC5] and on a device of the nMOS-flavored SmartArray B in Fig. 6.9 (right)
with dimensions
at 4.2 K with
.
Figure 6.9: Resonant tunneling measured at cryogenic temperatures on nMOS of Tech. A using SmartArray A (left)
and SmartArray B (right). The oscillations caused by resonant tunneling fade out with increasing . Figure taken from [MJC5].
These humps in the (
) curves can be extremely prominent and can even lead to a negative transconductance
, as can be seen in Fig. 6.10.
Figure 6.10: Distinctive oscillations, as can be seen in Fig. 6.9 (right), can lead to a negative transconduc-
tance. This effect fades out with increasing and towards higher temperatures.
The resonances are caused by quantum confinement in the channel due to ionized dopants, impurities or defects. Resonant tunneling leads then to the occurring resonance in . In literature, this is often referred to as Coulomb oscillation [227, 224, 223]. The resonance shows a large
and temperature dependence. A larger
leads to a fade out of the resonance, as can be seen in Fig. 6.9 (left). This is caused by the
drain-induced barrier lowering (DIBL) effect which allows more carriers to overcome the barrier and diminishes the reduced conductance caused by the resonance [227]. Towards higher temperature the resonance fades out,
because the higher thermal energy allows more carriers to overcome the occurring tunneling barriers [227].