Charge Trapping and Variability in CMOS Technologiesat Cryogenic Temperatures

6.3 Resonant Tunneling at Cryogenic Temperatures

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 \( W \times L = \SI {100}{\nano \meter }\times \SI {28}{\nano \meter } \) at 4.2 K with \( \vd =\SI {50}{\milli \volt } \) [MJC5] and on a device of the nMOS-flavored SmartArray B in Fig. 6.9 (right) with dimensions \( W \times L = \SI {100}{\nano \meter }\times \SI {70}{\nano \meter } \) at 4.2 K with \( \vd =5,10,...,\SI {50}{\milli \volt } \).

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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 (math image). Figure taken from [MJC5].

These humps in the (math image)((math image)) curves can be extremely prominent and can even lead to a negative transconductance (math image), 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 (math image) 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 (math image). In literature, this is often referred to as Coulomb oscillation [227, 224, 223]. The resonance shows a large (math image) and temperature dependence. A larger (math image) 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].