Operation of short-channel depletion-mode MOS devices at liquid-nitrogen temperature

Experimental observations that depletion-mode MOS devices optimized for room temperature can also work well when immersed in liquid nitrogen are reported in which the classical impurity freeze-out effect seems to vanish on short-channel devices if the drain voltage is not too small. This is attributed to field-assisted ionization mechanisms such as the Poole-Frenkel effect, with possible enhancement by self-heating. The MINIMOS 4 device simulator was modified to introduce this effect and then to check the validity of this assumption by comparison with experimental results. To prove that it is possible to take advantage of this effect a 3-bit feedback adder, used as a benchmark circuit, has been processed in an enhancement-depletion 0.5- mu m NMOS technology optimized for room temperature wherein the cooling from 300 to 77 K results in an improvement from 1 to 1.3 GHz for the maximum clock frequency of operation.<
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