Quantum Transport Simulations of Sub-60-mV/Decade Switching of Silicon Cold Source Transistors

A steep-slope switch of silicon (Si) transis-tor with a "cold" source (CSFET) with a metal betweenp-type and n-type Si (pSi-M-nSi) is investigated by quan-tum transport simulations, which breaks the subthresholdswing (SS) limit by manipulating the density of states(DOS) of injected carriers. The injected current from thejunction of pSi-gold (Au)-nSi is calculated to be over 10(3) mu A/mu m by first-principles quantum-transport simula-tions. Then, Si CSFETs are investigated and compared withconventional FETs and tunneling FETs. It is demonstrated that SS reaches 23 mV/decade in 15-nm Si CSFETs inthe ballistic limit. ON-state current is as large as 7.9x10(2) mu A/mu m at V-D=0.5 V with I-off fixed at 10 pA/mu m.The SS of CSFETs degrades with decreasing gate lengthand cannot be smaller than 60 mV/decade at L-G=6 nm. CSFETs have temperature-independent SS due to the coldelectron injection, which is different from FETs and Diracsource FETs. The output characteristics demonstrate that CSFETs exhibit negative differential resistance and canachieve current rectification. Finally, the effects of coldelectron rethermalization due to electron-phonon scatter-ing in 15-nm CSFETs are found to degrade the SS to 50 mV/decade in six orders of magnitude of drain current. The SS can be improved to 42 mV/decade by using shorter lengths of the metal layer and n-type Si in the source tominimize the scattering.