Design and simulation of two-dimensional superlattice steep transistors
As modern metal-oxide-semiconductor field-effect-transistors (MOSFETs) scale to smaller dimension, the power density of microprocessors rises up quickly. The supply voltage has scaled with device dimension. To maintain an acceptable ON/OFF ratio, the subthreshold swing (S.S.) of transistors need to be reduced. The 60mV/dec limit of MOSFET comes from the the e-kT variation in f(E) above the Fermi level. This limit can be overcome by changing the transmission probability T(E) so that it decades sharply above the source Fermi level. Such examples have been demonstrated. Tunneling FET (TFET) can have a steep S.S., but its on-current (Ion) is limited by tunneling probability from conduction band to valence band. Utilizing the high transmission of miniband and low transmission in the minigap, nanowire superlattice MOSFETs have been demonstrated to have been both steep S.S. and high Ion. Adding a superlattice between the N+ source and the channel suppresses the injection of high-thermal-energy electrons, reduces S.S., in simulations to 13mV/dec. However, high Ion per unit die area demands small nanowire pitch and such devices might be hard to fabricate. In this work, a double-gate MOSFETs having InGaAs/InAlAs superlattices between the N+ source and a planar InGaAs channel is proposed. As with nanowire superlattice transistors, the 2-D superlattice bandgap reduces injection into the channel of electrons having energy above the source Fermi energy. Simulated ballistic transport characteristics of FETs using a three-well superlattice show 29-37.5mV/dec. minimum subthreshold swing and 390 A/m on-current given 0.1 A/m off-current and a 0.2V power supply. Utilizing the high conduction band offsets, InAs/InAlAs superlattice MOSFETs have higher ON/OFF ratios than InGaAs/InAlAs superlattice MOSFETs. Furthermore within the same length we can fit in more superlattice periods, so a steeper S.S. and even higher ON/OFF ratio can be achieved.
Klimeck, Purdue University.
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