Quantum-mechanical effects on the threshold voltage of undoped double-gate MOSFETs

Quantum-mechanical (QM), or carrier energy-quantization, effects on the subthreshold characteristics, including the threshold voltage (V_t), of generic undoped double-gate (DG) CMOS devices with ultrathin (Si) bodies (UTBs) are physically modeled. The analytic model, with dependences on the UTB thickness (t_Si), the transverse electric field, and the UTB surface orientation, shows how V_t is increased, and reveals that 1) the subthreshold carrier population in higher-energy subbands is significant, 2) the QM effects in DG devices with {110}-Si surfaces, common in FinFETs, are comparable to those for {100}-Si surfaces for t_Si>∼4 nm, 3) the QM effects can increase the gate swing, and (iv) the QM effects, especially for t_Si<∼4 nm in nMOSFETs with {110}-Si surfaces and in pMOSFETs, will strongly influence DG CMOS design and scalability.