Toward the End of the MOSFET Roadmap: Investigating Fundamental Transport Limits

MOSFET scaling and performance has progressed rapidly in recent years, with physical gate lengths for electrostatically sound devices reaching 30 nm or below: near the prospective scaling limits for traditional bulk MOSFETs. This work investigates several key issues for this "end of roadmap" regime. Focus is on understanding the limitations to carrier velocity in MOSFET inversion layers as channel lengths are scaled well below 100 nm, and on relaxing these limits through architectural alternatives to bulk MOSFETs. It has been proposed that drain current is ultimately limited by the rate at which carriers can be thermally injected from the source into the channel. In this work it is shown that commonly used techniques for experimentally determining carrier velocity are insufficient to determine how close modem MOSFETs operate to this ballistic or "thermal limit". A new technique is proposed, and applied to two advanced industry technologies with deep-sub-100-nm channel lengths. It is shown that a IV NMOS technology with Leff < 50 nm operates at no more than-40% of the limiting thermal velocity. Furthermore, no indication is found that continued scaling is bringing us closer to the thermal velocity limit. Via simulation, the relationship between mobility and scaling is investigated for bulk silicon NMOSFETs and FDSOI (Fully-Depleted Silicon-On-Insulator) alternatives, focusing on the 50 and 25 nm Leff generations. Scaling of bulk MOSFETs well below 100 nm Leff requires heavy channel doping, leading to degraded low-field mobility. Provided that the gate workfunction is used to determined the threshold voltage, FDSOI devices do not suffer from this trade-off, by virtue of the fact that their channel can be undoped. It is shown that single-gate FDSOI is, accordingly , an attractive alternative down to 50 nm Leff. For deeper scaling, double-gate FDSOI should have approximately a 3X mobility advantage over bulk NMOS. With careful determination of channel length, inversion-layer charge, and series resistance, it is possible to study experimentally the relation between channel length and mobility in deep-sub-100-nm MOSFETs. With the aid of inverse modeling techniques, evidence is found that in the very shortest modern MOSFETs, mobility is less then would be expected from "universal" mobility , and independent of transverse field. This may be indicative of a transition in the dominant scattering mechanism, from surface-to Coulomb-scattering. 3 The relevance of low-field mobility to the performance of deep-sub-100-nm MOSFETs is not well understood. In this work this relationship is studied experimentally: mobility (with low …