A workshop sponsored by the US National Science Foundation, The International Centre for Materials Science (Bangalore, India), Purdue University (West Lafayetee, IN USA)
Electrical and Computer Engineering
Co-Authors: Ahmad Ehteshamul Islam, Muhammad Ashraful Alam
Fluctuation/variation in different observables is common in different types of electrical, biological, chemical, thermal, etc. systems. In a MOS transistor, such phenomena is also observed due to the many sources of threshold voltage (VT) variation (an entity), which leads to significant variation in transistor’s drive current (the observable). Such variations have recently been accelerated due to aggressive scaling of transistors following Moore’s law. Therefore, the study of transistor’s parametric variation and its effect on its performance are of significant importance.
In general, one can compensate the effect of any fluctuation of a certain entity – if and only if – the fluctuation of that entity gives rise to opposite changes in the two quantities, on which the observable depends on. Application of such principle is naturally present in chemical and biological systems in the form of the enthalpy-entropy (two quantities) compensation during a physiological process e.g., protein unfolding (the entity), which explains why such systems always operate near equilibrium (i.e., change in free energy, the observable, is negligible). In addition, the above principle can be artificially incorporated in a physical system by choosing two materials having opposing responses to a certain entity, as can be found in: (a) compensation of optical birefringence by randomly copolymerizing two monomers having opposite birefringence, (b) reducing sub-threshold slope below the thermal limit by using two dielectric materials having opposite capacitances during sub-threshold switching, etc.
In this work, we use similar principle to inherently compensate the effect of VT-fluctuation (entity) on drive current (observable) in nanoscale transistors. By choosing appropriate steepness of the mobility-field relationship of a transistor, it can be shown that the VT-fluctuation can give rise to opposite signs of fluctuations in carrier concentration and mobility, which are the two quantities that affects the drive current of a transistor. We experimentally demonstrate the existence of such compensation of VT-fluctuation, originating from the generation of defects at the oxide/substrate interface or within the oxide. In addition, our theoretical analysis also justifies the validity of the compensation principle for spatial sources of VT fluctuations arising from fluctuations in oxide thickness, metal-gate work-function, etc. Moreover, transistors designed for such inherent compensations in VT-fluctuation (a variation-resilient transistor) can minimize/eliminate the need for extra (guard-band) voltage currently being used in existing CMOS technology, for handling the effect of these fluctuations. Therefore, it would be possible to reduce supply voltage (and corresponding power consumption) of CMOS technology, which has been close to 1V for the last few CMOS generations.
Ahmad Ehteshamul Islam was born in Sylhet, Bangladesh in 1979. He received the B.S. degree in Electrical and Electronic Engineering (EEE) from Bangladesh University of Engineering and Technology (BUET) in 2004. He is currently pursuing the Ph.D. degree in the School of Electrical & Computer Engineering, Purdue University, West Lafayette, IN, USA. During 2004–2005, he was a Lecturer with the Department of EEE, BUET. He is currently working under the supervision of Prof. Ashraful Alam, where his research mainly focuses on studying the impact of variabilities in semiconductor devices. He has authored and coauthored more than 15 journals and conference papers. Mr. Islam has been a Student Member of the IEEE Electron Devices Society (since 2002) and American Physical Society (since 2008) and also serves as a Reviewer for several IEEE, Elsevier, and Electrochemical Society journals. He is the recipient of Kintar-Ul-Haque Gold Medal for his undergraduate result in 2005 and IEEE EDS Ph.D. Fellowship for his work on transistor reliability in 2008. He was also involved as student team leader (2002-2003) and faculty co-supervisor (2004-2005) of EEE, BUET team in International Future Energy Challenge 2003 and 2005 competitions, respectively.
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