Effect of MOSFET threshold voltage variation on high-performance circuits

Abstract

The driving force for the semiconductor industry growth has been the elegant scaling nature of CMOS technology. In future CMOS technology generations, supply and threshold voltages will have to continually scale to sustain performance increase, limit energy consumption, control power dissipation, and maintain reliability. These continual scaling requirements on supply and threshold voltages pose several technology and circuit design challenges. One such challenge is the expected increase in threshold voltage variation due to worsening short channel effect. This thesis will address three specific circuit design challenges arising from increased threshold voltage variation and present prospective solutions. First, with supply voltage scaling, control of die-to-die threshold voltage variation becomes critical for maintaining high yield. An analytical model will be developed for existing circuit technique that adaptively biases the body terminal of MOSFET devices to control this threshold voltage variation. Based on this model, recommendations on how to effectively use the technique in future technologies will be presented. Second, with threshold voltage scaling, sub-threshold leakage power is expected to be a significant portion of total power in future CMOS systems. Therefore, it becomes imperative to accurately predict and minimize leakage power of such systems, especially with increasing within-die threshold voltage variation. A model that predicts system leakage based on first principles will be presented and a circuit technique to reduce system leakage without reducing system performance will be discussed.

(cont.) Finally, due to different processing steps and short channel effects, threshold voltage of devices of same or different polarities in the same neighborhood may not be matched. This will introduce mismatch in the device drive currents that will not be acceptable in some high performance circuits. In the last part of the thesis, voltage and current biasing schemes that minimize the impact of neighborhood threshold voltage mismatch will be introduced.