Sub-Threshold Operation: Theory and Challenges

Abstract

In order to design circuits operating at ultra-low supply voltages, an understanding about the sub-threshold behavior of CMOS transistors must first be obtained. Therefore, this chapter discusses the fundamentals of sub-threshold operation, by looking at the general principles of transistor theory and by briefly giving some adequate background of the device physics. The different operating regions of a CMOS transistor are examined, as well as the definition of the threshold voltage and the parameters by which it is influenced. This chapter also provides an overview of the different challenges which are introduced by operating a circuit in the sub-threshold or weak inversion region. Insight in these circuit-level challenges is essential to efficiently and successfully design ultra-low-voltage systems. Subsequently, the impact of CMOS technology scaling on circuits operating in the ultra-low-voltage region is studied. This chapter aims to provide an answer to the benefits and disadvantages of scaling on such implementations. First, an equation to determine the minimum feasible supply voltage for digital circuits is derived. Out of this equation, a theoretical minimum as well as a practical minimum supply for a specific technology can be calculated. Second, scaling analysis focuses on the two CMOS technologies at hand of this book. Furthermore, the (in)accuracy of weak inversion transistor models is also explored. To conclude, the chapter explains what the difference is between the various transistor types offered by modern CMOS technologies and which type of transistors is used throughout the prototypes presented in this work.