Analog VLSI Circuits for the Perception of Visual Motion

Appendix C

Transistors and Basic Circuits

C.1 Large-signal MOSFET Model

The metal–oxide–semiconductor field-effect transistor (MOSFET) is the basic computational device of the implementation of the presented analog networks. Standard complementary metal–oxide–semiconductor (CMOS) process technology provides two complementary types of MOSFETs, the native and the well transistor. Labeling of the two types is often based on their channel type: thus the n-channel transistor is called the nFET and the p-channel transistor the pFET. In a typical p-type substrate process, the nFET is equivalent to the native and the pFET to the well transistor.

Schematic symbols for both transistors can vary in the literature. In this book symbols are used as depicted in Figure C.1. A MOSFET has four terminals called gate (g), bulk (b), drain (d), and source (s). It is important to note, however, that it is a symmetric device because the drain and source are not intrinsically defined by the device. The definition only depends on the voltage distribution on the two terminals. The source is defined as the terminal, where the net current of majority carriers originates. In Figure C.1, I_ds indicates the positive channel currents of the transistors in a typical p-type substrate process.

Sub-threshold regime (weak inversion)

In sub-threshold, the channel current I_ds is in first order defined by the difference of the diffusion current densities in the forward and the reverse direction, thus

where q is the ...