TABLE OF CONTENTS The dynamic MOSFET model we have presented so far assumes the transistor to be in quasi-static operation, i.e., the charge densities at any position in the channel are assumed to depend on the instantaneous values of the terminal voltages only [1], [2], [3]. If, however, the rate of variation of the terminal voltages is high, the quasi-static approximation is no longer valid. In fact, the charge densities along the channel depend not only on the voltage values at a certain time but also on the history leading to the charge densities at that time [2]. The model used for fast varying signals is called the non-quasi-static model, which is the subject of this chapter.
In chapter 5 we described the intrinsic quasi-static charge-conserving model. The quasi-static approximation assumes that the charge density at any position in the channel changes instantaneously with the applied voltages [1], [2], [4], i.e., it assumes the transit time in the channel to be zero [4]. Since the channel transit time is not zero, analyses based on the quasi-static approximation introduce errors for rapidly changing terminal voltages due to the distributed nature of the MOSFET. Models that take into account the distributed nature of the transistor are described as non-quasi-static (NQS) models. The derivation of NQS models is not a simple task since it requires the solution of both transport and continuity equations. In addition, the transport equation is non-linear, making the problem even more difficult to solve.
In the sections that...
