Tham khảo tài liệu 'mosfet modeling for vlsi simulation - theory and practice episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 256 6 MOSFET DC Model Though accurate this is a complicated expression and not suitable for CAD models. However the following simplified form of Eq. has been used in the drain current model 28 -5 . . 1 ự2ộf L 30 2ự f Ki J This approximation though accurate has Ô as a function of the variable V so Ô must be calculated for each V. The effect of approximating the function F V vo with different Ô expressions is most sensitive at zero Vsb. Therefore a comparison is made between the exact and approximate functions at zero Vsb by calculating the relative errors between them using different 5 expressions. The results are shown in Figure where the error Er is defined as 77 _ E1 E 100 X 1 exact f approx F exact where Fexact and Fapprox are values of F given by Eqs. and respectively. Note from this figure that the simplest approximation for Ô cf. Eq. has maximum error therefore this approximation will underestimate the depletion layer charge Qb the most. However the resulting error in lds calculations is not usually significant because Qb is much smaller than Qị. In fact for Ids calculations any of the 5 functions discussed above can be used depending upon the desired accuracy and speed of calculation. However accuracy in Ỗ approximations are important for Fig. Error between the exact and approximate square-root function F V y0 for different Ỗ approximations Piece-Wise Drain Current Model for Enhancement Devices 257 MOSFET capacitance calculations where small error in Qb can cause large errors in the capacitances. For this reason Eqs. or are most appropriate for circuit design although these expressions can create problem in the capacitance calculations as we shall see in Chapter 7. Drain Current Equation with Square-Root Approximation With the square-root approximation Eq. for Qh y becomes Qb y - Coxy ỏ 7 y j2ộf Vsb while Eq. for 2 y reduces to Qi y - C0XtVgs - vth - aF y where we have made use .
