Ee2021

NATIONAL UNIVERSITY OF SINGAPORE

Department of Electrical and Computer Engineering

EE2021 : Tutorial 4 : Solutions

• Unless otherwise stated, you may assume temperature, T = 300 K, thermal voltage, VT = 0.025 V, and silicon intrinsic carrier concentration, ni = 1.5 × 1010 cm-3  and make use of the equations given in the lecture notes directly, without having to derive them.
• All the symbols are as defined in lecture notes.

Homework 3:

Homework 3 is Question 5 of Tutorial 4 and you will need to submit it in class on Wednesday 12 July 2017.   

Q1.         An n-channel MOSFET M1 is connected as shown in Fig. 1a to perform ID - VGS measurements.  The gate-source voltage VGS is changed by varying VDD and the resulting drain current ID is measured through a current meter.

[pic 1]

                        Fig. 1a        

        (a)        Show that the transistor M1 is operating in the saturation region.

        (b)        The measured ID - VGS characteristic is shown in Fig. 1b.  Also shown is the corresponding tabulated data.  Due to experimental error, the measured points do not exactly follow the square law predicted by the drain current equation in the saturation region, i.e.,  ID = Kn (VGS-VTHN)2.  Transform the drain current equation and devise a way such that you can easily extract the device parameters Kn and VTHN, which are the conductance parameter and threshold voltage of the n-channel MOSFET, respectively. (Hint: Transform the drain current equation such that it follows a straight line equation format, such as y = mx+C.)

[pic 2]

                                                                 Fig. 1b

        (c)         Assuming that VTHN is unchanged, describe three ways of increasing Kn in the design of the MOSFET.

1.        (a)        In the circuit, VDS = VGS.  For an n-channel MOSFET, VTHN > 0.

                Therefore, VDS > VGS – VTHN.

                Hence the MOSFET is operating in the saturation region.

        (b)        Since the MOSFET is operating in the saturation region, the equation[pic 3] applies.

                [pic 4]         [pic 6]     [pic 5]

                This is an equation of the form y = mx + C where

                 [pic 7].

                Plot [pic 8]versus VGS and draw a best-fit straight line.

[pic 9]