BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI K K BIRLA GOA CAMPUS

Microelectronics Circuits (ECE/EEE/INSTR F244) Tutorial on MOSFET operation

Question 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A 0.18-μm fabrication process is specified to have tox = 4 nm, μn = 450 cm2 /V·s, and Vt = 0.5 V. Find the value
of the process transconductance parameter kn′ . For a MOSFET with minimum length fabricated in this process,
find the required value of W so that the device exhibits a channel resistance rDS of 1 kΩ at vGS = 1 V.

Question 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
For a 0.18-μm process technology for which tox = 4 nm and μn = 450 cm2 /V·s, find Cox , kn′ , and the overdrive
voltage VOV required to operate a transistor having W/L = 20 in saturation with ID = 0.3 mA. What is the
minimum value of VDS needed?

Question 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A circuit designer intending to operate a MOSFET in saturation is considering the effect of changing the device
dimensions and operating voltages on the drain current ID . Specifically, by what factor does ID change in each
of the following cases?

(a) The channel length is doubled.

(b) The channel width is doubled.

(c) The overdrive voltage is doubled.

(d) The drain-to-source voltage is doubled.

(e) Changes (a), (b), (c), and (d) are made simultaneously.

Which of these cases might cause the MOSFET to leave the saturation region?

Question 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Consider an NMOS transistor fabricated in a 0.18-μm process with L = 0.18 μm and W = 2 μm. The process
technology is specified to have Cox = 8.6 fF/μm2 , μn = 450 cm2 /V·s, and Vtn = 0.5 V.

(a) Find VGS and VDS that result in the MOSFET operating at the edge of saturation with ID = 100 μA.

(b) If VGS is kept constant, find VDS that results in ID = 50 μA.

(c) To investigate the use of the MOSFET as a linear amplifier, let it be operating in saturation with VDS =
0.3V. Find the change in iD resulting from vGS changing from 0.7 V by +0.01 V and by -0.01 V.

Question 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
′ = 50 V/μm of channel
An NMOS transistor is fabricated in a 0.4-μm process having μn Cox = 200 μA/V2 and VA
length. If L=0.8 μm and W=16 μm, find VA and λ. Find the value of ID that results when the device is operated
with an overdrive voltage VOV = 0.5 V and VDS = 1 V. Also, find the value of ro at this operating point. If VDS
is increased by 2 V, what is the corresponding change in ID ?

ECE/EEE/INSTR F244 Page 1 of 2

Question 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The PMOS transistor shown in figure below has Vtp = -1 V, kp′ = 60 μA/V2 , and W/L = 10.

(a) Find the range of VG for which the transistor conducts.

(b) In terms of VG , find the range of VD for which the transistor operates in the triode region.

(c) In terms of VG , find the range of VD for which the transistor operates in saturation.

(d) Neglecting channel-length modulation (i.e., assuming λ=0), find the values of |VOV | and VG and the
corresponding range of VD to operate the transistor in the saturation mode with ID = 75 μA.

(e) If λ = –0.02 V–1 , find the value of ro corresponding to the overdrive voltage determined in (d).

(f) For λ = –0.02 V–1 and for the value of VOV determined in (d), find ID at VD = +3 V and at VD = 0 V; hence,
calculate the value of the apparent output resistance in saturation. Compare to the value found in (e).

+5 V

VG

ID

VD

ECE/EEE/INSTR F244 Page 2 of 2