Field-Effect Transistors

PHYS-310 Homework #8 – FETs – Fall 2013 (20 points)

8.1) (4 points) Eggleston Exercise 5.1

8.2) (4 points) Eggleston Exercise 5.2

8.3) (2 points) nMOSFET in Cutoff - Write an inequality defining when an NMOS enhancement type

transistor is in cutoff. That is, no conduction channel is formed and the current flowing from Drain to

Source, ���, is equal to zero.

8.4) (4 points) nMOSFET in Triode Region – When Drain to Source voltage, ���, is less than the

overdrive voltage (i.e. ��� < ��� where ��� = � � − ��, � � is the Gate to Source Voltage and �� is the

threshold voltage), a continuous conduction channel between Drain and Source is formed and the

transistor is said to be operating in the triode region. The equation for the current flowing from Drain to

Source, ���, when an enhancement-type NMOS transistor is in the triode region is given by:

��� = 1

2 ��[�� � − ������ −

1

2 ���

� ]

If the MOSFET is operating in the triode region and given �� = 1 ��

�� , �� = 1.2� and � � = 2.7�:

a) Calculate the overdrive voltage ���,

b) Calculate ��� at two values of ��� of your choice (be sure to calculate these values in the triode

region!) and,

c) Plot ��� as a function of ��� from 0 to ���. You may do this by hand or using your choice of

program. For example, using wolframalpha.com, I generated a plot from for the above function

(but different values of �� and � �) using the following string:

plot[0.5*0.001*((1.5-1)*V-0.5*V^2),{V,0,0.5}]

8.5) (6 points) nMOSFET in Saturation - The equation for the current flowing from Drain to Source, ���,

when an enhancement-type NMOS transistor is in saturation is given by:

��� = 1

2 ���� � − ���

�

Where �� is the MOSFET Transconductance Parameter, � � is the Gate to Source Voltage and �� is the

Threshold Voltage for the device. The MOSFET only enters saturation when ��� ≥ � � − �� where

� � − �� is equal to the overvoltage, ���. Assuming that ��� ≥ ��� and the MOSFET is in saturation

and given �� = 1 ��

�� and �� = 1�:

a) find ��� for � � = 2.5�, 5� �� 10�,

b) Plot ��� as a function of � � from �� to 5V. Note especially that unlike the triode region, ��� is

independent of ��� and,

c) BONUS (2 points): Why are we starting our plot from � � = �� instead of zero?