EE 323 Lab 6
The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) as a Switch
Motivation
Transistors can be used to control current flow by applying appropriate voltages. Thus,
transistors can be used as electrically controlled switches. We will look at this particular
application of transistors in this lab.
Lab Objectives
● Design and simulate a MOSFET switch circuit (pre-lab exercises).
● Measure the threshold voltage and transconductance parameter of a MOSFET.
● Build and test the MOSFET switch circuit that you designed and simulated.
● Use the MOSFET to switch an LED on and off.
Materials
Supplies
Various resistors
2N7000 n-channel MOSFET
One red LED (Lite-On Technologies part
number LTL2R3KRD-EM)
Equipment
Breadboard
DC power supply
Digital multimeter
Function generator
Oscilloscope
Experiments
Fig. 1: Diode-connected MOSFET.
Part 1: Measure the MOSFET parameters
Although you were given the values of the threshold voltage Vt and the transconductance
parameter kn of the 2N7000 MOSFET in the pre-lab (Vt = 2.236 V and kn = 93 mA/V2) and in
the PSPICE model of the transistor, this value can vary for each fabricated device. Thus, this
value should be experimentally measured. To do this, construct a diode-connected MOSFET
circuit (Fig. 1).
Adjust the supply voltage 𝑉𝐷𝐷 to obtain a drain current ID1 ≈ 1 mA. Note that ID1 is
determined by measuring the voltage drop across the 1-kΩ resistor. Measure the corresponding
VGS. Record this value as VGS1.
Value of RD: _________________________________
Value of ID1: __________________________________
Value of VGS1: _______________________________
Increase Vdd so the drain current increases to ID2 = 4ID1. Measure the corresponding VGS, and
record this value as VGS2. Use the following equation to find the threshold voltage:
Vt = 2VGS1 - VGS2.
Value of ID2: __________________________________
Value of VGS2: _______________________________
Value of Vt: _______________________________
Use a drain current value and your value of Vt to determine kn using the saturation current
equation.
Value of kn: __________________________________
Part 2: MOSFET switch
Build the MOSFET switch circuit that you designed for your pre-lab exercise (Fig. 2). Note
that Rsource is internal to the function generator (do not add a discrete resistor). Do not connect
the function generator to the MOSFET yet.
Fig. 2: MOSFET switch with a resistive load.
Record the exact measured value for Rload here. Rload: ____________________
Set the function generator to output a square wave varying from 0 to +5 V, 50% duty cycle at
1 Hz. You will need to add a DC offset to obtain this square wave; ask your TA for help if you
do not know how to do this. Measure the output of the function generator with the oscilloscope
and verify that it is correct (use DC coupling on the oscilloscope).
With the output of the function generator off, connect it to MOSFET. Turn on the function
generator output, and measure the output of the function generator and the voltage across the
MOSFET (vDS), when the switch is open and closed. Both measurements will use DC coupling.
Output voltage of function generator (switch open): _______________
vDS (switch open): _________________
Output voltage of function generator (switch closed): _______________
vDS (switch closed): ________________
Part 3: LED switching circuit
Turn off the power to your circuit. Add an LED between Rload and the drain of the MOSFET
(Fig. 3). Turn on the DC power supply and the function generator, using the same square wave
as from Part 2. You should now be able to see the LED blinking at 1 Hz.
Fig. 3: MOSFET switch with an LED load.
Increase the frequency of the function generator until you can no longer tell the LED is blinking
(it will look like it is constantly on). What frequency does this occur at (might be different for
your lab partner)?
Maximum visible frequency for lab partner #1: _____________________________
Maximum visible frequency for lab partner #2: _____________________________
Maximum visible frequency for lab partner #3: _____________________________
Measure the voltage across the MOSFET (vDS) when the switch is open and closed. Use DC
coupling on the oscilloscope.
vDS (switch open): _________________
vDS (switch closed): ________________
Analysis / Discussion
1. In Part 2, you measured vDS when the MOSFET switch was closed. Use this to calculate
the voltage, current, and power of the load resistor. Is the current the same as what you
calculated in the pre-lab exercise? If not, explain why.
2. In Part 3, you measured vDS when the MOSFET switch was closed. Use this to calculate
the voltage, current, and power of both the load resistor and the LED. Is the current the
same as the current in Part 2? If not, explain why.
Concluding Remarks
MOSFETs are very useful as switches. Imagine trying to use a mechanical switch to turn the
LED on and off faster than you can distinguish with your eye. Now imagine the switches in
CPUs, which can switch billions of times a second…this is all made possible by transistor
switches.