LAB 5
MOSFET CHARACTERISTICS
LEARNING OBJECTIVES
By the end of this experiment, you should be able to:
1. Determine the effect of gate-to-source bias voltage VGS and drain-to-source voltage
VDS on drain current ID.
2. Determine the different mode of operations of the MOSFET from its I-V
characteristics.
3. Identify the key transistor parameters of the MOSFETs as well as the important
characteristics of the MOSFET transistor.
MATERIALS
Transistor:
1  2N7000 (NMOS)
Please bring your own Graph papers
EQUIPMENT
 Tektronix PS280 DC Power Supply
 Fluke 45 Dual Display Multi-meter
BACKGROUND
MOSFETs are the most popular type of transistors used in consumer electronics, due to its
superiority in integrated digital processes and suitability for mixed-signal (digital and analog)
applications in integrated circuits. Depending on the mode of operation, MOSFETs are used
as amplifiers and switches, and can also model the behavior of other devices such as resistance
and capacitors.
The actual device has 4 terminals; gate, drain, source and bulk (or substrate). However, to
determine the correct mode of operation, only the biasing of the gate, drain and source are of
importance. Hence, the discrete MOSFETs have only these three terminals. For design using
MOSFETS, the values of VGS and VDS must be appropriately chosen, to obtain suitable drain
current ID for the required functionality.
The important parameters identifiable from the I-V curves of the MOSFET to determine the
mode of operation are the threshold voltage VTH and the overdrive voltage or VDS(sat), as
well as the saturated drain current ID(sat).
Figure 5-1: The n-channel enhancement-mode MOSFET (NMOS); (a) conventional circuit
symbol, and (b) simplified circuit symbol used.
PRE-LAB ASSIGNMENT
1. Define enhancement mode MOSFETs, specifically the N-channel type, i.e. NMOS.
2. Define the threshold voltage VTH, and overdrive voltage VDS(sat) of an NMOS.
3. Run a simulation of the circuit in Figure 5-2 using a selected MOSFET model 2N7002
(Fairchild) in LTSPICE. Simulate the circuit for the following conditions:
a. While keeping the VDD fixed at 3V and VGG settings in Table 5-1, simulate the
circuit to complete the table. What is the value of threshold voltage of this
circuit?
b. Perform a DC sweep on the circuit to compute the data for Table 5-2.
Attach the simulation results with the pre-lab.
R = 100 Ω
Figure 5-2: Circuit setup for MOSFET I-V characterization
VGS (V)
-2.0
-1.0
-0.5
0
0.5
1.0
2.0
3.0
ID (mA)
Table 5-1
VGS (V)
-1.0
0
VDS (V)
1
2
ID (mA)
-0.5
0.0
1.0
`
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Table 5-2
3
4
5
4.0
5.0
IN-LAB ACTIVITIES
1.
2.
3.
4.
5.
Construct the circuit as shown in Figure 5-2. Note that VDD = VDS and VGG = VGS.
Measure and record the drain current ID for VDS and VGS as indicated in Table 5-3 and
5-4.
A load resistor of of 100 ohm between the Positive Terminal of the DC power supply
and the Drain terminal of the MOSFET. This is to limit the current flowing from the
drain to the source Ids.
An ammeter should be connected in series with this resistor if you want to measure
the current flowing from the drain to the source Ids
Measure all the value required in table 5-3 and 5-4
EEED211 ELECTRONICS DESIGN LAB, LAB 5
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RESULTS
VGS (V)
-2.0
-1.0
-0.5
0
0.5
1.0
2.0
3.0
4.0
5.0
ID (mA)
Table 5-3
VGS (V)
-1.0
0
VDS (V)
1
2
3
4
5
ID (mA)
-0.5
0.0
1.0
`
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Table 5-4
EEED211 ELECTRONICS DESIGN LAB, LAB 5
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In Lab Questions
1. Based on the results that you obtained in Table 5-3, determine the threshold voltage
of the device.
2. Plot I-V characteristics graph (ID (mA) versus VDS (V)) for this MOSFET (all VGS
in one graph). Indicate the different modes of operations on the curves.
3. From the I-V characteristic plots of the MOSFET, list the values of VDS(sat) and
ID(sat) for the following VGS in Table 5-5.
VGS (V)
ID (sat) (mA)
VDS(sat) (V)
-1
0
1
2
3
4
5
Table 5-5
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POST LAB DISCUSSIONS
1. Evaluate the value of the transconductance parameter Kn from the data in the I-V
characteristics. It is given that in saturation mode, ID=Kn(VGS-VTN)2
2. What is the value of the gate current IG? Discuss the implication of this value.
CONCLUSIONS:
Identify THREE (3) main understandings that you have gained from this experiment.
EEED211 ELECTRONICS DESIGN LAB, LAB 5
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