Introduction to Electronic Circuits & Labs
Lab #5: MOSFET I-V Characteristics
Seoul National University Spring 2023
Introduction to Electronic Circuits & Labs 2
Objectives
1. Measure I-V characteristics of a MOSFET and fit its model VDD
• Understand the behavior of a MOSFET ID
• Understand how each model parameter (Vth, mCox, l) affects
its characteristics VDS
Vin VGS
Introduction to Electronic Circuits & Labs 3
MOSFET Model Parameters in LTSPICE
• MOSFET model:
Parameter Symbol Description Units Default
VTO Vth Threshold voltage V 0
KP mCox Transconductance paramet V/A2 2e-5
er
LAMBDA l Channel-length modulation 1/V 0
• Example:
.model MN1 NMOS(VTO=0.8 KP=100u LAMBDA=0.1)
.model MP1 PMOS(VTO=0.7 KP=50u LAMBDA=0.2)
Introduction to Electronic Circuits & Labs 4
Pre-Lab Report
1. Simulate the ID-VGS characteristics of a MOSFET VDD
• Pick any NMOS transistor from the LTSPICE component library ID
• Set VDD at a high voltage (e.g. 5V) to keep MOSFET in saturation
VDS
• Sweep Vin from 0 to VDD and measure the ID-VGS characteristics
Vin VGS
2. Plan on how to setup the testbench to
make the same measurements in the lab
• How are you going to use the equipments
to apply stimuli and make measurements?
• Tip: you may use TinkerCAD (www.tinkercad.com)
to describe your testbench setups
Introduction to Electronic Circuits & Labs 5
Pre-Lab Report (2)
3. Fit the MOSFET model parameters (VTO and KP) from the simulated results
• Assuming ID = (1/2)KP(VGS-VTO)2
• Note 1: W/L of a discrete MOSFET is fixed at 1 (not a design parameter)
• Note 2: Your model only needs to be accurate for the range where the MOSFET operates in the
saturation region
4. Simulate the I-V characteristics of your own model and see if they match
• Error <10% within the saturation region is good enough
• Indicate the range of VGS that meets this accuracy requirement
• Note: your fitted KP and VTO values may not exactly match those in the model file. Key is to
learn how to determine those parameters from the measured results.
Introduction to Electronic Circuits & Labs 6
Pre-Lab Report (3)
5. Simulate the ID-VDS characteristics of a MOSFET ID
• For a fixed VGS that yields ID=~0.1mA with VDS=5V Vin
• Sweep VDS from 0 to 5V and measure ID VDS
VGS,fixed VGS
6. Plan on how to setup the testbench to
make the same measurements in the lab
7. Fit the MOSFET model parameter (l) from the ID-VDS characteristics
• Based on ID = ID,0(1 + lVDS) when MOSFET is in saturation region with VDS VGS-Vth
8. Simulate the ID-VDS characteristics of your model to see if they match
• Indicate the range of VDS that keeps the error < 10%
Introduction to Electronic Circuits & Labs 7
Lab Experiments
VDD
1. Measure the ID-VGS characteristics of a MOSFET
ID
for VGS ranging 0~5V and VDD=5V
VDS
Vin VGS
2. Measure the ID-VDS characteristics of a MOSFET ID
for VDS ranging 0~5V and fixed VGS that yields
Vin
ID=0.1mA at VDS=5V VDS
VGS,fixed VGS
Introduction to Electronic Circuits & Labs 8
Final-Lab Report
1. Describe your testbench setup and show the measured ID-VGS characteristics
2. Describe your testbench setup and show the measured ID-VDS characteristics
3. Fit a MOSFET model (KP, VTO, LAMBDA) from the measurement results
• If it's hard to achieve good fitting over a wide range, you can reduce the range so long as it
contains ID=0.1mA and VDS=2.2~2.8V (the operating condition of this MOSFET in Lab #6)
4. Compare the measurement results with the simulated results using your model
• Indicate the ranges of VGS and VDS that keep the error < 10%
5. Discuss any lessons you've learned
