LAB 3: MOSFET I-V CHARACTERISTICS

LEARNING OUTCOME:

In this lab, the students discover two voltage-control terminals of the four-terminal

MOSFET. The students will construct a circuit to observe the change in threshold voltage

of a MOSFET transistor due to the change in substrate-to-source voltage. The students will

collect and analyze data to find the transconductance parameters of a particular MOSFET.

The students will acquire the MOSFET I-V by setting up a simple circuit connecting to the

Analog Discovery equipment.

MATERIAL AND EQUIPMENT

Material Equipment

MIC94050 p-channel MOSFET Analog Discovery

BS107 n-channel MOSFET Digilent Waveforms software

Resistors Breadboard

BACKGROUND:

The MOSFET is actually a four-terminal device, whose substrate, or body terminal must be

always held at one of the extreme voltage in the circuit, either the most positive for the

PMOS or the most negative for the NMOS. One unique property of the MOSFET is that the

gate draws no measurable current. Another is that either polarity of voltage maybe applied

to the gate without causing damage to the transistor. Although enhancement-mode

MOSFETs respond to only one polarity, the students need not fear the consequences of

using the opposite polarity.

A MOSFET with its gate and drain connected together always operates in the constant-

current region, its iD-VGS relationship is

TRGSDVKvKi −= (1)

where the threshold voltage depends on the source-body potential vSB as

FFSBTRTR VVV φφγ 22[0 −+±= (2)

Using the typical values γ=0.4 V0.5

and φF=0.3 V gives the following values for the change

in threshold voltage of an nMOS.

vSB (V) VTR-VTR0

1 0.20

2 0.34

3 0.45

OBSERVE THE CHANGE IN THRESHOLD VOLTAGE DUE TO SUBSTRATE-TO-

SOURCE VOLTAGE:

Set up the circuit as shown in Figure 1 below. Use the MIC94050 p-channel MOSFET, this

transistor has 4 terminals in which the substrate is marked on the mounting PCB. Connect

the circuit to the Analog Discovery equipment, pay attention to the polarities of the scope

channels 1 and 2. Start Digilent Waveforms software and WaveGen (out). Set-up the 2

output waveforms AWG1 and AWG2 using the parameters provided in Table 1. Consult the

detailed information at the end of this document of how to set-up the desired waveforms in

the Analog Discovery. Start the Scope (in) and set-up the channels (C1, C2, and Math

Channel M1) as shown in the example waveforms in Figure 3. Add XY in the Oscilloscope

1 window and select X=C1 and Y=M1. Run two waveforms generated in the WaveGen (Run

all). Run the oscilloscope and observe the waveform of the window XY#1. The waveform

should be similar to the samples in Figure 3.

Table 1: Initial Waveforms parameters set-up

Waveform Gen. Frequency Amplitude Offset

AWG1 (sawtooth) 10 2.5V -2.5V

AWG2 (5 steps) 1 1.5V 1.5V

Channel Math Offset Range

C1 -1.1V 100mV/div

C2 -2V 500mV/div

M1 C2/RD -5mA 1mA/div

Time Start 0 Base 200ms/div

Figure 1: Circuit set-up to observe the substrate voltage

The plots of Di vs vGS will look like these in Figure 2. Each curve corresponds to a

different value of vSB. The slope of each curve is K . Extrapolating each curve to iD=0

gives threshold voltage, VTR, for each value of vSB. The curves are not equally spaced

because the change in VTR is proportional to SBV . Measure the voltage vGS, current iD with

the Digilent scope for different values of vSB. Plot as shown in Figure 2.

Obtain a printout of the XY waveform, label the values of VSB on each curve. The

waveform can be exported as data and can be saved into your computer. Use appropriate

software to plot the linear region of the curves. The slope of each curve is K . Find K in

mA/V2. Find VTR0 (i.e., VSB=0V).

RD,560

G

D

S

Substrate To AWG2

To AWG1

+

Channel 2

_

+

Channel 1

_

MIC940500

Setup of step voltage on AWG:

- Digilent waveforms 1 � Analog � Out Wave Gen � Open new.

- DWF1 – Arbitrary Waveform Generator 1 will appear.

- Generator 1 � Select Channels � Channel 1 (AWG1) � Chanel 2 (AWG2)

- Selecting Ch. 2 AWG2 Generator opens the second Generator. Run All, Stop All will

control both Generators.

- Generator AWG2 � Custom � File � Four Setps.csv � open � open. This file was

created in exel (1 column with 4 rows of values: 1, 2, 3, 4).

- Adjust Gen. Frequency, Amplitude and Offset.

Use Figure 1:

- Connect substrate to source

- Disconnect AWG2

- From XY Plot determine VTR.

- Substrate – Source Files.

Figure 2: Effect of bulk (substrate) voltage on the drain current

VGS

Increasing vSB

Di

Figure 3: Sample waveform generator and source current vs. substrate-source voltage

MIC94050 TRANSISTOR CHARACTERISTICS EXPERIMENT ON

ANALOG DISCOVERY

The experiment shows Id as function of Vds, with Vgs parameter using the Analog Discovery

and Waveform tool.

- Build the circuit as shown in the schematic diagram of Figure 4 and connect the Analog

Discovery instruments as indicated. Connect Scope probes and power supply of the

Analog Discovery as shown.

- Start the Digilent WaveForm software

The initial settings of the generated waveforms are:

• AWG1: generates Vss. A triangle browses the range from (0V…-5V) = (Amplitude =

2.5V, Offset=-2.5V, frequency=200Hz).

• AWG2: generates VG. There are 11 steps uniformly distributed in the range (create an

exel file to generate 11 steps). Adjust Amplitude=100mV, Offset=300mV,

frequency=10Hz.

• Set Vsb=1.5V (i.e., Vb=-1.5V).

Figure 4: Schematic diagram to obtain I-V characteristics of the p-channel MOSFET

Scope channels:

• C2: the voltage drop across R1

• C1: the source to drain voltage drop

• M1: Math channel calculating C2/R1 = Id current. Since C2 is expressed in Volts and R1

value is given as Ohm, M1 is expressed in Amps

• M2: Math channel calculating C1/R1*C2 = Is*Vs = P, the power dissipated by the transistor.

Expressed in Watts.

• M3: Math channel calculating (C1+C2) = Vss, as generated by AWG1.

• Main time plot: shows the time diagrams. Only C1, M1 are activated, to keep the image

clean.

• XY#1: an XY representation M1 which is Is(Vs) as function of C1. There are multiple

branches, corresponding to different values of Vg. Get the printout of the waveform, label the

values of VGS for each curve.

• Measurements: The average value of P (transistor dissipated power) = M3. Expressed in

Watts.

Results:

• Adjust Vsb (in step of 500mV) by adjusting the potentiometer P1. Observe, record, describe,

and explain the change in IV characteristic of the transistor due to the change of Vsb.

• Can the source is connected to +5V instead of GND? If so, re-draw the schematic diagram of

the circuit (no need to set-up the circuit).

AWG1 (Scope 2-)

MIC94050

VG

(AWG2)

(Analog GND,

Scope 1+)

Vd (Scope 2+, Scope 1-)

R1, 560

Vb

-5V (from

Diligent)

P1, 50K

Sample of the scope windows of MIC94050 characteristic:

In the AWG window, observe the preview for the two generated signals: Vg and Vss.

• in the oscilloscope Main Time plot, activate M2 to see Vss. Notice the voltage drop at high

currents, as AWG limitation. The triangle signal distortion does not affect the XY

representation, except the high current branches are a bit shorten (upper right end).

Uncheck M2 to return to the clean image.

• in the oscilloscope Main Time plot, C1 shows the source voltage, Vs, while M1 shows the

source current, Is.

• the XY#1 window shows the Is(Vds) characteristic. Each branch is generated during a single

step in the Vgg signal. The direction of browsing Vss (rising or falling) does not matter in the

XY representation.

o Quiz: what happens if you change the wave shape of AWG2 from triangle to

sinusoid?

• the Measurement window shows the transistor dissipated power. This value is computed as

average for the displayed time frame.

• You can start a scope ZOOM window to see the time domain and XY view corresponding to

the Zoom1 rectangle in the main time window. Click and drag the Zoom1 rectangle to see

what portion of the time diagram corresponds to each branch in the XY view.

• Explore further in amplitude, frequency, offset of AWG1 and AWG2. Change time scale,

offset, range, … on C1,C2,M1,M2,M3,M4 as necessary. Report what you have explored and

observed with clear explanation the characteristic of an n-MOSFET.

OPTIONAL:

BS107 N-CHANNEL TRANSISTOR CHARACTERISTICS EXPERIMENT

ON ANALOG DISCOVERY

Revise the above method to obtain an IV characteristic of the n-channel MOSFET

BS107. Describe your work, draw the schematic diagram, build the circuit, take measurements,

and obtain the IV characteristic.

Figure 5: Setup for BS107A IV characteristic experiment

RD,560

G D

S

To

AWG2

To AWG1

+

Channel 2

_

+

Channel 1

_ BS107A

Sample scope window of BS107 IV characteristic:

SOME DETAILED INFORMATION IN THE EXPERIMENT:

Some hints for experiment building:

- To generate the “stairs” signal of AWG2, start from an Xcel file with 1,2,3,4,5 in 5 rows.

Save that in .csv or .txt format (source file) and then import in the AWG (set Channel 2 to

“Custom”, click “File” and select the source file). When imported, the data in the source

file is scaled both in time and range domain:

o Each value in the source file is replicated as needed, such a way to fulfill the

AWG buffer (in our case, each of the 100 records generated 20 (or 21) samples, to

fill the 2048 samples in the AWG buffer.

o Each value in the source file is scaled to (-100%...+100%) range. The smallest

value results in -100%, the biggest one results in +100% and all other data is

linearly interpolated.

- After importing the file, the amplitude, frequency and offset can be set to fit the needed

signal specs. Frequency is here understood as the “buffer iteration frequency”; 10Hz

means that the whole stair sequence takes 100mS (10ms/step).

- To generate the “triangle” signal of AWG1, set the initial Phase to 270 degrees. This way

the project uses each rising and falling ramp for a value of Vbb. Frequency is set to 50Hz,

to result in 10ms for each rising or falling ramp.

- To hide the “connections” between branches of the XY view of the characteristic family,

the XY view of the Oscilloscope is set (by default) to show only points. You might

change to the curve mode by clicking in the Oscilloscope window: Settings-Options-

Display-XY dots = False.

- To keep absolute synchronism between AWG1 and AWG2, I set “Auto sync” mode,

With AWG2 as Master. I also set “Repeat: Infinite”, for AWG2 channel. Then I clicked

“Run All” instead of individually starting the two channels. “Auto sync” mode re-

synchronizes channels at the largest period of the two channels (100ms from AWG2, in

this case).

Appendix 1 – MIC94050 Specifications: http://www.micrel.com/_PDF/mic94050.pdf