REPORT EE331-LAB3 Laboratory-3 JFET and MOSFET Characterization Introduction: The objectives of this experiment are to observe the operating characteristics of junction field-effect transistors (JFET's) and metal-oxide-semiconductor field-effect transistors (MOSFET's). Some basic methods for extracting device parameters for circuit design and simulation purposes are also examined. Procedure 1 Discrete MOSFET gate lead, sex, and mode identification Measurements: - Using the DMM in the resistance mode to find the lead on the 2N7000 MOSFET which does not conduct to any of the other leads, in either polarity Measuring the resistance between each pairs of the leads assigned as 1, 2 and 3 from above gives the following results: Table 1
Transcript
REPORT EE331-LAB3
Laboratory-3
JFET and MOSFET Characterization
Introduction: The objectives of this experiment are to observe the operating characteristics of
junction field-effect transistors (JFET's) and metal-oxide-semiconductor field-effect
transistors (MOSFET's). Some basic methods for extracting device parameters for
circuit design and simulation purposes are also examined.
Procedure 1 Discrete MOSFET gate lead, sex, and mode identification
Measurements:
- Using the DMM in the resistance mode to find the lead on the 2N7000 MOSFET which does not conduct to any of the other leads, in either polarity
Measuring the resistance between each pairs of the leads assigned as 1, 2 and 3 from above gives the following results:
Table 1
leadx leady Resistance between x and y
1 2 ∞
1 3 1.406 MΩ
2 3 ∞
From the table 1, lead 2should bethe gate.
- Switching the DMM to the diode test function and determining the source and drain leads from the direction that the transient protection diode allows current to pass (from source to drain). For the 2N7000 MOSFET, this also verifies that it is an n-channel device.
Table 2
Lead x Lead y Voltage drop from x and y indicated by the
diode test function
1 3 0.5544 V
3 1 3.2476 V
Figure 1 MOSFET design
In a diode test mode, the larger voltage reading indicates the reverse-bias polarity of the diode, while a
forward-bias polarity of the diode would be indicated by a typical turn-on voltage of
about 0.6 Volts or so. Therefore, lead 1 should be the Source and lead 2 should be
the Drain.
- Use the DMM to measure the conduction from drain to source
R = 4.5 Ω, which is negligible => the MOSFET is in an enhancement mode.
Questions
a) Refer to the above measurements! b)
c) From the datasheet, 2N7000 is an n-channel enhancement mode field effect
transistor having the placements of the leads shown below. This suggests that all of our “educated” speculations are correct.
d)
e) If the gate oxide were destroyed, electric current could pass the Gate to the Drain
and Source, which would make Resistance measured between lead 1, 2 and lead 2, 3 different (not infinity anymore).
Procedure 2 Measurement of I-V Characteristics of a MOSFET
Set-Up + We build the MOSFET circuit as shown below in the schematic of Fig. E3.2a.
Figure E3.2a
VGG M1
DUT DC PS2
VGS
RC 100
VDD
VDS
GND DC PS
DMM2 (+)
DC PS 1
GND DMM2(-)
DMM1 (-)
VRC DMM1 (+)
+ The excitation voltage VDD is from DC power supply. This voltage is applied
across the series connection of a current limiting resistor RC = 100 Ω, the drain-source leads of the device
under test (DUT), and the drain current sensing resistor R = 100 Ω. Thus, VDD = VDS + VRC.
+ The voltage of DMM1 is equivalent to the voltage VRC, while the voltage from
DMM2 is the voltage VDS.
Measurement-2
Working Steps:
+ Adjust the DC power supply 2 VGG to produce voltages +1.0 Volts. Then adjust
DC power supply 1 from 0.0 V to 10.0 V with increment 1.0 V.
+ Measure the VRC and VDS with DMMs.
+ Record the drain current and VDS in a table.
+ Calculate the drain current: ID =VRC/100.0.
+ Then we increase the DC power supply 2 0.5 V and repeat till measuring four
different (ID,VDS) pairs.
From the 4 above tables, we plot the I-V characteristics (ID vs VDS) of MOSFET.
+ For VGS=1 V, we can see that the drain current is very small or we can tell that its value
is nearly zero.
+ For VGS=1.5 V, the drain current is also very small, nearly zero amp.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9x 10
-8 I-V characteristics of MOSFET
Drain-source voltage
Dra
in-s
ourc
e c
urr
ent
+ For VGS=2.0 V, the drain current is larger than the two previous cases.
+ For VGS=2.5 V, the drain current increases. The drain current reaches its saturation
value at VDS ≈0.901V
0 2 4 6 8 10 120
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1x 10
-7 I-V characteristics of MOSFET
Drain-source voltage
Dra
in-s
ourc
e c
urr
ent
0 1 2 3 4 5 6 7 8 9 100
1
2
x 10-4 I-V characteristics of MOSFET
Drain-source voltage
Dra
in-s
ourc
e c
urr
ent
+ For VGS=3.0 V, the drain current increases apparently compared to previous cases. The
drain current reaches its saturation value at VDS ≈1.61V
0 1 2 3 4 5 6 7 8 90
0.002
0.004
0.006
0.008
0.01
0.012
0.014
X: 0.901
Y: 0.012
I-V characteristics of MOSFET
Drain-source voltage
Dra
in-s
ourc
e c
urr
ent
0 1 2 3 4 5 6 7 80
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
X: 1.61
Y: 0.0794
I-V characteristics of MOSFET
Drain-source voltage
Dra
in-s
ourc
e c
urr
ent
Question-2 (a) Scan through the measurement results and find the value of VGS which just starts
to produce a non-zero drain current. This is a first approximation to the threshold voltage VT of the
MOSFET under test.
(b) Pick a few values of VGS for which the drain current ID shows a clearly defined
saturation. Find the value of VDS at which the drain current ID reaches its saturation
value and then compare this actual value of VDS,sat to a computed value of VGS – VT.
Comment on how close these values agree. How well does the textbook theory
predict the measured behavior of a real MOSFET?
(c) Using an electron mobility value of n = 800 cm2/V-s and a gate oxide thickness
of xox = 80 nm, compute the value of k = nCox, and from this value and a few of the
measured data points (where the drain current is saturated) make a rough estimate of
the W/L ratio for the 2N7000 MOSFET. Does this W/L ratio seem reasonable?
Answer:
a. From the data in 4 tables, we can see that the value of VGS which starts to
produce a non-zero drain current is 2.0V, which is approximation to the
threshold voltage VT of the MOSFET.
b. The drain current ID shows a clearly defined saturation at VGS being 2.5V
and 3.0V
With VT = 2.0 V, VDS = 0.901V => VDS, sat = 2.5 – 2.0 = 0.5V
This value is approximate to the actual value of VDS, sat = 0.901V
With VT = 2.0 V, VDS = 1.61V => VDS, sat = 3 – 2.0 = 1.0V
This value is approximate to the actual value of VDS, sat = 1.61V The textbook theory predicts the measured behavior of a real MOSFET pretty well.
We came up with that conclusion because from the datasheet, the width and
length value of 2N7000 MOSFET are W = 9.7mm, L = 2µm,respectively
Or
=
Procedure 3 Output conductance effects
Set up: Repeat previous procedure (procedure 2) to produce (ID, VDS) plots but locate a 10 kΩ
5% 1/4 W resistor, and connect this resistor in parallel with the drain and source terminals of
the MOSFET on the solderless breadboard (pins 7 & 8).
Measurements: Adjust the DC power supply 2 VGG to produce voltages +1.0 Volts. Then
adjust DC power supply 1 from 0.0 V to 10.0 V with increment 1.0 V.
Measure the VRC and VDS with DMMs
Record the drain current and VDS in a table in your notebook. The drain current
is equal to VRC/100.0. Increase the DC power supply 2 0.5 V and repeat till
measuring four different (ID,VDS) pairs.
VGG = 1V Table 3.1:
VDD(V) VRC(V) VDS(V) ID(A)
0 0 0 0
1 0 0.99 0
2 0.02 1.98 0
3 0.01 3.03 0
4 0 4 0
5 0.1 4.91 0
6 0 5.99 0
7 0.04 6.96 0
8 0 8.02 0
9 0 8.99 0
10 0.1 9.97 0.001
The I-V Characteristic when VGG=1V
VGG=1.5V
Table 3.2:
VDD(V) VRC(V) VDS(V) ID(A)
0 0 0 0
1 0 1 0
2 0.1 1.97 0
3 0 3.03 0
4 0 4 0
5 0.11 4.98 0.001
6 0.1 5.99 0.001
7 0.09 7 0.001
8 0.1 8.02 0.001
9 0.05 8.96 0.001
10 0.09 9.91 0.001
The I-V Characteristic when VGG=1.5V
VGG = 2V Table 3.3:
VDD(V) VRC(V) VDS(V) ID(A)
0 0 0 0
1 0.01 0.98 0
2 0.02 1.99 0
3 0.05 3 0
4 0 3.97 0
5 0 4.98 0
6 0.02 5.99 0
7 0.02 6.98 0
8 0.05 7.96 0
9 0 8.96 0
10 0.04 9.97 0
The I-V Characteristic when VGG=2V
VGG=2.5V
Table 3.4:
VDD(V) VRC(V) VDS(V) ID(A)
0 0 0 0
1 0.881 0.119 0.00881
2 1.16 0.84 0.0116
3 1.4 1.6 0.014
4 1.7 2.3 0.017
5 2.03 2.97 0.0203
6 2.43 3.57 0.0243
7 2.79 4.21 0.0279
8 3.17 4.83 0.0317
9 3.54 5.46 0.0354
10 3.92 6.08 0.0392
The I-V Characteristic when VGG=2.5V
VDD(V) VRC(V) VDS(V) ID(A)
0 0 0 0
1 0.96 0.034 0.0096
2 1.92 0.068 0.019
3 2.88 0.11 0.0288
4 3.65 0.17 0.0365
5 4.81 0.22 0.048
6 5.71 0.28 0.0571
7 6.66 0.34 0.066
8 7.66 0.44 0.076
9 8.3 0.74 0.083
10 8.5 0.77 0.085
The I-V Characteristic when VGG=3V
Questions :
a) Discuss qualitatively what effect the addition of the 10 kΩ resistor has on the
MOSFET output characteristics
- When we added a 10Ω resistor in parallel with the MOSFET, it will make the output
resistor decrease the output conductance will increase.
- And the output voltage in this case is VDS’ = VDD-VRS.
b) From the first measurement of MOSFET M1 without the resistor being present, and
select a value of VGS which shows a clean saturation of the drain current. Select a
few points within the saturated region of the curve and calculate the slope of the
output characteristics in units of Ω-1
. Then take the reciprocal of this value to
obtain the inverse slope in units of Ω. These values are the output conductance and
resistance, respectively.
We choose VGS = 3V,
Point # 1: ID = 0.0794A, VDS = 1.61V
Point # 2: ID = 0.0801A, VDS = 2.52V
Hence the conductance:
And the resistance:
P = 1/ = 1300 Ω
c) From the second measurement of MOSFET M1 with the resistor added, and perform
the same analysis on the same VGS curve to find the output conductance and resistance for
this case.
Choose VGS = 3V,
Points # 1: ID = 0.076A, VDS = 0.44V
Points # 2: ID = 0.083A, VDS = 0.74V
Hence the conductance:
And the resistance:
P = 1/ = 42.86Ω
d) Discuss how closely these measured values match to the empirical device equation
IDsat = 0.5 k(VGS - VT)2 (1 + VDS)
(Where is the output conductance parameter in units of V-1.)
And out IDsat = 0.076 A
Procedure 4 JFET gate lead, sex, and mode identification
Measurements:
- Using the DMM in its ohmmeter setting to test pairs of leads on the JFET and therefore identify the gate lead on the device. From the polarity which causes the gate terminal to conduct, deducing whether the JFET is an n-channel or p-channel device.
Measuring the resistance between each pairs of the leads assigned as 1, 2 and 3 from
above gives the following results:
- Table 1
lead x lead y Resistance between x and yand Resistance
between y and x
1 2 6.37 MΩ and ∞
1 3 6.34 MΩ and ∞
2 3 15.43 KΩ and 14.49 KΩ
From the table 1, lead 1, 2 and 3 should be the Gate, the Drain and the Source, respectively.
- Using the DMM, again in its ohmmeter setting, to determine whether the device is a
depletion-mode (D-mode, or normally-ON) or an enhancement-mode (E-mode, or normally-OFF) device. The resistance between S and D is not negligible (15.43 KΩ and 14.49 KΩ) => the JFET is operating in Depletion-mode
Questions
a) Refer to the above measurements b) JPET package and the leads
c) Not, it is not, because the Drain and the Source are connected and are basically symmetric, hence having the same potential.
d) From the datasheet, MPF102 is an n-channel enhancement JFET having the placements of the leads shown below. This suggests that all of our “educated” speculations are correct.
e) Flow chart for testing any JFET with an ohmmeter which can be used to conclusively determine which lead is the gate, whether the device is an n-channel or p-channel, and whether the device is a D-mode or an E-mode.
Conclusion:
From this experiment, we learn how to identify the leads, sex and mode of some MOSFET and JFET
devices as well as some gate protection diode of MOSFETs in CMOS. Moreover, we pay much attention
the operating regions of some MOSFET and the conditions for cutoff region, triode region or saturation
