AN-957 (v.Int)
Measuring HEXFETCharacteristics
(HEXFET is the trademark for International Rectifier Power MOSFETs)
Topics covered:
• Converting the nomenclature from bipolars to MOSFETs
• P-Channel HEXFET Power MOSFETs
• Initial settings
• Breakdown
• Drain leakage
• Gate threshold
• Gate leakage
• Transconductance
• On-resistance
• Diode drop
• Characteristics in synchronous rectification
• Transfer characteristics
• Measurements without a curve tracer
• Device capacitances
• Switching times
• Gate charge
• Reverse recovery
• A fixture to speed-up testing time
• Related topics
1. General
Curve tracers have generally been designed for making measurements on bipolar transistors. While power MOSFETs can be
tested satisfactorily on most curve tracers, the controls of these instruments are generally labeled with reference to bipolar
transistors, and the procedure to follow in the case of MOSFETs is not immediately obvious. This application note describes
methods for measuring HEXFET Power MOSFET characteristics, both with a curve tracer and with special-purpose test circuits.
Testing HEXFET Power MOSFETs on a curve tracer is a simple matter, provided the broad correspondence between bipolar
transistor and HEXFET Power MOSFET features are borne in mind. Table 1 matches some features of HEXFET Power
MOSFETs with their bipolar counterparts. The HEXFET Power MOSFET used in all the examples is the IRF630. The control
settings given in the examples are those suitable for the IRF630. The user must modify these values appropriately when testing a
different device. The IRF630 was selected since it is a typical mid-range device with a voltage rating of 200 volts and a
continuous current rating of 9 amps (with TC = 25°C). For measurements with currents above 20 amps, or for pulsed tests not
controlled by the gate, the Tektronix 176 Pulsed High
Current Fixture must be used instead of the standard test fixture.
The IRF630 is an N-channel device. For a P-channel device, all the test procedures are the same except that the position of the
Polarity Selector Switch must be reversed—that is, for P-channel devices, it must be in the PNP position.
The curve tracer used as an example in this application note is a Tektronix 576, since this instrument is in widespread use.
However, the principles involved apply equally well to other makes and models. Figure 1 shows the layout of the controls of the
Tektronix 576 curve tracer, with major controls identified by the names used in this application note. Throughout this application
note, when controls are referred to, the name of the control is printed in capitals. For all tests, when the power is on, the initial
state of the curve tracer is assumed to be as follows:
• LEFT/ RIGHT switch in “off” position
• VARIABLE COLLECTOR SUPPLY at zero
• DISPLAY not inverted
• DISPLAY OFFSET set at zero
• STEP/OFFSET POLARITY button OUT (not inverted)
• VERT/HORIZ DISPLAY MAGNIFIER set at NORM (OFF)
• The REP button of the STEP FAMILY selector should be IN
• The AID button of the OFFSET selector should be IN
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AN-957 (v.Int)
• The NORM button of the RATE SELECTOR should be IN
Figure 1. Location of controls in a 576 curve tracer
The accuracy of all tests is predicated on the correct use of the Kelvin connections, as indicated in the instructions for the curve
tracer. This is particularly important for power semiconductors, as inductive and resistive drops across sockets and wiring are
significant.
Some tests require the use of high voltages. After the device is mounted in the test fixture as described for each test, the test
fixture safety cover should be closed and the curve tracer manufacturer's safety warnings heeded. The exposed metal parts of
many HEXFET Power MOSFETs (for example, the tab of TO-220 devices) are connected to the drain and are therefore at the
potential of the collector supply.
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As with any semiconductor device, some of the characteristics of HEXFET Power MOSFETs are temperature dependent. For
tests in which there is significant heating of the HEXFET Power MOSFET, a low repetition rate should be used. For tests
involving a slow transition through the linear region, a damping resistor of at least 10 ohms should be connected in series with
the gate, close to the gate lead to prevent oscillations. If frequent testing of MOS-gated devices is expected, the use of a test
fixture that plugs directly into the curve tracer would save a significant amount time. Such a fixture is descibed in Section 12.
MOS-gated transistors are static sensitive. Wrist straps, grounding mats and other ESD precautions must be followed, as
indicated in INT-955.
2. BVDSS
This is the drain-source breakdown voltage (with VGS = 0). BVDSS should
be greater than or equal to the rated voltage of the device, a t the specified
leakage current.
1. Connect the device as follows: drain to “C”, gate to “B”, source to
“E”.
2. Set the MAX PEAK VOLTS to 350V.
3. Set the SERIES RESISTOR to limit the avalanche current to a safe
value (i.e., tens of milliamps). A suitable value in this case would be
14 kOhms.
4. Set the POLARITY switch to NPN.
5. The MODE control should be set to NORM.
6. HORIZONTAL VOLTS/DIV should be set at 50 volts/div on the
"collector" range.
7. VERTICAL CURRENT/DIV should be set at 50 microA/div.
8. On the plug-in fixture, the CONNECTION SELECTOR should be set to “SHORT” in the “EMITTER GROUNDED” sector.
This action grounds the gate and disables the step generator.
9. Connect the device using the LEFT/RIGHT switch. Increase the collector supply voltage using the VARIABLE
COLLECTOR SUPPLY control until the current (as indicated by the trace on the screen) reaches 250 microA. (See Figure
2.) Read BVDSS from the screen.
3. IDSS
This is the drain current for a drain-source voltage of 100% of rated
voltage, with VGS = 0 . This measurement is made i n the same manner as
BVDSS, except that:
1. The MODE switch is set to “LEAKAGE”.
2. Connect the device using the LEFT/RIGHT switch and adjust the
collector supply voltage to the rated voltage of the HEXFET Power
MOSFET (200V for the IRF630). Read the value of IDSS from the
display (see Figure 3). The vertical sensitivity may need altering to
obtain an appropriately sized display. Often IDSS will be in the
nanoamp range and the current observed will be capacitor currents
due to minute variations in collector supply voltage.
4. VGS(th)
This is the gate-source voltage which produces 250 microA of drain current (VDS = VGS). At this gate-source voltage the device
enters the active region. In circuits where devices are connected i n parallel, switching losses can be minimized by using devices
with closely matched threshold voltages. This test requires the gate to be connected to the drain and conducted as follows:
1. Connect the device as follows: source to “E”, gate to “B”, drain to “C”. This connection arrangement may require the
construction of a special test fixture. Bending of the device leads can cause mechanical stress which results i n the failure of
the device.
50
µµ
A
PER
V
E
R
T
DIV
50
V
PER
H
O
R
Z
DIV
1
ηη
A
PER
V
E
R
T
DIV
50
V
PER
H
O
R
Z
DIV
Figure 2. Drain-source breakdown voltage
Figure 3. Drain-source leakage current
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2. Set the MAX PEAK VOLTS to 15V.
3. Set the SERIES RESISTOR to 0.3 ohms.
4. Set POLARITY to PNP. This causes the drain (collector) terminal
to be negative with respect to the source (emitter) terminal.
5. Set the MODE CONTROL to NORM.
6. Set the VERTICAL CURRENT/DIV to 50 microA/div.
7. Set the HORIZONTAL VOLTS/DIV to 500 mV/div.
8. Set the CONNECTION SELECTOR to “SHORT” in the
“EMITTER GROUNDED” sector.
9. DISPLAY should be inverted.
10. Connect the device using the LEFT/ RIGHT switch. Increase the
VARIABLE COLLECTOR VOLTAGE until the drain current
reaches 250 microA as indicated by the trace on the screen. Read
the voltage on the horizontal center line (since this line corresponds
to ID = 250 mA) (see Figure 4).
5. I GSS
This is the gate-source leakage current with the drain connected to the
source. An excessive amount of gate leakage current indicates gate oxide
damage.
1. The device is connected as follows: gate to “C”, drain to “B”, source
to “E”. This is not the usual connection sequence, and a special test
fixture will be required if bending of the leads is to be avoided.
2. Set MAX PEAK VOLTS to 75V.
3. Set the SERIES RESISTOR to a low value (for example, 6.5 ohms).
4. Set POLARITY to NPN.
5. Set the MODE switch to LEAKAGE.
6. Set the CONNECTION SELECTOR to the “SHORT” position i n the
“EMITTER GROUNDED” sector.
7. HORIZONTAL VOLTS/DIV should be set at 5V/div.
8. VERTICAL CURRENT/DIV should be set to an appropriately low range.
9. Connect the device using the LEFT/RIGHT switch. Increase the collector supply voltage using the VARIABLE
COLLECTOR SUPPLY control, but do not exceed 20V, the maximum allowable gate voltage. It may be necessary to adjust
the vertical sensitivity. Read the leakage current from the display (see Figure 5). I n many cases, the leakage current will be i n
the nanoamp range, in which case the trace will be dominated by currents which flow through the device capacitance as a
result of minute fluctuations in the collector supply voltage.
10. The above procedure is for determining gate leakage current with a positive gate voltage. To make the same measurement
using a negative voltage, reduce the VARIABLE COLLECTOR SUPPLY voltage to zero, change the POLARITY switch to
the PNP position, and reapply the voltage (see Figure 6). The trace will take time to settle because of the gate-source
capacitance.
6. gfs
This is the forward transconductance of the device at a specified value of
ID. gfs represents the signal gain (drain current divided by gate voltage) i n
the linear region. This parameter should be measured with a small ac
superimposed on a gate bias and the curve tracer is not the appropriate
tool for this measurement. Even with specific test equipment, as
indicated in Section 11, the dc bias tends to overheat the MOSFET very
rapidly and care should be exercised to insure that the pulse is suitably
short.
50
µµ
A
PER
V
E
R
T
DIV
500
m
V
PER
H
O
R
Z
DIV
1
ηη
A
PER
V
E
R
T
DIV
5
V
PER
H
O
R
Z
DIV
1
ηη
A
PER
V
E
R
T
DIV
5
V
PER
H
O
R
Z
DIV
Figure 4. Gate-source threshold voltage
Figure 5. Gate-source leakage current at +20 V
Figure 6. Gate-source leakage current at -20 V
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Figure 8. Drain-source resistance
Figure 9. Source-drain voltage (diode)
7. RDS(on)
This is the drain-source resistance at 25°C with VGS = 10V. Since RDS(on) is temperature-dependent, it is important to minimize
heating of the junction during the test. A pulse test is therefore used to measure this parameter. The test is set up in the following
manner:
1. Connect the device as follows: gate to “B”, drain to “C”, source to “E”.
2. Set the MAX PEAK VOLTS to 15 V.
3. Set the SERIES RESISTOR to 0.3 Ohms.
4. The POLARITY switch should be set to NPN.
5. The MODE switch should be set to “NORM”.
6. Set the STEP AMPLITUDE to 1V.
7. Set NUMBER OF STEPS to 10.
8. Set OFFSET MULT to 0.
9. The CURRENT LIMIT should be set to 500 mA.
10. The STEP MULTIPLIER button should be OUT—that is, 0.1X not
selected.
11. On the PULSED STEPS selector, the 80 microsec button should be IN
(or the 300 microsec, if the 80 is not available).
12. On the RATE selector, the 0.5X button should be IN.
13. Set VERTICAL CURRENT/DIV at 1 amp/div (IRF630). This scale
should be chosen according to the on-resistance of the device being
tested
14. Set the CONNECTION SELECTOR to the “STEP GEN” position in
the “EMITTER GROUNDED” sector.
15. Connect the device using the LEFT/RIGHT switch and raise the
VARIABLE COLLECTOR SUPPLY voltage until the desired value of
drain current is obtained. RDS(on) is obtained from the trace by reading
the peak values of current and voltage (see Figures 7and 8).
RDS(on) = VDS/ID.
Logic level devices would have different settings for 6, 7 and 8 so that the
on-resistance is measured at the specified gate voltage
8. VSD
This is the source-drain voltage at rated current with VGS = 0. It is the forward voltage drop of the body-drain diode when
carrying rated current. If pulsed mode testing is required, use high current test fixture.
1. Connect the device as follows: gate to “B”, drain to “C”, source to “E”.
2. Set the MAX PEAK VOLTS to 15V.
3. Set the SERIES RESISTOR at 1.4 ohms or a value sufficiently low
that rated current can be obtained.
4. Set POLARITY to PNP.
5. Set MODE to "NORM".
6. The 80 microsec button of the PULSED STEPS selector should be IN
(or the 300 microsec, if the 80 is not available).
7. The CONNECTION SELECTOR should be set to the "SHORT"
position in the “EMITTER GROUNDED” sector.
8. HORIZONTAL VOLTS/DIV should be on 200 mV/div.
9. VERTICAL CURRENT DIV should be on 1 amp/div.
10. The DISPLAY button should be set to invert.
1
A
PER
V
E
R
T
DIV
500
mV
PER
H
O
R
Z
DIV
1
A
PER
V
E
R
T
DIV
500
mV
PER
H
O
R
Z
DIV
1
A
PER
V
E
R
T
DIV
200
mV
PER
H
O
R
Z
DIV
Figure 7. Drain-source resistance, pulsed mode
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11. The device is connected using the LEFT/RIGHT switch. Increase the VARIABLE COLLECTOR SUPPLY voltage until rated
current is reached (9A for the IRF630). Read VSD from the trace (see Figure 9).
9. Composite Characteristics
The forward and reverse characteristics of the HEXFET Power MOSFET may be viewed at the same time. This display can be
used to obtain an appreciation of the HEXFET Power MOSFET’s behavior i n applications in which current flows i n the channel
in either direction. such as synchronous rectifiers and analog waveform switching. The procedure is the same as for on-resistance
except that:
1. OFFSET is set to zero.
2. The POLARITY control is set at "AC".
3. The device is connected using the LEFT/RlGHTswitch. The VARIABLE COLLECTOR SUPPLY voltage is increased to
obtain the required peak value of ID. Beware of device heating. Figure 10 shows the trace obtained with the IRF630. To obtain
the reverse characteristics of the diode alone, invert the step polarity. The FET is inoperative, and the display will resemble
that shown in Figure 11. The step polarity should also be inverted to obtain the composite characteristics of P-channel
devices.
10. Transfer Characteristics
The transfer characteristic curve of ID versus VGS may be displayed using
the pulse mode. The test is set up in the same manner as the on-
resistance test, except for the following:
1. OFFSET MULTIPLY should be set at zero.
2. Set HORIZONTAL VOLTS/DIV on “STEP GEN”.
3. The 300 microsec button of the PULSED STEP SELECTOR should
be IN.
4. Increase the VARIABLE COLLECTOR SUPPLY voltage to obtain
the trace shown in Figure 12. The transfer characteristic is outlined
by the displayed points.
2
A
PER
V
E
R
T
DIV
200
mV
PER
H
O
R
Z
DIV
2
A
PER
V
E
R
T
DIV
200
mV
PER
H
O
R
Z
DIV
1
A
PER
V
E
R
T
DIV
PER
H
O
R
Z
DIV
Figure 10. Operation in first and third
quadrant (synchronous rectification) Figure 11. Operation in first and third
q
uadrant without
g
ate drive
Figure 12. Transfer characteristic (ID vs Vgs)
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11. Measurement Of HEXFET Power Mosfet Characteristics
Without A Curve Tracer
HEXFET Power MOSFET parameters can be measured using
standard laboratory equipment. Test circuits and procedures for
doing this are described in the following sections, with the IRF630
used as an example. the test arrangement should be varied
appropriately for other devices.
BVDSS, Drain-Source Breakdown Voltage
The test circuit is shown in Figure 13. The current source will typically consist of a power supply with an output voltage
capability of about 3 time BVDSS in series with a current defining resistor of the appropriate value.
When testing high voltage HEXFET Power MOSFETs it may not be practical or safe to use a supply of 3 times BVDSS. In such
cases, another type of constant current source may be used.
VGS(th), Threshold Voltage
The test circuit is shown in Figure 14. The 1 kohm gate resistor is required to suppress potentially destructive oscillations at t he
gate. the current source may be derived from a voltage source equal to the gate voltage rating of the HEXFET Power MOSFET
and a series resistor.
VDS(on), On-Resistance
The test circuit is shown in Figure 15. The pulse
width should be 300 microsec at a duty cycle of less
than 2%.
The value quoted is at a junction temperature of 25ºC.
RDS(on) is calculated by dividing VDS(on) by I D. Connect
the ground of the gate supply as close to the source
lead as possible.
gfs, Transconductance
Connect a 50V power supply
between the device drain and
source, as shown in Figure 16. Use
a current probe to measure ID.
A signal generator operating at low
duty cycle to prevent heating of the
device, is used to obtain 80
microsec pulses of the required
voltage (VGS) to obtain the
following currents:
0.015 x ID , 0.05 x ID , 0.15 x ID , 0.5
x ID , and 1.5 x ID where ID is the
rated value at TC = 25ºC. Plot a
graph of VGS versus ID.
The transconductance is equal to the slope of the graph at the appropriate value of drain current.
DVM
BV
DSS
S
D
G
250
µµ
A
DVM
VGS(th)
S
D
G
250
µµ
A
1K
ΩΩ
BV
DSS
S
D
G
ID
PULSE COMMAND
10V
OSCILLOSCOPE
or DVM
+
-
Figure 13. Test circuit for BVDSS
Figure 14. Test circuit for gate threshold voltage
Figure 15. Test circuit for drain on-state voltage
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Figure 17. Test circuit for input capacitance
Figure 18. Test circuit for output capacitance
Ciss, Coss, and Crss. Output,
Input and Reverse
Transfer Capacitances
A 1 MHz capacitance bridge
is used for all these tests.The
capacitance to be measured
is connected in series with a
capacitance of known value
to provide dc isolation. If Cu
is the unknown capacitance,
Ck is the known capacitance,
and Cm is the measured
capacitance, then Cu can be
calculated as follows:
(1)
Figures 17, 18 and 19 show the circuit connections for the three capacitances that characterize power MOSFETs.
50V
OSCILLOSCOPE
+
-
100
ΩΩ
CP
C = C C
C -C
uk m
k m
HIGH
+
-
10 M
ΩΩ
S
G
LOW
CB
2000 pF
CK
1.0
µµ
F
HIGH
+
-
10 M
ΩΩ
S
G
LOW
CB
2000 pF
CK
1.0
µµ
F
D
Figure 16. Test circuit for transconductance
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Figure 19. Test circuit for transfer capacitance
Turn-on Delay
Time, Rise Time,
Turn-Off Delay
Time, Fall Time
Data sheet value
are for a resistive
load, as shown in
Figure 20, as well
as individual data sheets. The gate pulses
should be just long enough to achieve
complete turn-on, with a duty cycle of the
order of 0.1%.
The series resistor is as specifyed in the data
sheet. The definitions of rise, fall and delay
times are given in Figure 21.
Power MOSFETs can switch in nanoseconds. Unless the test circuit is laid-out with RF techniques, the measurements will be
totally unreliable. Switching time measurements frequently amount to a characterization of the test circuit, rather than the device
under test. Gate charge provides a better indication of the switching capability of power MOSFETs.
Qg, Qgs, Qgd Total Gate Charge, Gate-Source Charge, Gate-Drain Charge
The total gate charge has two
components: the gate-source charge
and the gate-drain charge (often
called the Miller charge).
INT-944 gives more details on this
test. Figures 22 and 23 show the test
circuit and waveforms.
From the relationship Q = ∫i, the
following results are obtained:
25V
10M
LOW
HIGH
CB or
DVM
2000 pF
10:1
S
G
DVDS
VGS
RD
RG
10V +
-
TO THE
OSCILLOSCOPE
0.5 x RATED V
DS
td(on)
ON DELAY
TIME
tdoff
OFF DELAY
TIME
tr
RISE TIME
tf
FALL TIME
VDS
VGS
90%
10%
Figure 20. Test circuit for switching times
Figure 21. Switching time waveforms
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Figure 22. Test circuit for gate charge
Figure 25. Test fixture for HEXFET test
Qg = (t3 - t0) ig,
Qgd = (t2 - t1) ig,
Qgs = Qg - Qgd
VSD Body-Drain Diode Voltage Drop
The current source may consist or a voltage
source and a series resistor, as shown in Figure
24.
The voltage should be applied in short pulses
(less than 300 microsec) with a low duty cycle
(less than 2%).
trr, Qrr Body-Drain Diode Reverse Recovery Time and Reverse Recovery Charge
Several test circuits are commonly used to charac-terize these parameters. Some have been qualified by JEDEC. The data sheet
indicates the test method used for the specific device.
12. A fixture to speed-up
testing time
The most commonly tested
parameters in a MOS-
gated transistor are gate-
source leakage (IGSS),
drain-source resistance
(RDS(on)), breakdown
voltage (BVDSS), drain
current (IDSS), source-
drain voltage (VSD),
threshold (VGS(th)), and so
on. These tests can be
greatly simplifyed with the
fixture shown in Figure25.
VGS
10V
V2
V1
t0t1t2t3t
OSCILLOSCOPE
or DVM
S
D
G
IS
330
ΩΩ
G
30V
30V
S
DSENSE
SSENSE
1
2
3
4
1
2
3
4
1
2
3
4
CSENSE
ESENSE
C
B
E
TO CURVE TRACER TO PVT
Figure 24. Test circuit MOSFET diode dropFigure 23. Gate charge waveforms
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POSITION MEASUREMENT COMMENT
1I
GSS C sense disconnected, Drain Source S/C connected, Collector Voltage applied to
gate via 330Ω resistor. Note: Gate protected by back to back 30V zeners.
2R
DS(on) Collector Voltage applied to DrainBased Voltage applied to Gate via 330Ω resistor.
3B
VDSS / IDSS / VDS Collector Voltage applied to Drain, Gate Source S/C connected via 330Ω resistor.
4V
GS(th) Collector Voltage applied to Drain, Gate Drain S/C connected via 330Ω resistor.
Related topics:
Parameter definition in IGBTs
Gate Charge
Thermal characteristics
ESD sensitivity
ESD test methods
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Data and specification subject to change without notice.
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