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25
Gm
(transconductance) Test
FET Leakage Tests
1.
Be
sure
the
ON-OFF switch
is
in
the
OFF
(out)
position_
2_
Identify each lead
of
the
F
ET
to
be
tested;
then
connect
the
F ET
to
the
Tester.
3_
Press
the
F ET switch_
4_
Press
the
Gm =a switch_
5.
Press
the
100
pA
switch.
6.
Identify
the
FET
type
(N
channel
or
P channel).
Then
place
the
P CHAN - N CHAN switch
in
the
appropriate
position.
7. Press
the
ON-OFF switch
to
the
ON
(in) position.
8. Adjust
the
SET
Gm = a
control
to
place
the
meter
pointer over
the
a mark on
the
Gm scale.
9. Press
the
Gm switch. Read
the
Gm directly from
the
Gm scale. Multiply this reading by
1000
to
obtain
Gm.
If
the
Gm
is
00
or
0,
the
F ET
is
defective.
10. Press
the
GATE 1 switch.
The
Gm should decrease.
If
the
Gm does
not
change
or
increases, first check
to
make sure
the
F ET
is
correctly
connected
to
the
Tester and
the
P CHAN - N CHAN switch
is
in
the
proper position.
If
the
Gm still does
not
change,
or
if
it increases,
the
FET
is
defective.
11.
If
the
FET being tested has
two
gates, press
the
GATE
2 switch.
The
Gm should decrease.
If
the
Gm does
not
change,
or
if
it
increases,
the
F ET
is
defective.
The previous steps have explained
the
Gm testing procedure.
If
the
device
is
out
of
the
circuit,
the
following leakage
test
will further
test
the
FET
. Disregard
the
next
two
steps if
you are
gOing
to
perform
the
leakage tests.
12. Release
the
ON-OFF switch
to
the
OFF
(out)
position.
13. Disconnect
the
FET from
the
Tester.
Leakage tests must always be
performed
with
the
FET
out
of
the
circu it, since
the
resistances
of
the
circu it could cause
erroneous
readings.
Two
leakage measurements (lgss and Idss) will be made
in
the
following steps.
Igss
is
the
measurement
of
the
leakage
current
(I) between
the
gate
(g)
and
the
source
(s)
with
the
drain
shorted
(s)
to
the
source. Since
this
leakage
current
is
nominally
in
the
nanoampere range,
any
measurable leakage indicates
the
FET
is
defective.
Idss
is
the
measurement
of
the
current
(I) between
the
drain
(d) and
the
source
(s)
with
the
gate
shorted
(s)
to
the
source.
This
measurement
is
not
an actual leakage
current,
but
more
of a forward
current
between
100
pA
and
lOrnA
to
indicate
that
the
F ET
is
conducting.
The Idss measu
rement
is
often
helpful
when
selecting
FET's
(matching)
that
have similar
characteristics.
NOTE :
Be
sure
the
"Gm
Test"
has been
completed
before
you perform
the
following leakage tests.
1.
Press
the
Igss switch.
2.
Be
sure
the
100
/1A
switch
is
pressed. Read
the
leakage
current
directly
from
the
leakage scale.
Any
measurable leakage indicates
that
the
F
ET
is
defective.
3. Press
the
lamA
switch.
4. Press
the
Idss switch. Read
the
current
directly from
the
leakage scale.
If
necessary, select a lower
current
range
to
obtain
a
meter
indication.
5.
Release
the
ON-OFF switch
to
the
OFF
(out)
position.
6.
Disconnect
the
F ET from
the
Tester.
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