(Figure 25C). The entire family of curves moves in
the direction of higher collector current as the tem
perature build-up continues. This regenerative or
"runaw
ay
" phenomena may easily destroy the
transistor if not stopped immediately.
CAUTION
Testing procedures should not be con
ducted at sweep voltage levels and
base current step levels that cause any
of the previously mentioned conditions.
Testing at values which cause exces
sive temperature is probably not repre
sentative of typical operating condi
tions for the transistor and may damage
or destroy the device.
LOOP IN CURVES
j
I
j
...
;
, ......
,
I--· j·--·------, ...... .....
, .................
.... ,l··--·····:1::
/J
/
/
I
/:.
-
'
, k.?"
,
.....
�.�r:...
i,....-
-
V
.
'
r,,;,;.
-
-
�
--·C
i,:-;:
!
1
�
....
L)
'
;
.
...
..
..
CURVES MOVING OFF SCALE
, ..........
•
COLLECTOR CURRENT CONTINUOUSLY INCREASING
11
VERTICAL ROLL
11
EFFECT
TESTING FET'S
Comparison to Bipolar Tests
In many respects, the testing of field effect transis
tors (FET's) including metal-oxide-silicon field effect
transistors (MOS FET's), is similar to testing NPN
and PNP transistors A family of curves is displ
ay
ed
on the oscilloscope in each case and the curves have
a similar appearance. N channel FET's have a fam
ily of curves similar to NPN transistors (Figure 26),
and P Channel FET's curves are similar to PNP tran
sistors. Transistor curves are a graph of collector
current vs collector voltage at various base currents;
FET curves are a graph of drain current vs drain
voltage at various gate voltages. FET breakdown
voltage may be observed and measured by the
same method used for transistors.
In several other respects, testing FET's is different
from testing transistors. For testing FET's, the STEP
SELECTOR switch of the curve tracer is placed in the
"Volts per Step" positions. The curve tracer sup
plies constant voltage steps rather than constant
current steps. Also, the polarity of the step voltage
is reversed in relation to the sweep voltage. While
the zero base current step of a transistor usually pro
duces no collector current, the zero volt step at the
gate of an FET produces the highest drain current.
Each reverse bias voltage step results in less drain
current, and when the gate voltage is sufficiently
high, drain current is pinched off. The point of pinch
off can be measured with the curve tracer. The
method of gain measurement of an FET is similar to
the gain measurement of a transistor, but the for
ward transconductance of a FET has a voltage input
characteristic which cannot be directly compared
with the beta of a transistor which has a current
input characteristic.
Typical Test Procedure
1. Before plugging in or connecting the FET to the
curve tracer, set the controls as follows:
SWEEP VOLTAGE:
zero
VERTICAL SENSITMTY: 1 ma/Div
STEP SELECTOR:
Ioss/ICES
POLARITY:
Depends on device
21
