Keithley 7022, Instruction Manual

Page: 66 / 107

Operation
5-12
Common-emitter characteristic curves
A profile of the transistor operating characteristics can be
obtained by measuring the collector current over a specified
voltage range (V
CE
) for different base bias currents (I
B
). For
example, Figure 5-10 shows the characteristics of a typical
NPN silicon transistor at base bias currents (I
B
) of 20µA,
40µA, 60µA, and 80µA.
Extensive trigger capabilities facilitate synchronization of
the Keithley Source Measure Unit operations. By perform-
ing a subordinate sweep, SMU #1 will perform a staircase
sweep at every base bias current level set by SMU #2. On
every step of each staircase sweep, SMU #1 will source a
voltage level (V
CE
) and measure the subsequent collector
current (I
C
). For the characteristics shown in Figure 5-10,
four staircase sweeps were performed; one staircase sweep at
each base bias level.
Figure 5-10
Common-emitter characteristics of an NPN silicon
transistor
Refer to a Keithley Source Measure Unit instruction manual
for details on performing sweeps.
Measurement considerations
Many measurements made with the Model 7022 are subject
to various effects that can seriously affect low-level measure-
ment accuracy. The following paragraphs discuss these
effects and ways to minimize them.
Path isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system. Ideally,
the path isolation should be infinite, but the actual resistance
and distributed capacitance of cables and connectors results
in less than infinite path isolation values for these devices.
Path isolation resistance forms a signal path that is in parallel
with the equivalent resistance of the DUT, as shown in
Figure 5-11. For low-to-medium device resistance values,
path isolation resistance is seldom a consideration; however,
it can seriously degrade measurement accuracy when testing
high-impedance devices. The voltage measured across such
a device, for example, can be substantially attenuated by the
voltage divider action of the device source resistance and
path isolation resistance, as shown in Figure 5-12. Also,
leakage currents can be generated through these resistances
by voltage sources in the system.
Figure 5-11
Path isolation resistance
0 1 2 3 4 5
10
8
4
2
6
V , volts
CE
I
, ma
c
+80
µ
a
+60
µ
a
+40
µ
a
+20
µ
a
I = 0
B
R
E
DUT
DUT
R
PATH
V
R
IN
DUT
Matrix
Card
Measure
Instrument
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Measuring Instrument
R
DUT
E
DUT
R
PATH
R
IN
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