Transistor Testing - Keithley 7014 Manual

Operation
4.4.3 Transistor testing:
Typical transistor tests that can be performed with the
aid of the Model 7014 include current gain tests, leak-
age tests, as well as tests to determine the common-
emitter characteristics of the device. The following
paragraphs discuss these tests and give typical equip-
ment configurations for the tests.
For example, a resistor that measures exactly 10061 at
25°C with a temperature coefficient of 100ppm/"C
should not change more than 1OmR per "C of temper-
ature change. That resistor measured at 35°C should
read between 99.900 and lOO.lOOR (lOOn +lOOmQ.
Temperahwe coefficient is calculated from the follow-
ing equation:
Current
gain tests
The DC or static common-emitter current gain of a
transistor can be determined by biasing the transistor
for a specific value of base current, IB, and then mea-
suring the collector current, E. The DC common-emit-
ter current gain, p, of the transistor is then determined
as follows:
Figure 4-8 shows the test configuration and equivalent
circuit for the current gain test. The Model 224 Current
Source is used to source the base current, IB, The Model
230 Voltage Source supplies the collector-emitter volt-
age, VCE, and the collector current, F, is measured by
the Model 196 DMM. Switching among the transistors
being tested is, of course, performed by the Model 7014
multiplexer card.
In order to perform the current gain test, the voltage
source is first set to the desired value of VCB The cur-
rent source is then set to a base current value that will
result in the desired value of k as measured by the
DMM. The current gain can then be calculated as out-
lined above.
TC = (AR) (lob)
(RI (AT)
where:
TC = temperature coefficient in ppm/'C
AR = change in resistance (reference resistance
test resistance)
R = actual resistance at the reference temperature
AT = change in temperature (reference tempera-
ture - test temperature)
Typically, several samples of a particular resistor from
a vendor will be tested to verify the specifications. The
temperature coefficient is usually checked at several
temperature points to ensure its integrity over a range
of temperatures.
Evaluation of resistors can be done with a Model 7014
card in a Model 7001 mainframe, along with a Model
2001 multimeter to make temperature and 4-terminal
resistance measurements. Temperature coefficients are
calculated with respect to the resistance measurement
made at a reference temperature. Thermal EMFs gene
ated by connections in the test circuit are cancelled by
the offset compensated ohms feature of the Model
2001.
In order to reduce errors caused by voltage burden, use
a higher current range on the Model 196 DMM. Doing
so will result in the loss of one or two decades of reso-
lution, but 3% or 4M-digit resolution will probably be
adequate for most situations.
Figure 4-8 shows a system that can test accuracy and
temperature coefficient of up to 29 resistors that have
the same specifications (resistance and temperature co-
efficient.
4.4.4 Resistor temperature coefficient testing
Temperature coefficient is the rate of change of resis-
tance with respect to temperature, typically expressed
as ppm/"C (parts per million per degree centigrade).
For further information on resistor temperature coeffi-
cient testing, see the following reference:
Simplified Resistor Temperature Coefficient Test Sys-
tern Using Model 196. Keithley Instruments, AppIica-
tion Note 811, 1987.
4.10