As a first step, the forward Gummel plot is measured. In
order to measure the forward Gummel characteristics of the
transistor, the Agilent 4156C is programmed as a voltage or
current source. For measuring the Gummel characteristics, the
instrument is programmed to sweep emitter-base voltage from
o to I V. Both the base and collector currents are measured.
The forward current gain is plotted as the ratio of the measured
collector current to the base current. The output characteristics
of the BJTs are measured using either forced base-emitter
voltage (kept constant), or a forced base-current (kept
constant). In the measurement collector-emitter voltage is
swept and the collector current is measured. Figure 3 is a
screenshot showing the Gummel plot and the extracted SPICE
parameters; the current gain and other associated parameters
for a NPN BJT.