Semiconductor Laboratory














NPN bipolar transistor measurements with a sourcemeter

This page describes how to use a Keithley 2600 Series Sourcemeter to measure the characteristics of an NPN bipolar transistor. We will start with a common base configuration.

Common base
The base is grounded and a current is passed from the emitter to the base so that the emitter-base junction is forward biased (\(I_E\) is negative for an NPN transistor). The current between the collector an base is then plotted as a function of collector-base voltage. Most of the electrons injected into the base diffuse across it and are collected at the reverse biased collector-base junction. The collector current is almost equal to the emitter current. Although the currents are the same at the input and output, the voltage at the output can be much larger than at the input. This circuit exhibits power gain.

The base is the middle lead of the transistor. Connect the Hi of channel A to one of outside leads of the transistor, the Hi of channel B to the other outside lead of the transistor and the common ground to the base at the middle lead.

Download the Python script and the Python library Keithley 2600 Python library and save them in your bipolar-transistor directory .

Review the script to see what it does and then run it. Swap the outer leads (emitter and collector) and run the script again. One of these orientations is forward-active mode (which is usually what is wanted) and the other is reverse-active mode. The transistor will work in either direction but it suffers from punchthrough at low voltages in reverse-active mode. In forward-active mode, the emitter is connected to channel B. For negative collector-base voltages, both junctions are forward biased and the collector current is dominated by the forward biased collector-base junction. When both junctions are forward biased, this is called saturation.

Common emitter
A bipolar transistor is put in a common emitter configuration to determine the current gain. Connect the Hi of channel A to the collector, the Hi of channel B to the base, and the common Lo to the emitter. Download the script.

Review this script to see what it does then run it. The current gain is the ratio of the collector current to base current,


It is also possible to swap the emitter and collector terminals here and you will once again see that punchthrough is an issue.

The slope of the curves is caused by the Early effect. Use the data file to make plots of \(\beta=I_C/I_B\) as a function of \(V_{CE}\).