Semiconductor Fundamentals Unit 5 – Transistor Junctions & PNP DC Bias
Exercise 1 – Testing the Junctions of a Transistor
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to test a transistor by forward biasing
and reverse biasing the junctions. You will verify your results with an ohmmeter.
DISCUSSION
- A transistor is operating properly if it amplifies correctly, does not go into breakdown under
operating voltages, and leakage current is within tolerance. - Commercial transistor testers exist; however, an ohmmeter can also be used to test a
transistor. - Simple ohmmeter tests can show if a transistor is open or shorted, is a PNP or NPN type, or
has excessive leakage. - Transistors, for testing, can be considered to consist of two diodes the base-emitter PN
junction and the base-collector PN junction. - A good NPN transistor has base-emitter and base-collector junctions that conduct when
forward biased and do not when reverse biased. No current flow should be present between
the collector and emitter. - The same is true for a PNP transistor; however, care should be taken when connecting the
ohmmeter. For example: to forward bias the base-emitter junction, connect the positive lead
to the emitter and the negative lead to the base. - If an ohmmeter indicates “overload” or off-scale when connencted to forward bias a junction,
the transistor is damaged. - If an ohmmeter reading indicates current flow (low resistance) when connected to reverse
bias a transistor junction, the transistor is damaged. - A very low ohmmeter reading across the collector and emitter indicates a short or high
leakage current and, therefore, a damaged transistor. - Directions for testing with LAB-VOLT DIGITAL MULTIMETER, DIGITAL
MULTIMETER WITH A DIODE TEST FUNCTION, and an ANALOG OHMMETER are
presented in this exercise.