TABLE OF CONTENTS The bipolar junction transistor (BJT) was the first solid-state active electronic device. Before the BJT, electronic amplifiers were based on vacuum tubes. The concepts on which BJTs were based were experimentally and theoretically established by Bardeen, Brattain, and Schokley at the Bell Telephone Laboratories during 1948. The era of semiconductor-based electronics, which has had an enormous influence on the way we live today, actually began with the invention of the BJT.
A number of alternative semiconductor devices have been developed since the first BJTs, notably MOSFETs and MESFETs. Nonetheless, the BJTs are still used, as there are applications in which the BJTs still offer the best performance. In addition, there are applications in which the BJTs are combined with MOSFETs, even in integrated-circuit technology. It should also be noted that the BJT principles are frequently used in a number of specifically designed semiconductor devices.
This section introduces the BJT principles. The BJT can be used as both a voltage-controlled current source (an analog device) and a voltage-controlled switch (digital device). Both modes of operation are considered to present a complete description of the device. At the end of the section, the BJT is compared to the MOSFET.
The essential characteristic of a current source is that its current does not depend on the voltage drop across the current source. In other words, it delivers a constant current at any voltage.
A reverse-biased P-N junction is a semiconductor implementation of a...
