Chapter 5 Bi - polar Junction Transistor

5.1. BJT Fundamentals. In this section we discuss the main principles behind the operation of the BJT. The NPN bipolar transistor consists of 2 NP-PN junctions, as depicted in Fig. 5.1.a. The bipolar device is composed of two diodes back to back with different doping profiles; the BJT has 3 terminals labeled as E(Emitter), B(Base) and C(Collector). The emitter is the most doped terminal and emits the majority of the carriers to flow through the base-emitter junction, for that reason this is the most doped terminal. The base-emitter diode is usually forward biased such that large amount of current can be flow through this junction. The base is a very thin layer such that most of the carriers being injected from the emitter into the base have enough energy to travel throughout the base and are collected at the collector terminal if this terminal has enough potential energy; for that purpose, the potential at the collector terminal must be more positive than the emitter’s potential (typically > 0.3 V). A small number of carriers are recombined in the base generating a small base current, but the idea is that most of the carriers reach the collector terminal generating enough output current. The number of electrons reaching the collector terminal is determined by the base-emitter potential (similarly to the current generated in a forward biased diode). Thus the output current (ic) is controlled by the base-emitter voltage, leading to an operation similar to a voltage controlled current source, or transconductance amplifier: input is voltage and output is current.