A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an NPN transistor, a thin and lightly doped P-type base is sandwiched between a heavily doped N-type emitter and another N-type collector; while in a PNP transistor, a thin and lightly doped N-type base is sandwiched between a heavily doped P-type emitter and another P-type collector. In the following we will only consider NPN BJTs.
In many schematics of transistor circuits (especially when there exist a large number of transistors in the circuit), the circle in the symbol of a transistor is omitted. The figures below show the cross section of two NPN transistors. Note that although both the collector and emitter of a transistor are made of N-type semiconductor material, they have totally different geometry and therefore can not be interchanged.
All previously considered components (resistor, capacitor, inductor, and diode) have two terminals (leads) and can therefore be characterized by the single relationship between the current going through and the voltage across the two leads. Differently, a transistor is a three-terminal component, which could be considered as a two-port network with an input-port and an output-port, each formed by two of the three terminals, and characterized by the relationships of both input and output currents and voltages.
Depending on which of the three terminals is used as common terminal, there can be three possible configurations for the two-port network formed by a transistor:
Two voltages and are applied respectively to the emitter and collector , with respect to the common base , so that the BE junction is forward biased while the CB junction is reverse biased.
Note that the polarity of and direction of associated with the PN-junction between E and B are the same as those associated with a diode, voltage polarity: positive on P, negative on N, current direction: from P to N, but and the direction of associated with the PN-junction between the base and collector are defined oppositely.
The behavior of the NPN-transistor is determined by its two PN-junctions:
The current gain or current transfer ratio is defined as the
ratio between the emitter (input) current and the collector (output)
The CB configuration can be considered as a 2-port circuit. The input port is formed by the emitter and base, the output port is formed by the collector and base. The relationships between the current and voltage of both the input and output ports are described by the following input and output characteristics.
The input current is a function of as well as the input
voltage , which is much more dominant:
The output current is a function of the output voltage
as well as the input current , which is much more dominant:
Two voltages and are applied respectively to the base
and collector with respect to the common emitter . As typically
, the BE junction is forward biased but the CB junction is
reverse biased, same as the CB configuration. The voltages of CB and CE
configurations are related by:
The base current is treated as the input current, and the collector
current is treated as the output current:
The two parameters and are related by any of the
The CE configuration can be considered as a 2-port circuit. The input port is formed by the base and emitter, the output port is formed by the collector and emitter. The relationships between the current and voltage of both the input and output ports are described by the following input and output characteristics.
Same as in the case of common-base configuration, the EB junction of the
common-emitter configuration can also be considered as a forward biased
diode, the current-voltage characteristics is similar to that of a diode:
The CB junction is reverse biased, the current depends on the current . When , , the current caused by the minority carriers crossing the PN-junctions. When is increased, is correspondingly increased by fold.
The relationship between the input and output currents of both CB and CE configurations is summarized below:
The collector characteristics of the common-base (CB) and common-emitter (CE) configurations have the following differences:
Various parameters of a transistor change as functions of temperature. For example, increases along with temperature.