Bipolar Junction Transistors
Spring 2019

ECE 110

Course Notes

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A bipolar junction transistor (BJT) has three terminals, called base, collector, and emitter. The BJT exhibits a tremendously useful property: a small current flowing into the base causes a much larger current to flow into the collector. Moreover, if the small current into the base increases, so does the large current into the collector (up to a maximum saturation value). If the current into the base decreases to zero, so does the current into the collector.

Figure 1

The symbol for an NPN BJT
Fig. 1: The symbol for an NPN BJT. The currents $I_B$ and $I_C$ are labeled as flowing into the base and collector, respectively. The respective voltage drops $V_{BE}$ and $V_{CE}$, from base and collector to emitter, are labeled in standard labeling. The power absorbed by a BJT is given by $P = V_{BE} I_B + V_{CE} I_C$.

The idea that the current into the base causes a larger current into the collector can be expressed as:
I_C = \beta I_B, \label{BJT-EQ1}
where $\beta$ is called the DC current gain. As long as $I_C$ has not saturated at its maximum, we treat $\beta$ as a constant (usually between 30 and 100). This property means that a BJT can be used as an amplifier or a switch.

Every BJT has a datasheet published by its manufacturer. The datasheet provides comprehensive information about the physical characteristics and maximum ratings of the device. For example, you can look up which terminals are the base, collector and emitter. You can also find approximate values of the DC current gain (which we call $\beta$) listed on the datasheet as $h_{FE}$.

Figure 2

Fig. 2: A BJT. You can read the model number on the body of the BJT; this one is model P2N2222A. To find its datasheet, search the web for P2N2222A datasheet.

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BJTs are made in two different semiconductor structures, called NPN and PNP. NPN BJTs are the kind we consider in ECE 110.

Figure 3

Semiconductor structure of an NPN BJT
Fig. 3: Semiconductor structure of an NPN BJT. An NPN BJT is fabricated as a thin layer of p-type semiconductor sandwiched between two thicker n-type semiconductors. The base is connected to the p-type and the collector and emitter to the n-types.

The base-emitter connection is a p-n junction. If this p-n junction is OFF with $V_{BE}$ less than a threshold voltage $V_{BE,\text{on}}$, the charge carriers are separated. This separation blocks both currents into the base and collector, so $I_B=I_C=0$. If the base-emitter p-n junction becomes ON and $V_{CE}$ is large enough, the charge carriers in this junction spread and allow both $I_B$ and $I_C$ to flow. In ECE 340: Solid State Electronic Devices, you will learn why increasing $I_B$ also increases $I_C$ until it reaches its maximum saturation value.