The Bipolar Junction Transistor (BJT)

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The Bipolar Junction Transistor (BJT)
The transistor, derived from transfer resistor, is a three terminal device whose resistance between two terminals is controlled by the third. The term bipolar reflects the fact that there are two types of carriers, holes and electrons which form the currents in the transistor. If only one carrier is employed (electron or hole), it is considered a unipolar device like field effect transistor (FET).
The transistor is constructed with three doped semiconductor regions separated by two pn junctions. The three regions are called Emitter (E), Base (B), and Collector (C).
Physical representations of the two types of BJTs are shown in Figure (1–1). One type consists of two n -regions separated by a p-region (npn), and the other type consists of two p-regions separated by an nregion (pnp).
Figure (1-1) Transistor Basic Structure The outer layers have widths much greater than the sandwiched p– or n–type layer. The doping of the sandwiched layer is also considerably less than that of the outer layers (typically, 10:1 or less). This lower doping level decreases the conductivity of the base (increases the resistance) due to the limited number of “free” carriers. Figure (1-2) shows the schematic symbols for the npn and pnp transistors 1 College of Electronics Engineering - Communication Engineering Dept.

Figure (1-2) standard transistor symbol
Transistor operation
Objective: understanding the basic operation of the transistor and its naming
In order for the transistor to operate properly as an amplifier, the two pn junctions must be correctly biased with external voltages. The basic operation of the transistor will now be described using the npn transistor. The operation of the pnp transistor is the same as for the npn except that the roles of the electrons and holes, the bias voltage polarities and current directions are all reversed.
Figure (1-3) shows both the pnp and npn transistors with the proper
DC biasing. Notice that in both cases the base-emitter junction is forward biased and the base-collector is reverse biased
Figure (1-3) transistor forward reverse bias Before the transistor is biased there are two depletion regions. What is happens inside the transistor when it is forward and reverse bias is the forward biased from the base to emitter narrows the BE depletion region as shown in figure (1-4). This will result in a heavy flow 2 College of Electronics Engineering - Communication Engineering Dept.

of majority carriers (electrons) from the emitter to the base as indicated by the wide arrow.
The base region is slightly doped and very thin so that it has a very limited number of holes. Thus, only small percentage of all the electrons flowing across the BE junction combine with the available holes. These relatively few recombined electrons will form the small base current (IB).
The reverse biased from base to collector widens the BC depletion region as shown in figure (1-4). Consider the similarity between this situation and that of the reversed biased diode. Recall that the flow of majority carriers is zero, resulting in only a minority carriers flow. The free electrons move through the collector region, into the external circuit, and then return into the emitter region along with the base current, as indicated. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter.
Transistor currents
Applying Kirchhoff’s current law to the transistor of figure (1-4) we obtain
𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
The collector current comprises two components
𝐼𝐶 = 𝐼𝐶 𝑚𝑎𝑗𝑜𝑟𝑖𝑡𝑦 + 𝐼𝐶𝑂 𝑚𝑖𝑛𝑜𝑟𝑖𝑡𝑦
Those for a pnp transistor notice that the arrow on the emitter inside the transistor symbols points in the direction of conventional current (holes current).
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Figure (1-4) transistor operation 4 College of Electronics Engineering - Communication Engineering Dept.

Transistor Categories

Manufacturers generally classify bipolar junction transistors into

three broad categories:

1- General-Purpose/Small-Signal



purpose/small-signal transistors are generally used for low- or

medium-power amplifiers or switching circuits. Figure (1-5) show

small signal transistors.

Figure (1-5) Small Signal Transistors
2-Power Transistors: - Power transistors are used to handle large currents (typically more than 1 A) and/or large voltages.
Figure (1-6) Examples of Power Transistors
3-RF Transistors: - RF transistors are designed to operate at extremely high frequencies and are commonly used for various purposes in communications systems and other high frequency applications. Figure (1-7) show examples of RF transistors.
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Figure (1-7) show examples of RF transistors
The DMM Diode Test Position
A digital multimeter can be used as a fast and simple way to check a transistor. For this test, you can view the transistor as two diodes connected as shown in Figure (1-8) for both npn and pnp transistors.
An ohmmeter or the resistance scales of DMM can be used to check the state of the transistor. A good diode will show an extremely high resistance (or open) with reverse bias and a very low resistance with forward bias.
A defective open diode will show an extremely high resistance (or open) for both forward and reverse bias.
A defective shorted or resistive diode will show zero or a very low resistance for both forward and reverse bias.
Many digital multimeters (DMMs) have a diode test position that provides a convenient way to test a transistor. In Figure (1-8a), the red (positive) lead of the meter is connected to the base of an npn transistor and the black (negative) lead is connected to the emitter to forward-bias the base-emitter junction.
If the junction is good, you will get a reading of between approximately 0.6 V and 0.8 V, with 0.7 V being typical for forward bias.
In Figure (1-8b), the leads are switched to reverse-bias the baseemitter junction, as shown. If the transistor is working properly, you will typically get an OL indication. The process just described is repeated for the base-collector junction as shown in Figure (1-8c) and (d).
For a pnp transistor, the polarity of the meter leads are reversed for each test.
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Figure (1-8) Typical DMM test of a properly functioning npn transistor. Leads are reversed for a pnp transistor.
Transistor configurations
Objective: shows the transistor connection configurations and the difference between them
As we have seen, the bipolar transistor is a three-terminal device. Three basic single transistor amplifier configurations can be formed; depending on which of the three transistor terminals is used as signal
ground (i.e. which terminal is common to both the input and the output side of the configuration). These three basic configurations are appropriately called common emitter, common collector (emitter follower), and common base. Figure (1-9)
shows the three basic configurations for npn transistor.
Figure (1-9) transistor configuration
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Common Emitter Configuration
Figure (1–10) shows a common-emitter configuration for pnp and npn transistors. The common-emitter (CE) configuration has the
emitter as the common terminal, or ground, to an ac signal. VBB forwardbiases the base-emitter junction, and VCC reverse-biases the basecollector junction. This configuration is the most frequently encountered transistor configuration.
Figure (1-10) Common Emitter Configuration DC Beta (βDC)
The common-emitter, forward-current, amplification factor
(or dc current gain) is the ratio of the dc collector current (IC) to the dc base current (IB) and is designated DC Beta (βDC)
𝛽 = 𝐼𝐶 ≅ 𝐼𝐸 𝐷𝐶 𝐼𝐵 𝐼𝐵
Typical values of 𝛽𝐷𝐶 range from less than 20 to 200 or higher. 𝛽𝐷𝐶 is usually designated as an equivalent hybrid (h) parameter, ℎ𝐹𝐸, on transistor datasheets
𝛽𝐷𝐶 = ℎ𝐹𝐸 Example Determine the dc current gain 𝛽𝐷𝐶 and the emitter current 𝐼𝐸 for a transistor where 𝐼𝐵 = 50µ𝐴 and 𝐼𝐶 = 3.65 𝑚𝐴.
Two sets of characteristics are necessary to describe fully the behavior of the common-emitter configuration: one for the output or
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collector-emitter circuit and the other for the input or base-emitter circuit. Both are shown in Fig. (1-11).
Figure (1-11) Characteristics of a silicon transistor in the common-emitter configuration: (a) collector characteristics; (b) base characteristics.
When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of transistor in the “on” or active region the base-to emitter voltage is 0.7 V
𝑉𝐵𝐸 = 0.7 𝑉 𝑓𝑜𝑟 𝑠𝑖𝑙𝑖𝑐𝑜𝑛 𝑎nd 0.3V for ge𝑟𝑚𝑎𝑛𝑖𝑢𝑚 There are three basic regions as indicated in the figure (1-11a). These regions are
1-The Active region: the collector-base junction is reverse-biased, while the base-emitter junction is forward-biased.
2-The Saturation region: the collector-base and base-emitter junctions are forward-biased. When the base-emitter junction becomes forward-biased and IB is increased, IC also increases and VCE decreases as a result of more drop across the collector resistor (VCE =VCC-ICRC). This is illustrated in Figure (1–12). When VCE reaches its saturation value, VCE(sat), the basecollector junction becomes forward-biased and IC can increase no further even with a continued increase in IB. VCE(sat) for a transistor occurs somewhere below the knee of the collector curves, and it is usually only a few tenths of a volt.
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Figure (1–12)

3-The cutoff region: the collector-base and base-emitter junctions of a transistor are both reverse-biased. When IB=0, the transistor is in the cutoff region of its operation. This is shown in Figure (1–13). With the base lead open, resulting in a base current of zero. Under this condition, there is a very small amount of collector leakage current, ICEO, due mainly to thermally produced carriers. Because ICEO is extremely small, it will usually be neglected in circuit analysis so that
Figure (1–13)


DC Bias

Bias establishes the dc operating point operation of an amplifier. The dc operation of a transistor circuit can be described graphically using a dc load line. This is a straight line drawn on the characteristic curves from the saturation value where IC=IC(sat) on the y-axis to the cutoff value where VCE=VCC on the x-axis, as shown in Figure (1–14).

(Q-point) for proper linear Figure (1–14) DC load line

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The Bipolar Junction Transistor (BJT)