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Transistor Construction
The transistor is a three-layer semiconductor device consisting of either two n and one p -type layers of material or two p - and one n -type layers of material
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Both types are shown
in Figure 3
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The emitter layer is heavily doped, with the base and collector only lightly
doped
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As shown in Figure 3
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150/0
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The doping of the sandwiched layer is also considerably less than
that of the outer layers (typically, 1:10 or less)
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Again shown in Figure 3
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The abbreviation BJT, from bipolar junction
transistor, is often applied to this three-terminal device
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If only one carrier is employed (electron or hole), it is considered a unipolar
device is considered

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0 The transistor construction
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2 Transistor Operation
The basic operation of the transistor will now be described using the pnp transistor
of Figure 4
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The operation of the npn transistor is exactly the same if the roles played by
the electron and hole are interchanged
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0(a), the pnp transistor has been
redrawn without the base-to-collector bias
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The depletion region has been
reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from
the p - to the n -type material
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0(a)
as shown in Figure 4
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Consider the similarities between this situation and that of the
reverse-biased diode
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One p–n junction of a transistor is reversebiased, whereas the other is forward-biased
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0 Biasing a transistor: (a) forward-bias; (b) reverse-bias
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0 both biasing potentials have been applied to a pnp transistor, with the
resulting majority- and minority-carrier flows indicated
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0 the widths of the
depletion regions, indicating clearly which junction is forward-biased and which is reversebiased
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0, a large number of majority carriers will diffuse across
the forward-biased p–n junction into the n -type material
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Since the sandwiched n -type material is very thin and has a low conductivity, a
very small number of these carriers will take this path of high resistance to the base
terminal
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The larger number of
these majority carriers will diffuse across the reverse-biased junction into the p -type
material connected to the collector terminal as indicated
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In other words, there has been an injection of
minority carriers into the n -type base region material
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Applying Kirchhoff’s current law (KCL) to the transistor of Figure 5
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The
collector current, however, comprises two components—the majority and the minority
carriers as indicated in Figure 5
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The minority-current component is called the leakage
current and is given the symbol 𝐼𝐶𝑂 (𝐼𝐶 current with emitter terminal 𝑂pen)
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𝐼𝐶𝑂 , like 𝐼𝑆 for a reverse-biased diode, is
temperature sensitive and must be examined carefully when applications of wide

temperature ranges are considered
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Improvements in construction techniques have
resulted in significantly lower levels of 𝐼𝐶𝑂 , to the point where its effect can often be
ignored
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0 Majority and minority carrier flow of a pnp transistor



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