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Fig. 7 shows
that a p-n junction biased in the reverse direction is equivalent to a high-resistance element (low current for a given applied voltage), while a junction biased in the forward direction is equivalent to a low-resistance element (high current for a given applied voltage). Because the power developed by a given current is greater in a high-resistance element than in a low-resistance element (P = I2R),
power gain can be obtained in a structure containing two such resistance elements if the current flow is not materially reduced. A device containing two p-n junctions biased in opposite directions can operate in this
fashion.
Bipolar Transistors
All bipolar transistors consist of three layers of semiconductor material (usually silicon) referred
to as emitter, base, and collector. The resultant structure forms two back-to-back p-n junctions. The input (emitter-base) junction serves as the source, or injector, of current carriers; the output
(base-collector) junction collects the injected current carriers. During normal operation, the emitter-base p-n junction is forward-biased, and the collector-base p-n junction is reverse-biased.
As explained in the section on Silicon Rectifiers, a p-n junction biased in the reverse direction
is equivalent to a high-resistance element, while a junction biased in the forward direction is equivalent to a low-resistance element. The electric field across the forward-biased junction
overcomes the energy barrier at the junction and causes holes to be injected into the n-type region and electrons to be injected into the p-type region. Because of the large number of free
electrons in the n-type region and of holes in the p-type region, the injected holes and electrons are referred to as minority-charge carriers. A forward-biased p-n junction, therefore, is a
minority-carrier injector, and the number of minority carriers injected is dependent upon the magnitude of the forward-bias voltage.
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Charge-Carrier Flow-When a symmetrical p-n junction is forward-biased, the lifetime of the injected minority carrier is very short.
Because of the many free electrons in the n-type region, a hole injected into this region is not likely to penetrate very far before it meets an electron and is annihilated (i.e., neutralized), as shown in Fig. 13. Similarly, any electron injected into the p-type region is usually quickly neutralized by one of the numerous holes in this region. In a symmetrical p-n junction, therefore, injected minority carriers cannot penetrate very far or last very long before they are annihilated
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Fig. 14(a) shows a nonsymmetrical p-n junction in which the n-type region is made very thin and the p-type region is much more heavily
doped. When this junction is forward-biased, an injected hole is much less likely to be annihilated by an electron before it crosses to the end of the thin n-type region. Moreover, because of the heavy dopmg of the p-type region, more holes are injected into the n-type region than there are free electrons in this thin region. Consequently, even though some injected holes are annihilated by free electrons, most of them are able to survive and penetrate the full width of the n-type region, as shown in Fig. 14(a). Similarly, in a forward-biased p-n junction in which the p-type region is very thin and the n-type region is much more heavily doped, an injected electron is unlikely to meet (and be neutralized by) a hole before it penetrates to the end of the thin p-type region, as shown in Fig. 14(b)
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In bipolar transistors, a thin lightly doped semiconductor layer (base region). is sandwiched
between two wider (emitter and collector) semiconductor layers that are much more heavily doped with an opposite type of impurity from the dopant used in the thin base layer. The two
nonsymmetrical back-to-back p-n junctions that result may form either a p-n-p or an n-p-n transistor
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Fig. 15 shows the layer structure and the corresponding schematic symbol for each type of transistor.
N-P-N Types
Fig. 16 shows the
basic biasing arrangements for an n-p-n bipolar transistor. External batteries bias the emitter-base (n-p) junction in the forward
direction to provide a low-resistance input section, and bias the base-collector (p-n) junction in the reverse direction to provide a high-resistance output section.
Electrons flow easily from the n-type emitter region to the p-type base region as a result of the forward biasing. Most of
these electrons diffuse through the thin p-type region, however, and are attracted by the positive potential of the external bias supply across the base-collector
(p-n) junction. In practical devices, approximately 95 to 99.5 per cent of the injected electrons reach the n-type collector region. This high percentage of
current penetration makes possible power gain in the high-resistance output circuit and is the basis for the amplification capability of a transistor. |
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P-N-P Types
The operation of a p-n-p
transistor is essentially identical to that of an n-p-n transistor except that the polarities of the bias voltages are reversed and the main current carriers are holes instead of electrons. Fig. 17
shows the basic biasing and input-signal connections for a p-n-p transistor.
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. On to Diacs
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