NPN Transistor
Operation: Just as in the case of the PN
junction diode, the N material comprising the two end sections of the NP N transistor
contains a number of free electrons, while the center P section contains an
excess number of holes. The action at each junction between these sections is
the same as that previously described for the diode; that is, depletion regions
develop and the junction barrier appears. To use the transistor as an
amplifier, each of these junctions must be modified by some external bias
voltage. For the transistor to function in this capacity, the first PN junction
(emitter-base junction) is biased in the forward, or low-resistance, direction.
At the same time the second PN junction (base-collector junction) is biased in
the reverse, or high-resistance, direction. A simple way to remember how to
properly bias a transistor is to observe the NPN or PNP elements that make up
the transistor. The letters of these elements indicate what polarity voltage to
use for correct bias. For instance, notice the NPN transistor below:
1. The emitter, which is the first letter in the
NPN sequence, is connected to the negative side of the battery while the base,
which is the second letter (NPN), is connected to the positive side.
2. However, since the second PN junction is
required to be reverse biased for proper transistor operation, the collector
must be connected to an opposite polarity voltage (positive) than that indicated
by its letter designation(NPN). The
voltage on the collector must also be more positive than the base, as shown
below:
We now have a properly biased NPN
transistor. In summary, the base of the NPN transistor must be positive with
respect to the emitter, and the collector must be more positive than the base.
NPN
FORWARD-BIASED JUNCTION: An important point to bring out at this time, which
was not necessarily mentioned during the explanation of the diode, is the fact
that the N material on one side of the forward-biased junction is more heavily
doped than the P material. This results in more current being carried across
the junction by the majority carrier electrons from the N material than the
majority carrier holes from the P material. Therefore, conduction through the
forward-biased junction, is mainly by majority carrier electrons from the N
material (emitter).With the emitter-to-base junction in the figure biased in
the forward direction, electrons leave the negative terminal of the battery and
enter the N material (emitter). Since electrons are majority current carriers
in the N material, they pass easily through the emitter, cross over the
junction, and combine with holes in the P material (base). For each electron
that fills a hole in the P material, another electron will leave the P material
(creating a new hole) and enter the positive terminal of the battery. NPN
REVERSE-BIASED JUNCTION.The second PN junction (base-to-collector), or
reverse-biased junction as it is called (fig. 2-6), blocks the majority current
carriers from crossing the junction. However, there is a very small current,
mentioned earlier, that does pass through this junction. This current is called
minority current, or reverse current. As you recall, this current was produced
by the electron-hole pairs. The minority carriers for the reverse-biased PN
junction are the electrons in the P material and the holes in the N material.
These minority carriers actually conduct the current for the reverse-biased
junction when electrons from the P material enter the N material, and the holes
from the N material enter the P material. However, the minority current
electrons (as you will see later) play the most important part in the operation
of the NPN transistor.At this point you may wonder why the second PN junction
(base-to-collector) is not forward biased like the first PN junction
(emitter-to-base). If both junctions were forward biased, the electrons would
have a tendency to flow from each end section of the N P N transistor (emitter
and collector) to the center P section (base). In essence, we would have two
junction diodes possessing a common base, thus eliminating any
amplification and defeating the
purpose of the transistor. A word of caution is in order at this time. If you
should mistakenly bias the second PN junction in the forward direction, the
excessive current could develop enough heat to destroy the junctions, making
the transistor useless. Therefore, be sure your bias voltage polarities are
correct before making any electrical connections.
NPN JUNCTION
INTERACTION:
We are now ready to see what happens when we place the two junctions of the NPN
transistor in operation at the same time. For a better understanding of
just how the two junctions work
together.base voltage supply. Also notice the base supply battery is quite
small, as indicated by the number of cells in the battery, usually 1 volt or
less. However, the collector supply is generally much higher than the base
supply, normally around 6 volts. As you will see later, this difference in
supply voltages is necessary to have current flow from the emitter to the
collector. As stated earlier, the current flow in the external circuit is
always due to the movement of free electrons. Therefore, electrons flow from
the negative terminals of the supply batteries to the N-type emitter. This
combined movement of electrons is known as emitter current (IE). Since
electrons are the majority carriers in the N material, they will move through
the N material emitter to the emitter-base junction. With this junction forward
biased, electrons continue on into the base region. Once the electrons are in
the base, which is a P-type material, they become minority carriers. Some of
the electrons that move into the base recombine with available holes. For each
electron that recombines, another electron moves out through the base lead as
base current IB (creating a new hole for eventual combination) and returns to
the base supply battery V BB. The electrons that recombine are lost as far as
the collector is concerned. Therefore, to make the transistor more efficient,
the base region is made very thin and lightly doped. This reduces the
opportunity for an electron to recombine with a hole and be lost. Thus, most of
the electrons that move into the base region come under the influence of the
large collector reverse bias. This bias acts as forward bias for the minority
carriers (electrons) in the base and, as such, accelerates them through the
base-collector junction and on into the collector region. Since the collector
is made of an N-type material, the electrons that reach the collector again
become majority current carriers. Once in the collector, the electrons move
easily through the N material and return to the positive terminal of the
collector supply battery VCC as collector current (IC). To further improve on
the efficiency of the transistor, the collector is made physically larger than
the base for two reasons: (1) to increase the chance of collecting carriers
that diffuse to the side as well as directly across the base region, and (2) to
enable the collector to handle more heat without damage. In summary, total
current flow in the NPN transistor is through the emitter lead. Therefore, in
terms of percentage, IE is 100 percent. On the other hand, since the base is
very thin and lightly doped, a smaller percentage of the total current (emitter
current) will flow in the base circuit than in the collector circuit. Usually
no more than 2 to 5 percent of the total current is base current (IB) while the
remaining 95 to 98 percent is collector current (IC). A very basic relationship
exists between these two currents: IE = IB + IC
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