(4) In C of figure 1-3, battery Ebb is reversed, making the plate of the
tube negative.
The plate repels the negative electrons, and, as the
meter indicates, no current is flowing.
(5) In D of figure 1-3, the negative charge of the battery on the P-type
material attracts the positive charges of the P material away from the
junction.
Likewise, the positive charge of the battery on the N-type
material attracts the negative charges of the N material away from the
junction. The concentration of unlike charges at the junction is thus
reduced (strengthening the junction barrier), so the current flow
across the junction is reduced. When the battery is connected in this
manner, it impedes the flow of current across the junction, and we say
that the diode is reverse biased.
(6) If the battery in B of figure 1-3 is replaced by an ac source, the
charges will move to the junction on the positive alternation (forward
voltage), and from the junction on the negative alternation (reverse
voltage) of the ac wave. Since the junction diode has low opposition
to the forward voltage and high opposition to the reverse voltage, the
ac wave is changed to dc.
d. Transistors.
Transistors are composed of semiconductor materials.
Both
the N-and P-type material are used in the same transistor.
Two NP junctions and
two barriers are created.
The conductance can be varied by external control
voltages and signals. The transistor can be compared with a triode tube. Instead
of a cathode, control grid, and plate, the transistor has an emitter, base, and
collector, respectively.
The emitter and collector are made of the same type of
semiconductor material, with the same electrical characteristic, both either N or
P.
The base must always be of the opposite electrical characteristic.
The
transistor is enclosed in a case, and three leads are usually brought out of the
case, one lead connected to each of the three elements.
Transistors are made of
small bits of solid-state material.
They are normally light in weight, small in
size, and free from the effects of vibration. They are, however, sensitive to heat
changes. There are two types of transistors.
(1) The NPN transistor is constructed with the emitter and collector made
of N-type semiconductor material, while the base is made of P-type
material. The schematic symbol is shown in figure 1-4.
(2) The PNP transistor is constructed with the emitter and collector made
of P-type semiconductor material, while the base is made of N-type
material.
The schematic symbol for the PNP transistor is shown in
figure 1-4.
e. Transistor Operation.
When the signal is applied to the base of a
transistor, the immediate effect is to either strengthen or weaken the junction
barrier.
This, in turn, will cause the current to decrease or increase with the
input signal. In both cases the input signal is amplified (strengthened).
309 L1
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