long as the battery is pumping out. Notice that the
CARRIERS.--Electrons in N materials are called
ammeter is showing current flow.
majority carriers because they are primarily
responsible for the current flow. Holes are called
In view C the battery has been reversed to show
minority carriers because they conduct a much smaller
how a diode blocks current flow. The electrons are
current.
trying to enter the P section; the electrons in the N
In P materials the opposite is true. Holes (or
section move in the direction of current flow. Looking
positive charges) are the primary current carriers.
at view C, you can see there are no holes or electrons
Electrons are minority carriers, conducting a much
near the PN junction. Therefore, there can be no
smaller current flow.
current flow. The holes move in the opposite direction
away from the PN junction. No reading is shown on the
Semiconductor Devices
meter. Remember that electrons can flow from N to P,
but not from P to N.
As the application of semiconductor devices
The most important point to remember about the
expands throughout the support equipment field, the
PN junction diode is its ability to offer very little
requirement for a working knowledge of these devices
resistance to current flow in the forward-bias direction,
becomes increasingly important. Because NEETS,
but maximum resistance to current flow when reverse
Module 7, describes in detail the theory and
biased. A good way of illustrating this point is by
application of many devices, only the diode, Zener
plotting a graph of the applied voltage versus the
diode, transistor, and the silicon-controlled rectifier
measured current. Figure 7-3 shows a plot of this
(SCR) are covered here.
voltage-current relationship (characteristic curve) for
DIODES.--The basic function of the diode is to
a typical PN junction diode.
act as a rectifier (one-way valve for electricity). The
To determine the resistance from the curve in this
diode is the simplest kind of semiconductor. Figure 7-2
figure, you can use Ohm's law, where R = E I. For
shows how a diode conducts; view A shows the P
example, at point A the forward-bias voltage is 1 volt
junction and N junction mated together, and view B
and the forward-bias current is 5 milliamperes. This
shows current flow from the battery to the N section of
represents 200 ohms of resistance (1 volt 5 mA = 200
the diode. These negatively charged electrons repel the
ohms). However, at point B the voltage is 3 volts and
free negative electrons already there, forcing them
the current is 50 milliamperes. This results in 60 ohms
toward the PN junction. At the same time, electrons are
of resistance for the diode. Notice that when the
being withdrawn from the P section, creating new
forward-bias voltage was tripled (1 volt to 3 volts), the
holes. The new holes repel the old ones, moving them
current increased 10 times (5 mA to 50 mA). At the
toward the PN junction. The holes in the P section and
same time the forward-bias voltage increased, the
the electrons in the N section move toward each other.
resistance decreased from 200 ohms to 60 ohms. In
As the holes and electrons meet at the junction, the
electrons fall into the holes. This action continues as
other words, when forward bias increases, the junction
P
N
P
N
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++
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+ ++
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+
P-TYPE
N-TYPE
MATERIAL
MATERIAL
_
_
+
+
MILLIAMMETER
MILLIAMMETER
A
B
C
FORWARD BIAS VOLTAGE
REVERSE BIAS VOLTAGE
CURRENT FLOW
ZERO CURRENT
ASf07002
Figure 7-2.--Junction diode.
7-2
