The
first electronic devices to be introduced is called the diode. It is the
simplest of semiconductor devices but plays a very vital role in electronic
systems, having characteristic that closely match those of a simple switch. It
will appear in
range of applications,
extending from the simple to the very complex. In addition to the details of
its construction and characteristics, the very important data and graph to be
found on specification sheets will also be covered to ensure an understanding
of the terminology employed and to demonstrate the wealth of information typically
available from manufacturers.
The term ideal will be used
frequently in this text as new devices are introduced. It refers to any device
or system that has ideal characteristic – perfect in every way. It provides a
basis for comparison, and it reveals where improvements can still be made. The
ideal diode is a two-terminal device having the symbol and characteristics
shown in figs. 1.1a and b,
respectively./ ideal diode ; symbol, characteristics
Ideally, a diode will conduct
current in the direction defined by the arrow in the symbol and act like an
open circuit in any attempt to establish current in the opposite direction. In
essence :
The characteristics of an ideal
diode are those of a switch than can conduct current in only one direction.
In
the description of the elements to follow, it is critical that the various
letter symbols, voltage polarities, and current directions be defined. If the
polarity of the applied voltage is consistent with that shown in fig. 1.1a, the portion of the characteristics
to be considered in fig. 1.1b is to
the right of the vertical axis. If a reverse voltage is applied, the
characteristics to the left are pertinent. If the current thorough the diode
has the direction indicated in fig. 1.1a,
the portion of the characteristics to be considered in above the horizontal
axis. While a reversal in direction would require the use of the
characteristics below the axis. For the majority of the device characteristics
that appear in this book, the ordinate (or ‘y’axis ) will be the current axis,
while the abscissa (or ‘x’ axis ) will be the voltage axis.
One of the important parameters for
the diode is the resistance at the point or region of operation. If we consider
the conduction region define by the direction of Id and polarity of Vd in fig.
1.1a ( upper-right quadrant of fig. 1.1b ), we will find that the value of the
forward resistance, Rf as defined by ohm’s law is
Rf = Vf/If = 0V/2,3,mA,…..or any
positive value = 0 ohm ( short circuit )
Where
Vf is the forward voltage across the diode and If is the forward current
through the diode.
The ideal diode, therefore, is a
short circuit for the region of conuction. Consider the region of negatively
applied potential ( third quadrant ) of fig. 1.1b,
Rr = Vr/Ir = -5, - 20, or any
reverse-bias potential/0mA = ~ ohm ( open circuit )
Where Vr is reverse voltage across the diode and Ir is reverse current in the
diode.
The
ideal diode, therefore, is an open circuit in the region of non conduction. In
review, the conditions depicted in fig.
1.2 are applicable. / a. conduction and b. nonconduction states of the
ideal diode as determined by the applied bias.
In general, it is relatively simple
to determine whether a diode is in the region of conduction or nonconduction
simply by nothing the direction of the current Id established by an applied
voltage. For conventional flow ( opposite to that of electron flow ), if the
resultant diode current has the same direction as the arrowhead of the diode
symbol, the diode is operating in the conducting region as depicted in fig. 1.3a. if the resulting current has the opposite
direction, as shown in fig. 1.3b,
the open circuit equivalent is appropriate./ conduction and b. nonconduction
states of the ideal diode as determined by the direction of conventional
current established by the network.
As indicated earlier, the primary
purpose of this section is introduce the characteristics of an ideal device for
comparison with the characteristics of the commercial variety. As we progress
through the next few section, keep the following question in mind :
How close will the forward or ‘on’
resistance of a practical diode compare with the desired 0 ohm level ?
Is the reverse-bias resistance
sufficiently large to permit an open-circuit approximation
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