Semiconductor Diodes

It is now some 50 years since the first transistor was introduced on December 23,

1947. For those of us who experienced the change from glass envelope tubes to the

solid-state era, it still seems like a few short years ago. The first edition of this text

contained heavy coverage of tubes, with succeeding editions involving the important

decision of how much coverage should be dedicated to tubes and how much to semiconductor

devices. It no longer seems valid to mention tubes at all or to compare the

advantages of one over the other—we are firmly in the solid-state era.

 Figure 1.1 Ideal diode: (a)
symbol; (b) characteristics.

The miniaturization that has resulted leaves us to wonder about its limits. Complete

systems now appear on wafers thousands of times smaller than the single element

of earlier networks. New designs and systems surface weekly. The engineer becomes

more and more limited in his or her knowledge of the broad range of advances—

it is difficult enough simply to stay abreast of the changes in one area of research or

development. We have also reached a point at which the primary purpose of the container

is simply to provide some means of handling the device or system and to provide

a mechanism for attachment to the remainder of the network. Miniaturization

appears to be limited by three factors (each of which will be addressed in this text):

the quality of the semiconductor material itself, the network design technique, and

the limits of the manufacturing and processing equipment.

The first electronic device to be introduced is called the diode. It is the simplest of
semiconductor devices but plays a very vital role in electronic systems, having characteristics
that closely match those of a simple switch. It will appear in a 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 graphs 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 characteristics—perfect in every way.
It provides a basis for comparison, and it reveals where improvements can still be
made.

Ideally, a diode will conduct current in the direction defined by the arrow in the
symbol and act like an open circuit to any attempt to establish current in the opposite
direction. In essence:
The characteristics of an ideal diode are those of a switch that 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 through
the diode has the direction indicated in Fig. 1.1a, the portion of the characteristics to
be considered is 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 defined 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
V
IF
F
0 (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 conduction.
Consider the region of negatively applied potential (third quadrant) of Fig. 1.1b,
RR
V
IR
R
(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 nonconduction.
In review, the conditions depicted in Fig. 1.2 are applicable.
5, 20, or any reverse-bias potential

0 mA
0 V

2, 3, mA, . . . , or any positive value
Figure 1.2 (a) Conduction and (b) nonconduction states of the ideal diode as
determined by the applied bias.