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ELEC 350L
Electronics I Laboratory
Fall 2002
Lab 7: Semiconductor Diode Characteristics
Introduction
Semiconductor diodes are employed in a wide range of electronic circuit applications. However,
unlike resistors, capacitors, inductors, and most sources, diodes are nonlinear devices; that is, the
voltage across their terminals is not directly proportional to the current that flows through them.
(For capacitors and inductors in the frequency domain, the linear relationship is in the form of a
simple multiplication by 1/jωC or jωL, respectively. In the time domain, the linear relationship
is in the form of derivatives or integrals.) One consequence of this is that diode circuits do not
obey the principle of superposition, and linear analysis techniques such as nodal and mesh
analysis cannot be applied to them. Unless some simplifying approximations are made, other
analysis approaches must be used, and these depend upon knowing the i-v characteristic of the
diode or diodes in question. In this lab experiment you will determine the i-v characteristic of a
semiconductor diode made of silicon.
There will be times in your career when you will need information about devices and circuits that
is not given in manufacturers’ data sheets. In these situations it will be up to you to plan a series
of measurements in order to obtain the data you need. The experimental procedure for this lab is
not outlined as explicitly as those of past labs. Part of the purpose of this experiment is to help
you practice devising test procedures on your own.
Theoretical Background
The semiconductors found in most modern electronic devices are made of tiny crystals of silicon,
germanium, or other similar elements that have been tainted slightly with small concentrations of
other elements. The purpose of these impurity elements, usually called dopants, is to create a
supply of free electrons or holes not bound in the crystal structure. In a crystal of pure silicon all
of the valence electrons are trapped in covalent bonds near to the silicon nuclei, but a doped
crystal has either an excess or a deficiency of valence electrons. These impurities impart a
significantly higher conductivity to the crystal. If the dopant creates a supply of free electrons,
the modified crystal is called n-type material (because electrons are negatively charged). If the
modified crystal has a supply of holes, the material is called p-type (because holes are positive).
The structure of a basic semiconductor diode is shown in Figure 1. A section of n-type material
abuts a section of p-type material to form a pn junction. The diode symbol is shown just below
the pn junction in the orientation corresponding to the arrangement of the p- and n-type material.
If a resistor and voltage source are connected to the diode as shown in Figure 1, a significant
amount of current will flow through the circuit. If the polarity of the voltage source is the
reverse of that shown, a very small amount of current will flow (on the order of nanoamps). The
voltage vD across the diode and the current iD that flows through it are related to each other, but
not linearly.
1
p
n
diode
+
_
vD
iD
_
+
R
vs
Figure 1. Physical construction, symbol, and forward-biasing of a semiconductor diode.
The interaction of the holes and electrons on both sides of the pn junction under the influence of
an applied voltage or current leads to a nonlinear i-v characteristic for the diode given by


iD  I S e v D /VT  1 ,
which is known as the diode equation. The constant IS is called the saturation current (or
sometimes the reverse current) and is the amount of current that flows through the diode when it
is reverse-biased, the state that corresponds to negative vD. Its value usually varies between 10–8
and 10–14 A for discrete devices (stand-alone diodes not found in integrated circuits) at room
temperature. Typically, IS is measured in nanoamps. The constant  is called the emission
coefficient and is empirically (experimentally) determined. Its value is usually close to unity (1)
for germanium diodes and close to two for silicon diodes in normal operation. The constant VT is
called the thermal voltage or the volt-equivalent of temperature and is given by
VT 
T
,
11,600
where T is measured in degrees Kelvin. At room temperature (300 K, 27 C, or 81 F) VT is
approximately 26 mV. Note that the diode equation applies whether the diode is forward-biased
(vD > 0) or reverse-biased (vD < 0). If the diode is reverse-biased, then e v D /VT  1 ; the diode
current iD has a negative value (approximately -IS); and the actual direction of the tiny current is
opposite that indicated by the arrow in Figure 1.
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A diode’s i-v characteristic, which is mathematically expressed by the diode equation, has the
general shape shown in Figure 2. The distance between the horizontal axis and the curve for
negative vD is exaggerated to show that the current is negative in the reverse-bias regime. If the
left-hand (negative-vD) part of the curve were drawn to scale, it would be so close to the
horizontal axis that it would be impossible to discern from the plot that iD is negative when vD is
negative.
iD
IS
vD
Figure 2. Typical i-v characteristic for a semiconductor diode.
Experimental Procedure
Your main task is to determine the i-v characteristic of a 1N914 silicon diode using a test
procedure of your own design. Note that the benchtop multimeters do not measure current; you
will have to devise another way to obtain readings of iD. You should gather enough data in order
to produce a curve like the one shown in Figure 2 for positive values of vD only. (The reverse
saturation current IS is too small to measure easily. It will be determined indirectly later.) The
“knee” in the curve (where it turns sharply upward) occurs at a vD value of several tenths of a
volt, so your maximum value of vD should be at least that large. Remember that vD is the voltage
measured across the terminals of the diode, not the value of any applied voltage source. Your
measurement approach should prevent excessive current (more than a few tens of milliamps)
from flowing through the diode. Have the instructor or TA check your proposed test procedure
before you begin taking measurements. Make sure you include in your notebook a complete but
concise description of the test procedure you devise. There should be a sufficient amount of
detail so that another engineer could read your description and repeat your measurements
successfully. Sketch or print out a copy of the i-v characteristic for your diode. After you have
finished taking measurements, you may ask the instructor to verify your i-v curve using a piece
of test equipment known as a curve tracer.
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From the i-v data you have collected, determine the values of the constants IS and , assuming
that the value of VT obtained using the equation given above in the “Theoretical Background”
section is exact. (You’ll need to estimate the temperature of the room in degrees K.) This could
be accomplished via trial-and-error, but there is an easier way. Note that, when vD is “large,” the
diode equation simplifies to
i D  I S e v D /VT .
Taking the natural logarithm of both sides of this approximation leads to


ln iD  ln I S e v D /VT  ln I S  ln e v D /VT  ln I S 
vD
.
VT
This implies that a log-linear plot of iD vs. vD should be a straight line over the range of vD values
for which the approximation given above is valid. Does this suggest to you a simple way to find
IS and ? Clearly explain in your notebook the procedure you use to find IS and . Comment on
the degree to which your calculated value of IS agrees with the range of typical values given by
the 1N914 data sheet. Also, does your calculated value of  fall in the range of typical values for
silicon diodes?
A data sheet for the 1N914 can be found at:
http://www-us.semiconductors.philips.com/acrobat/datasheets/1N914_3.pdf
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