ELEC 350L
Electronics I Laboratory
Fall 2005
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 in
general 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 (absences of electrons) that are not bound in the crystal
structure. In a crystal of pure silicon all of the valence electrons are trapped close to the silicon
nuclei in covalent bonds, but a doped crystal has either an excess or a deficiency of valence
electrons. Whether p-type or n-type, however, a doped sample of silicon is still electrically
neutral; that is, the numbers of electrons and protons in the sample are the same. The 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 (since electrons are negatively charged).
If the modified crystal has a supply of holes, the material is called p-type (since holes are
positive). Because free electrons and holes are not held in place by tight covalent bonds, they
move easily under the influence of an applied voltage. Hence, the conductivity increases.
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
materials. If a resistor and a voltage source are connected to the diode as shown in Figure 1, a
significant amount of current will flow through the circuit. In this case, the diode is said to be
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forward-biased. If the polarity of the voltage source is reversed, a very small amount of current
will flow (on the order of nanoamps). The diode is then said to be reverse-biased. The voltage
vD across the diode and the current iD that flows through it are related to each other, but not
linearly.
p
n
diode
+
_
vD
iD
_
+
R
vs
Figure 1. Physical construction, symbol, and forward-biasing of a semiconductor
diode.
The behavior of the holes and electrons on both sides of the pn junction under the influence of an
external source leads to a nonlinear i-v characteristic for the diode given by


i D  I S e vD / nVT  1 ,
which is known as the diode equation. The constant IS is called the saturation current (or
sometimes the scale current or 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, the value of IS is on the order of nanoamps. The
constant n is called the emission coefficient and is empirically (experimentally) determined. Its
value is usually close to unity (one) 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
T
,
VT 
11,600
where T is measured in degrees Kelvin. At warm temperatures (300 K, 27 C, or 81 F) VT is
approximately 26 mV. Note that the diode equation applies whether the diode is forward-biased
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(vD > 0) or reverse-biased (vD < 0). If the diode is reverse-biased, then e vD / nVT  1 ; the diode
current iD has a negative value (approximately –IS); and the actual direction of the tiny current is
opposite to that indicated by the arrow in Figure 1.
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 for the reverse-bias case. 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 two types of silicon diodes using a
test procedure of your own design. 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 (or perhaps
many tenths) of a volt, so your maximum value of vD should be well beyond that point.
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
from flowing through the diode; consult the relevant data sheets to find out what the current
limits should be. 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.

Using the test procedure you devise, determine the i-v characteristic of a 1N914 or 1N4148
diode, whichever type is available. You are highly encouraged to organize your measured
data in tabular form.
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
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.

Apply your measurement procedure again to find the i-v characteristic for a 1N4007 rectifier
diode. This is a heavy-duty diode used in power supplies, so it is possible that you will have
to allow more current to flow through it than the 1N914/1N4148 to get past the “knee” in the
i-v characteristic. Note that the value of vD at which the “knee” occurs could be significantly
higher than it is for the first diode. Sketch or print out a copy of the i-v characteristic.

From the i-v data you have collected, determine the values of the parameters IS and n for each
diode, 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.) For reasons explained below, you are not likely to get good results if
you blindly use the “trend line” (curve-fitting) feature of your calculator or spreadsheet to do
this. You can find the parameters 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 vD / nVT .
Taking the natural logarithm of both sides leads to


ln i D  ln I S e vD / nVT  ln I S  ln e vD / nVT  ln I S 
vD
.
nVT
Because IS, n, and VT are constants, this implies that a semi-log plot of iD vs. vD should be a
straight line over the range of vD values for which the approximation given above is valid.
(Note that the natural logarithm is used here, not the common logarithm.) This should
suggest to you a simple way to find IS and n. Clearly explain in your notebook the procedure
you end up using.
Note: The diode equation is not applicable over the entire range of safe operation of a diode,
although it is usually valid over the typical range of operation. Of course, vD must be large
enough so that the approximation given above is valid. But as vD increases well above the
“knee” in the curve the i-v characteristic also departs from the simple exponential
relationship. In the latter case a diode begins to take on the characteristics of a resistor
(which has a linear, rather than exponential, i-v characteristic). One of your tasks is to
determine the range of your measurements over which the diode equation is applicable (and
over which your method to find IS and n should be applied).

Comment on the degree to which your calculated values of IS agree with the typical values
for that parameter given by the appropriate data sheets. Also discuss whether or not your
calculated values of n fall within the range of typical values (1-2) for silicon diodes.
Links to data sheets for the 1N914 and 1N4007 diodes are available on the “Laboratory” page.
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