13. THE SEMICONDUCTOR DIODE
In this experiment you will study the properties of a diode made from semiconductor
material. This is a basic activity in the subject of electronics.
Theory
Why Study the Semiconductor Diode?
Recall from the experiment “DC Circuits” that a
carbon composition resistor is an example of a
linear device. A resistor presents the same resistance to the flow of current in either direction
through it. Also, the voltage drop across a resistor
is a linear function of the current flow.
A diode on the other hand is a non-linear
device. The resistance a diode presents depends
critically on the polarity of the voltage drop across
it. In what is called the forward polarity (when the
anode is positive with respect to the cathode) the
resistance is relatively small. For the opposite
polarity the resistance is large. This amounts to the
diode passing current preferentially in one direction through itself. This behaviour results from the
fact that the diode is made from two types of semiconductor material (as opposed to a uniform block
of compressed carbon). Diodes have special
applications in AC circuits as rectifiers, devices
which assist in converting an AC voltage into a DC
voltage. A diode has also been used as a
rudimentary digital device, the high current in the
forward direction representing, say, a binary “1”,
the low current in the reverse direction representing a binary “0”.
Diode Facts
Three views of the diode you will be using in this
experiment are drawn in Figure 1. Your diode
(called a signal diode) is packaged in a cylindrical
body and looks little different from a carbon
composition resistor. It has two ends, an anode and
anode
cathode
a cathode. The cathode end may be marked by a
rounding of the diode’s body or by the placement
of a band. In Figure 3 is shown the diode
connected to a variable voltage power supply
much like any other load device.
+
+
+
–
–
–
(a)
(b)
(c)
Figure 1. The cathode end of a diode may be marked by a rounding of the diode’s body (a) or by the placement of a
band (b). In (c) is shown the circuit symbol for the diode.
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13 The Semiconductor Diode
The VI Characteristic of a Diode
The electrical properties of a diode are summarized in the graph that results when the voltage
across the diode, say Vd, is plotted versus the
current flow I through the diode (Figure 2). This
graph is called the VI characteristic. This graph can
be found in the same way as for a resistor.
I
(c)
reverse direction
Ir
forward direction
If
(b)
(a)
Vd
VPN
Figure 2. The VI characteristic of a typical diode. In the reverse direction (a) the current is very small. In the
forward direction (b) and (c) a current flows. If V d exceeds VPN (c) the current can be very large.
Diode Action
In Figure 3a the diode is used in the forward direction, that is, in such a way that the anode of the
diode is connected to the positive terminal of the
voltage source and the cathode is connected to the
negative terminal. In this orientation a current If is
seen to flow. If Vd is small, less than a certain voltage VPN, called the turn on voltage, If is relatively
small. But should Vd exceed VPN the current If can
become quite large.
If the diode is used in the reverse direction
shown in Figure 3b a very small current flows.
Often this current Ir is so small it requires a sophisticated ammeter to measure it. Thus the diode is
observed to pass current preferentially in one
direction through itself. This is just what is meant
by diode action. The resistance (the ratio Vd/I) of a
typical diode is sketched in Figure 4.
If
Ir
anode
anode
Vs
Vs
Vf
cathode
Vr
cathode
(a)
(b)
Figure 3. In the forward direction (a) the current I f is large in general. In the reverse orientation (b) the current I r is
very small.
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The Semiconductor Diode 13
R
reverse direction
forward direction
Vd
Figure 4. Resistance of a typical diode. In the reverse direction the resistance is larger than implied here.
The Diode Characteristic
There are three important sections to the diode
characteristic shown in Figure 2. In the reverse
direction (a) the current is very small. In the
forward direction (b) and (c) a current flows. In
region (b) the current is relatively small and is a
non-linear function of the applied voltage. Solid
state theory predicts that the relationship is
 VVd

I f = Ir  e T –1 ,


…[1]
where VT = kT/e is a constant at a given absolute
temperature T, k is Boltzmann’s constant (1.38 x
10–23 W.s.K–1) and e is the electronic charge (1.602 x
10–19 C). Eq[1] is known as the rectifier equation. Ir is
also a constant which depends on the particular
diode used. If Vd>> VT eq[1] takes on the simpler
form
Vd
VT
I f = Ir e .
…[2]
In the event that the voltage across the diode is increased far enough, eq[1] no longer applies and the
current becomes quite large. (This is the same as
saying that the forward resistance of the diode
becomes very small.) In this region (c) the current
increases linearly with voltage and the diode
behaves like a carbon resistor. Ohm’s Law applies
(more-or-less) in this region and we can write
Vd = I f Rf
…[3]
where R f is the diode’s forward resistance. If this
linear section of the curve is interpolated back to
the voltage axis (Figure 2) an approximate value
for VPN, the turn on voltage, can be obtained. VPN
is typically equal to about 0.2V for a diode made of
germanium and 0.6V for a diode made of silicon.
The Solid State Physics of the Diode
The solid state physics of the diode is extensive
and involves a good deal of mathematics and
quantum mechanics. We shall dwell on only those
qualitative aspects which will enable us to understand its general behavior. A semiconductor diode
can be imagined to be constructed from the fusing
together of two pieces of silicon. One piece is
doped with an impurity element so as to produce
an excess of holes. The other material is doped
with an impurity element so as to produce an
excess of free electrons. The former is called a Ptype material, the latter an N-type (Figure 5).
The situation after the pieces are fused is
drawn in Figure 6. Just after the sections are fused
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13 The Semiconductor Diode
together, holes (h) diffuse from the P- to the Nmaterial and electrons (e) diffuse in the opposite
direction. This results in the materials acquiring an
excess of charge, the P-type material becoming
more negative than the N-type. There arises across
the junction a small “intrinsic” electric field Ein and
a corresponding potential difference Vin. In the
region close to the actual junction electrons and
holes recombine and lose their identity. This
happens on both sides of the junction. This region
is therefore called the depletion region. Once the
system has reached equilibrium the diode can be
thought of (with care) as a charged parallel plate
capacitor.
holes
P-type
material
–
N-type
material
–
–
–
–
free electrons
–
–
Figure 5. Two pieces of silicon before being fused together. Both majority charge carriers, electrons and holes, form
the bulk of the diode current after the pieces are fused. Both materials are electrically neutral at this stage.
e
P
e
e
excess electrons
–
e
N
–
h
h
–
–
–
–
–
–
Ein
excess holes
–
–
h
Vin
–
+
h
–
(a)
Vin
Ein
+
+
+
+
(b)
Figure 6. (a) After the pieces are fused together, holes (h) migrate from the P- to the N-region and electrons (e)
migrate in the opposite direction. (b) shows that the diode at this stage can be thought of (with caution) as a charged
parallel plate capacitor.
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The Semiconductor Diode 13
e flow only in connecting wires
(conventional current is opposite)
–
+ load
V
h
flow
E
–
+
e
flow
P
V
h
flow
N
(a)
–
–
load
Ef
–
e
flow
(b)
Figure 7. (a) Conduction arises inside a load because of the existence there of an electric field. (b) Current results in a
diode from the movement of both types of majority charge carrier. (Only electrons flow in the connecting wires. The
direction of conventional current is opposite to that of the electrons.)
It is worthwhile here to review what is meant by
conduction inside a load (Figure 7a). The potential
difference developed by the voltage source and
applied across the load produces an electric field
inside the load. Free charges in the load respond to
this field by moving and thus form a current.
(They are subject to an electrostatic force.)
Electrons flow antiparallel to the field and holes (if
they exist) flow parallel to the field. Thus the
resultant electric field E f inside a diode must have
the polarity as shown in Figure 7b.
This means that in order to induce appreciable
conduction the intrinsic electric field Ein within the
diode shown in Figure 6 must first be “cancelled
and overcome” by the electric field produced by
the external source. Let us suppose that the voltage
Vs supplied by the source can be increased starting
from zero. In Figure 8a is shown the situation
where the external voltage results in just a reduction of Ein, call it Er. The depth of the depletion
region is reduced and a small current flows. (The
narrower the depletion region becomes the more
likely a charge will pass through it. Conversely, the
wider the depletion region becomes the less likely
a charge will pass throught it.) As the volt-age is
increased the depletion region continues to shrink
and Ein continues to decrease. The current in the
forward direction continues to increase, but only
slowly. This is the region in which the current
increases nonlinearly with voltage as is shown in
section (b) of Figure 2, and as described by eq[1].
As soon as the external voltage reaches VPN the
depletion region disappears altogether (Figure 8b)
and large currents flow. This puts us into the linear
region (c) of Figure 2.
Suppose now the polarity of the supply (or the
diode) in Figure 7 were reversed. As can be seen
from Figures 6a and 8a the internal electric field
produced by the source would add to Ein. The
depletion region would widen. The electric field
would not have the proper polarity to induce
conduction and the current flow expected would
be very small. (The only current that flows in this
direction is that produced by minority charge
carriers.) This is the reverse direction.
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13 The Semiconductor Diode
large I
–
small I
–
e
e
e
e
e
e
e
e
V ≥ VPN
V < VPN
Er < Ein
–
h
h
–
–
–
–
h
Ef
–
–
h
h
h
–
–
–
(a)
–
–
h
–
h
–
(b)
Figure 8. In (a) the voltage applied is less than VPN but greater than zero. A small current flows depending nonlinearly on V. In (b) the applied voltage just exceeds VPN and a large current flows. The current depends linearly on
V. Electron current is shown in the connecting wires.
The Experiment
Exercise 0. Preparation
Orientation
Identify the apparatus: one Heathkit Model EUW17 variable voltage power supply; two Digitek
Model DT-890D 3 1/2 digit multimeters; one box
containing four terminal points; one 100 kΩ resistor, one 20 kΩ resistor, one diode and connectors
of various lengths. (These connecting wires are
hanging up in the lab. You will have to find them.)
Checklist
Carry out the following cold start checks:
3 If ON, turn any and all instruments OFF. Turn
the voltage control on the power supply to
zero. This control should be zeroed before
turning the supply ON or OFF.
3 If present, clear away any and all connecting
wires from all apparatus.
3 In what follows you’ll be required to wire up
circuits. Therefore, after wiring up your circuit
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and before turning the power supply ON, have
your TA check your circuit for you. Don’t
worry, the voltages here are not dangerous.
In order to insert a diode correctly into a circuit
one must know the diode’s polarity. As discussed
in the theory section there are various methods to
choose from. Some of these methods work better
than others.
Ô By Inspection. With reference to Figure 1 identify your diode’s polarity.
Ô Using an Ohmmeter. The multimeter in ohmmeter function can be used to measure the
diode’s resistance in both directions (at least in
theory). Clearly, the resistance in the forward
direction should be less than the resistance in
the reverse direction. Simply connect the diode
directly to the “COM” and “Ω ” inputs of the
meter, select the appropriate function and
The Semiconductor Diode 13
range and read the resistance. (For forward
operation the anode of the diode should be
connected to the “Ω ” input.) What values of
resistance do you get? You may in fact be
surprised at how high the resistance is in the
forward direction. This is because the multimeter puts only a very small current through
the diode. Can you measure any resistance at
all in the reverse direction?
Ô Multimeter Diode Test. The Digitek DT-890D
multimeter has a function that will test most
diodes. You can find the diode test setting
(diode symbol) on the upper left side of the
rotary dial. In the forward direction of the
diode the multimeter will display the approximate forward voltage drop at a current of
1mA. In the reverse direction it will display
overrange.
Exercise 1. The VI Characteristic (Forward Direction)
A circuit for studying the VI characteristic of a
diode is drawn in Figure 9. Notice that the 100 kΩ
resistor is placed in series with the diode. This
resistor serves two functions, firstly it provides a
load for the power supply when the diode resis-
tance is very small and secondly it provides for an
indirect way for finding the diode current. You
measure the voltage drop across this resistor and
then calculate the current from Ohm’s Law (I =
VI/R).
VI
V
+
V
R = 100 kΩ
or 20 kΩ
Vd
direction of
conventional current
the voltmeters
are connected
in parallel
with
resistor
and
diode
direction of
electron current
Figure 9. A circuit to study the VI characteristic of a semiconductor diode. The current through the diode is
determined indirectly—by first finding the voltage drop across the series resistor R and then calculating I from
Ohm’s Law.
Plot I vs Vd for the silicon diode connected in the
forward direction. Take advantage of the full range
of the power supply. Tabulate your data. Tabulate
the resistance of the diode. From your VI graph
interpolate the value of VPN. How well does this
value agree with the value you found in the diode
test part of Exercise 0?
Fit your data to eq[1] or eq[2] and find Ir and
VT from the results of the fit. The trick is to note
that taking the natural logarithm of both sides of
eq[2] gives
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13 The Semiconductor Diode
ln I f = ln I r +
Vd
.
VT
…[4]
Thus a plot of lnIf vs Vd should yield a straight line
with slope = 1/VT and intercept = lnIr. Of course,
you also have the option of inputting the pairs (Vd,
I) into proFit. You can command proFit to change or
transform your data for you. At what value of Vd
does eq[2] cease to be an acceptable fit?
Exercise 2. Studying the Diode in the Reverse Direction
When using the diode in the reverse orientation,
which circuit should you use—the one for a small
or large resistance? (If necessary, review your
notes on the experiment “DC Circuits”.) Can you
measure any current at all in this direction?
Physics Demonstrations on LaserDisc
from Chapter 44 Non-Ohmic Resistance
Demo 18-10 Diode
Demo 18-11 Rectifier Circuit
Demo 18-12 Transistor Amplifier
Activities Using Maple
E13The Semiconductor Diode
This worksheet is a tutorial on the electrical properties of a semiconductor diode. You can input the data
you have collected in this experiment and fit and plot it.
Stuart Quick 94
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