The Semiconductor Diode
Department of Physics
Ryerson University
rev.2013
1
Introduction
A diode is a device which allows a large current to flow in one direction by properly
arranging the polarity of the applied voltage (”forward biased”’). When the polarity of
the applied voltage is reversed (”reverse-biased”) no current will flow through the diode.
In actual practice, in a semiconductor diode, there is always a very small reverse current
termed I0 . This current is normally so small that its effects may be ignored in a circuit.
The semiconductor diode usually consists of a piece of semiconductor material doped so
that one region is positively doped (P-type) and the adjacent region is negatively doped
(N-type) as shown in Fig. 1. Note that this does not mean the material carries a positive or
negative static charge. It means that the charge carriers in the P-type region are positive,
and the charge carriers in the N-type region are negative.
Diodes can be used to create two of the basic building blocks in Boolean logic circuits:
AND and OR gates. Other computer circuits may be constructed with the addition of
the transistor, which may be thought of as two diodes placed back-to-back, sharing a base
region in common as shown in Fig. 2.
a.
b.
p n
Figure 1: a. a p-n junction. b. The associated circuit schematic.
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a.
collector
b.
n p n
emitter
emitter
base
collector
base
Figure 2: a. an n-p-n bipolar junction transistor. b. The circuit schematic for bipolar
junction transistors.
In this experiment, an N-P-N transistor will be used to observe some characteristics
of a diode. By observing the current versus voltage characteristic of the forward biased
P-N junction, it is possible to measure Boltzmann’s constant kB (which is an important
fundamental constant you have encountered in the kinetic theory of gases).
A P-N semiconductor junction has a current-voltage relationship described by:
I = I0 (exp
qV
− 1)
kB T
(1)
This equation is plotted for both forward and reverse bias in Fig.3. It is interesting to
note that this equation applies to both reverse bias and forward bias. In this equation, T
is the temperature of the junction in Kelvin, kB is Boltzmann’s constant, V is the applied
potential across the junction, q is the charge on the charge carriers, and I is the actual
current that flows through the junction. I0 is a very small value, on the order of µA or nA.
When the diode is reverse biased, V is taken to be negative, and you can see by inspection
of the equation that the exponential term will be small, and the current I is thus always
less than the maximum for reverse bias, I0 . When the reverse-bias voltage becomes very
large, the exponential term approaches zero, and I approaches I0 . In forward bias, the
exponential term will quickly become large since the forward bias voltage is now positive,
and the −1 term in the equation can be ignored. For forward bias conditions, the equation
becomes:
I = I0 (exp
qV
)
kB T
(2)
In a silicon transistor (Fig. 2) the emitter current diffuses almost entirely across the
base region to the collector with only a small portion flowing out the base connection.
Hence the collector current will be proportional to exp kqV
representing the emitter-base
BT
2
Current (A)
junction current. In this experiment, we will measure the collector current, and the baseemitter voltage, to obtain a close approximation of the ”ideal”’ P-N junction currentvoltage relationship.
0
0
Voltage (V)
Figure 3: Characteristic diode current as a function of voltage, from Eq.1.
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2
Experiment
2.1
Equipment
• silicon bipolar junction transistor (NPN) (TIP41A)
• Dual multimeter box
• 200 mA current supply
• Thermometer (0 - 100 ◦ C)
• Insulated cup
• ice cubes
• hot water (∼50 ◦ C)
2.2
Procedure
1. Connect the emitter and the base of the NPN transistor to the multimeter supplied,
set on the ”diode” setting (see Fig. 1 for symbol). The diode setting is similar to the
resistance setting, but the difference lies in the test voltage applied. In the resistance
setting, the meter automatically applies a voltage which is sometimes too low (0.65
V) to cause the diode under test to go into full forward conduction. The diode setting
applies a voltage of 1.5 V to the diode being tested, which is adequate to cause full
forward conduction for most diodes (other than light emitting diodes, which require
more than 2.1 V). Note the reading on the meter, and then reverse the transistor
leads on the meter and again note the meter reading. Do the same for the collector
and base leads. This procedure should convince you that the transistor does consist
of diodes, and that diodes conduct in one direction only. The base region is common
to both diodes that form the transistor. It should be noted that the base region
is made very thin in a transistor to obtain effects not obtained by connecting two
separate diodes together.
2. Connect the transistor according to the circuit diagram shown (Fig. 4). Measurements of the collector current and emitter-base voltage are to be made, up to a current
of 200 mA. Start at the lowest current that can be measured, and record both voltage
and current. Take successive values of the current and voltage, doubling the current
each time until the 200 mA limit is reached. Be careful not to exceed the 200 mA as
this is the limit for the power supplies. Record the room temperature which will be
the approximate temperature of the transistor.
3. Repeat the procedure in 2, with the transistor immersed in a mixture of ice and
water. Record the temperature of the ice-water mixture.
4
_
current
supply
+
emitter
V
base
I
collector
Figure 4: The experimental setup to measure the I-V characteristic of the ideal diode.
4. , Repeat the procedure in 2 once more, with the transistor immersed in hot tap water
(∼50◦C). Record the temperature of the water.
5. Dismantle the circuit, and dispose of the water down the sink.
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3
3.1
Report
Analysis
By taking the natural logarithm of both sides of Eq. 2, the equation of a straight line
in the form of y = mx + b will result with ln I as the dependent variable, and V as the
independent variable. Find the expression that represents the slope of the straight line.
Plot ln IC vs. VEB on a graph. Use the method of least squares to obtain both the best
slope and the y-intercept for each line. Obtain the value of Boltzmann’s constant and the
reverse saturation current I0 from this data.
−E
For the small range of temperatures used in this experiment, the equation I0 = B exp kB Tg
shows the dependence of the reverse saturation current I0 on temperature T and band gap
Eg , where B is a constant. Plot ln I0 vs. (1/T ) and obtain a value of the band gap from
the best fit straight line. Compare this value to the band gap for silicon, 1.1 eV.
Be sure to include error calculations in your analysis, and sample calculations in your
report.
3.2
Discussion
Comment on the validity of dropping the −1 term in Eq. 1 to obtain Eq. 2. Verify this by
using one set of voltage and current measurements from your data.
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