Download Diode Lab

Kirsten Strandjord
Kevin Williams
Diode Lab
Abstract
The subject of this experiment is to study diodes and their effects in circuits. The main
conclusions of the experiment are that the ideality factor of the 1N4004 diode is 1.85, the saturation
current is 8.0 * 10-9 amps for the 1N4004 diode when in series with a 470 Ω resistor, and that the diodes
can act as rectifiers when configured in certain ways.
Introduction
Diodes are created when N-type and P-type semiconductors are combined. This combination
allows current through easily in one direction, and slows it significantly in the other direction. This
occurs near the depletion zone where some electrons cross over from the N-type and recombine with
‘holes’ on the P-type side. Cartoons depicting a diode are given in Figure .5a & b. This experiment was
designed to study a specific diode type (1N4004) in circuits.
N Type
P Type
-
Net
(+)
Net
(-)
+
+
Figure .5b shows the band location on the diode
1N4004 and the whereabouts of the N and P type
semiconductors.
+
+
-
+
N Type
P Type
depletion zone
Figure .5a shows a diode which consists of
both N-type and P-type semiconductors.
band
Experiment and Results
For the first set-up of the lab, the circuit was set up as shown in Figure 1 using a breadboard.
The power supply and the diode were connected with a forward bias. The voltage across the diode was
measured while the current from the power supply was manipulated. Figure 1.5 shows the resulting
currents and voltages measured. The equation of fit for Figure 1.5 suggests that the saturation current is
8.0 * 10-9 amps. It also suggests that the constants in the exponential are equal to 21.441 V-1. This
constant is equal to q/nkT. Where q is the fundamental charge, n is the ideality factor (which is between
1 and 2), k is the Boltzmann constant and T is the temperature (Kelvin). If we solve for n:
𝑞
21.441 = 𝑛𝑘𝑇 ⇒ 𝑛 =
𝑞
21.441 𝑘𝑇
⇒ 𝑛 = 1.85
“The ideality factor of a diode is a measure of how closely the diode follows the ideal diode
equation.” (Ideality Factor) . Our ideality factor is closer to 2. This means that the current is dominated
by the recombination of electrons with holes in the depletion zone.
Figure 1.75 is a graph of the reverse bias of the configuration in Figure 1. This means that the
positive and negative connections of the Power Supply were switched.
470 Ω
○
○
+
○
A
Power
Supply DC
Diode
Figure 1
+
○
V
Figure 1 shows a circuit involving
a resistor, an ammeter (A), a
diode, a voltmeter (V), and a DC
power supply connected as
shown. The Figure shows
- ) and positive (○
+ )
negative (○
ends.
Current vs. Voltage With Forward Bias
0.002
y = m2*(exp(m3*x) - 1)
Current (amps)
0.0015
m2
m3
Value
8.011e-9
21.441
Error
1.5342e-9
0.33727
Chisq
R
1.5503e-8
0.99883
NA
NA
Figure 1.5 shows the current vs. voltage
resulting from the set-up shown in Figure 1
by altering the power supply. The equation
for the fit is
I = 8.011 * 10-9 * (e(21.441 * V) – 1).
0.001
The maximum forward bias voltage the diode
can handle before it is destroyed is 1 Volt.
We were within 58% of this limit.
Figure 1.5
0.0005
0
0.44
0.46
0.48
0.5
0.52
0.54
Voltage (volts)
0.56
0.58
0.6
Current vs. Voltage With Reversed Bias
0
Current (amps)
Figure 1.75
-5 10
-7
-1 10
-6
-1.5 10
-6
-2 10
-6
Figure 1.75 shows the current vs. voltage
resulting from the set-up shown in Figure 1
by altering the power supply.
-25
-20
-15
-10
-5
0
Voltage (V)
For the second setup, the circuit schematic was as shown in Figure 2. We observed the
oscillating current using a two channel oscilloscope. Figure 2.5 shows a cartoon of what was observed
on the oscilloscope for voltage/time. We observed a sinusoidal curve with amplitude of .64 Volts and a
frequency of 892.8 Hz. Figure 2.5 shows both positive and negative voltages.
470 Ω
~
Figure 2 shows a circuit involving
a resistor, a two channel
oscilloscope (OSC), a Wavetek
Meterman FG2C function
generator (~), and a diode.
Diode
OSC
Figure 2
Figure 2.5
Amplitude
= .64 Volts
Figure 2.5 shows the sinusoidal
curve that results from our
configuration in Figure 2 and is
shown on our oscilloscope.
Frequency of oscillation =
892.8 Hz
For the third set-up, the circuit configuration is shown in Figure 3. Figure 3.5 shows a cartoon of the
voltage vs. time observed by the oscilloscope. A sinusoidal curve with only positive voltages appears on
the oscilloscope. The diode setup creates a rectifier, which only amplifies the positive voltages and
converts the alternating current into a direct current. This is why we see only the positive amplitudes.
The frequency of oscillation in the set-up in Figure 3 is the same for the set- up in Figure 2 because the
voltage is driven by the same function generator and is left at the same frequency.
Figure 3 shows a circuit involving
a resistor, a two channel
oscilloscope (OSC), a Wavetek
Meterman FG2C function
generator (~), and four diodes.
470 Ω
~
OSC
Figure 3
Amplitude
= .92 Volts
Figure 3.5 shows the positive
portions of a sinusoidal curve
that results from our
configuration in Figure 3 and is
shown on our oscilloscope.
Frequency of oscillation =
892.8 Hz
Conclusion
To conclude, our experimentally determined ideality factor was 1.85 (which lies between 1 & 2,
the limits for the factor), and the coefficient in the exponent was found to be 24.441 V-1. When the
circuit was arranged as shown in Figure 2, the voltage across the resistor was purely sinusoidal. This
varied greatly from the voltage across the resistor in the configuration shown in Figure 3, which only
showed the positive values of the sinusoidal wave. This suggests that diodes can be used as a rectifier to
change an AC current into a DC current. However, the frequency was not noticeably changed.