Figure 1a.
To study the I - V curve of a semiconductor diode with forward bias, to observe rectifying properties of the diode.
Semiconductor diode, DC source, 36 W resistor, Vernier Current-Voltage Probe and computer, switch.
Semiconductor devices such as diodes and transistors make use of n-type and p-type semiconductors joined together. In practice, the two types of semiconductor are often a single silicon crystal doped with donor impurities on one side and acceptor impurities on the other. The region in which the semiconductor changes from a p-type to n-type is called a junction. For more information about semiconductors see Tipler's "Physics", pp. 1301-1306.
When n-type and p-type semiconductors are not in contact, the n-region has free electrons, which are major charge carriers. For the p-type, major charge carriers are positive holes (absence of electron) (see Fig.1a).
Figure 1a.
The left side of the figure shows a p-semiconductor, and the right side shows a n-semiconductor, where l designates electrons (-e), and ¢ designates holes (+e).
When n-type and p-type semiconductors are placed in contact, the initially unequal concentration of electrons and holes result in the diffusion of electrons across the junction from the n-side to the p-side and holes from the p-side to the n-side until equilibrium is established. The electrons can not travel very far from the junction region because a semiconductor is not a very good conductor. The diffusion of electrons and holes therefore create a double layer of charge at the junction similar to that on a parallel plate capacitor (Fig.1b).
Figure 1b.
The left side of the figure is the p-side; the right side of the figure
is the n-side.
Thus, there is a potential difference V across the junction, which tends to inhibit further diffusion. In equilibrium, the n-side is at a higher potential than the p-side. In the junction region there are very few charge carriers of either type, so the resistance of this area is very high.
A semiconductor pn junction can be used as a simple diode rectifier.
When we connect the positive terminal of the battery to the p-side, the
potential across the junction will be lowered. The diffusion of electrons
and holes through the junction will increase as they attempt to reestablish
equilibrium, resulting in a current in the circuit (Fig.1c). The diode
is said to be forward biased.
Figure
1c. Forward bias.
The left side of the diode is the p-side; the right side of the diode is the n-side.
If we connect the positive terminal of the battery to the n-side, it
will increase the potential difference across the junction, diffusion will
be further inhibited (Fig. 1d).
Figure 1d. Reverse bias.
The left side of the diode is the p-side; the right side of the diode is the n-side.
A plot of current versus voltage for a typical diode is shown in Fig. 2. Note that if we apply considerably large reverse bias, current can suddenly increase, and the diode can be broken. It happens because in such large electric fields, electrons are stripped from their atomic bonds and accelerated across the junction. These electrons, in turn, cause others to break loose. This effect is called avalanche breakdown.
Figure
2.
Current versus applied voltage across a diode
When the applied voltage is equal to zero there is a small equilibrium
current of electrons I0 through the junction which is compensated
by the same current of holes in the opposite direction. Therefore total
current is equal to zero. When we apply forward bias, additional current
of holes from p-region to n-region is equal to
(1)
(2)
(3)
(4)
(5)START-UP PROCEDURE
1. Set up the computer as instructed in Instructions
for Computerized Experiments.
2. Plug in the Dual Channel Amplifier wires.
WARNING: DURING THE DATA COLLECTION, THE SMALL RESISTORS CAN GET VERY HOT BECAUSE OF THE LARGE CURRENT BEING PASSED THROUGH THEM. TO AVOID THIS, WHILE COLLECTING DATA, RAISE THE VOLTAGE FROM 0 TO 10 V, STOP COLLECTING DATA, THEN TURN THE VOLTAGE ON THE POWER SUPPLY BACK DOWN TO 0 VOLTS IMMEDIATELY.
1. Make sure that your power supply is set on the 10 V scale and that Short Circuit Current is set to 225 mA.
2. Connect the circuit shown in Figure 3. Use a 36 W resistor for R. Make sure that you will be applying forward bias through the diode.
Figure 3.
Diode on a diagram6. Reverse the positive and negative out of the power supply. Current will now flow in the opposite direction, and the diode will be in a reverse bias situation.
7. Change the axes of your graph so that the voltage axis reads -5 to 0 volts, and the current axis reads 0 to 0.05 amps. This is accomplished by clicking on the first and last values on an axis and entering the values that you would like to use.
8. Collect data by slowly raising the voltage from 0 to 5 Volts. Record your observations about this data. Does it look similar to that in figure 2?
9. Shut down the computer as instructed in Instructions for Computerized Experiments.
10. Disconnect all circuits and leave the materials as they were on the lab bench.
1. Find the average value for I0 and its standard deviation. Report the value as I0 ± I0 mA.
2. Find the average value of b1 and its standard deviation, Db1.
3. Calculate the theoretical value of (e/kBT), where T is assumed to be room temperature, approximately 298 K. Be careful of your units!
4. Find the correctional coefficient, b, for your diode, and its associated error, Db. Using correct error techniques (see Fundamentals of Data Analysis), we find that
In the lab report, you should include all of the original data generated in class in your lab report. Make sure to show your calculations for data analysis questions #1-4. In the conclusions, you should include your results with the associated errors for I0 and b. What kinds of errors are present in this experiment? Make sure you include your answer to data analysis question 5.