Rensselaer Polytechnic Institute
Class 26
Introductory Physics II
PHYS 1200 Fall 2008
p and n Semiconductors
You will investigate a rectifier that is formed by the junction of a p-type semiconductor with an
n-type semiconductor.
Equipment:
Mobile Card oscilloscope ( 2 channels) and function generator, breadboard, diode, 470 ohm
resistor, six wires.
1. Wire the diode and resistor in series on the breadboard. (For consistency, place the diode
to the right and the resistor to the left. Place the white line on the diode to the right side as
well.) We will use the diode to control the current flow through the device and the
resistor to monitor the current.
2. Start the Mobile Card and set up the FG1 to create a 2V peak to peak triangle wave. Use
FG1 to drive the resistor and diode in series by wiring the ground side of FG1 to the
white line (right) side of the diode and the AWG1 high side of FG1 to the other side of
the resistor.
3. Wire oscilloscope Ch1 to measure differential voltage across the diode (A1+ to the right
and A1- to the left). Wire Ch2 to measure differential voltage across the resistor (A2+ to the
right and A2- to the left).
The circuit is now set up to drive the whole series circuit with a triangle wave and then
monitor the current (voltage across the resistor/470 ohms) and the voltage across the diode.
The result on the oscilloscope should look like this:
When you get something like this, store it using the screenshot function of the Mobile Card.
4. Store your data in the comma-separated-value format. You need only save 1 point for
every 10 measurements.
5a) Switch the Horizontal Display mode on the oscilloscope to X-Y. It should look like the
figure under section 6 below. Show it to your TA or instructor. You have now finished
the in-class portion of the lab.
Turn the paper over. There is more on the back.
(C) 2009 Rensselaer Polytechnic Institute
Do NOT write on these sheets or take them with you! The next class needs them too!
**********************************************
5. Open the CSV file in Excel and edit it to keep just one sweep of data from about -0.6V to
about +0.6 V across the diode. (You should also see about 0 to 0.4V across the resistor.)
There should now only be about 400 data points.
6. Plot the diode voltage on the x-axis against resistor voltage on the y-axis. (The resistor
voltage is proportional to current (VR=I*470), so now you have a plot of current through
the diode as a function of voltage across the diode.)
It should look something like this:
Ch2 (A2 DIF):
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
According to theory, the current through the diode, is controlled by two separate processes.
One depends on the applied voltage according to the equation,
eV
I  I 0 (e kT  1) ,
and the other according to the equation,
eV
2 kT
I  I 0 (e  1)
where I is the current, the e in the exponent is the elementary charge, V is the applied voltage,
k is the Boltzmann constant, and T is the absolute (Kelvin) temperature (273 K = 0°C). I0 is a
constant that depends on the particular diode being used. We will use a combination of these
equations of the form,
eV
1.6 kT
I  I 0 (e
 1)
Notice that for very large negative values of V, the exponential goes to zero, and the reverse
current becomes – I0. This quantity is too small to measure with the apparatus that you are
using today, but you can get it from the function that the computer will fit to your data.
When V is positive, the exponential becomes much larger than one, so the data for positive V
can be fitted with the equation,
I  I 0e
eV
1.6 kT
.
7. Choose data points from your spreadsheet for VR>0.05 and make a separate plot of this
data. Fit this data with an exponential trendline. It should look like this.
(C) 2009 Rensselaer Polytechnic Institute
Do NOT write on these sheets or take them with you! The next class needs them too!
0.5
0.45
y = 1E-05e17.614x
0.4
0.35
0.3
Series1
0.25
Expon. (Series1)
0.2
0.15
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
8. What is the value of the constant
e
from your trendline fit?
1.6kT
e
, find the apparent temperature of the diode when you did the
1.6kT
e
 7.25  103 K/V ) Does the temperature seem reasonable?
experiment. (Use
1.6k
9. From your value of
Prepare the answers and plots in either a paper or Word file document and send to
persap@rpi.edu or deliver to the Physics Office by the time listed above.
(C) 2009 Rensselaer Polytechnic Institute