ECE 335: Laboratory Notebook - Diode Characterization
Lab
2
II. Diode Characterization
A.
Objective
The purpose of this lab will be to introduce you to semiconductor diodes. You will use LabView, Matlab,
and the associated pieces of equipment to obtain an I-V characteristic for your diode.
B.
Needed Hardware
C.
A 5V Zener Diode (or approximate)
1 Op-Amp chip (most any commercial Op-Amp should do, i.e. u741, OP27, etc.)
A solderless breadboard
Wire
Wire cutter/stripper
Pre-Lab
Read the lab.
Familiarize yourself with diodes.
Generate a SPICE netlist capable of simulating the Forward Bias Characteristic of the diode.
(Obviously, you will be using the data you collect from the lab to determine the model
parameters. Therefore you won't be able to accurately simulate the diode before coming to
lab. However, you should have a netlist where all you need are the model parameters and
you will be able to simulate.)
Generate a SPICE netlist capable of simulating the Reverse Bias Characteristic of the diode.
D.
Background
The relationship between the current through a diode and the voltage across it is given by the following
equation:
1
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Diode Characterization | Ethan Farquhar, Ph.D.
ECE 335: Laboratory Notebook -Diode Characterization
Where iD is the diode current, vD is the voltage across the diode, IS is the reverse saturation current, UT is
thermal voltage, and n is the ideality factor (usually n=1).
Your book uses VT for thermal voltage, however, in an effort to keep it unambiguous from terms like
threshold voltage (which you will learn about later), I will always refer to thermal voltage as UT. Thermal
voltage is defined by the following equation:
k = Boltzmann's constant = 1.38x10-23 J/K
q = Charge of an electron = 1.6 x 10-19 Coulombs
T = absolute temperature 300 @ room temp
At room temp, UT = 0.0258V or approximately 26mV. However, due to ease of calculation issues, you
will frequently see this estimated by 25mV. Also, be warned: k (Boltzmann's constant) is frequently
given as k = 8.617x10-5eV/K. This number is the number you see above already divided by the charge of
the electron. In other words, it is . Make sure you know which one you are dealing with when you
calculate UT.
Under forward bias conditions, you would expect to get a current versus voltage plot similar to that
shown in Figure II-1.
Figure II-1 Typical diode under forward bias conditions
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Diode Characterization | Ethan Farquhar, Ph.D.
ECE 335: Laboratory Notebook -Diode Characterization
Figure II-2 Diode under reverse bias conditions
Under reverse bias conditions, we usually assume that the diode will not let any real amount of current
to flow through it. However, as you can see from Figure II-2, the diode will break down at a certain
point. Normal diodes can have breakdown voltages greater than 100V. However, zener diodes are
special diodes that are designed to breakdown at specific voltages. A 5V zener will breakdown when
there is 5V reverse bias. It will “clamp” the voltage at that point too! This will be useful for making DC
power supplies.
E.
Curve fitting exponentials
Many times, you can gain insight into something by plotting it in a different way. This is certainly the
case with an exponential. Note the top plot in Figure II-3. This shows the plot of a generic diode plotted
in a way that you are used to, with both the x and y axes linearly spaced. However, if you plot that same
data with the y axis spaced logarithmically (and the x axis spaced linearly), something very interesting
happens. The exponential now looks like linear! This is shown in the bottom plot of Figure II-3
Ethan Farquhar, Ph.D. | Diode Characterization
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ECE 335: Laboratory Notebook -Diode Characterization
Figure II-3 Data from a diode plotted both linearly and with the y axis spaced logarithmically. Matlab knows how to plot a y
axis spaced logarithmically... all you have to do is, instead of using the plot command, use the semilogy command.
What happens if you take the ln() of both sides? (Remember that in Matlab the natural log, ln(), is
calculated when you issue the command log. To get the base 10 log, log(), you issue the command
log10.) First, which part of the equation dominates from = 0.1V to 0.8V? (Recall the equation)
1
The exponential does. The -1 contributes almost nothing to this equation in this region, so we are going
to drop it, yielding:
Now, if we take the natural log (ln()) of both sides we get:
ln ln e
ln !
ln ln Why do we take the ln() of the current? Well, if you notice from Figure II-3, we have the ln(current)
plotted vs. the voltage, and that gives us a straight line. Well if you look at the equation above, we have
the "! and in one equation. Remembering that the equation of a line is:
# $%
&
From this we can deduce that, for this device:
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Diode Characterization | Ethan Farquhar, Ph.D.
ECE 335: Laboratory Notebook -Diode Characterization
# ln $
1
!
% & ln You can simply curve fit this using the same polyfit and polyval functions that you used last week, if you
do a few special things. Since the diode current is linear when the y axis logarithmically spaced, the
Matlab command:
curve_ fit = polyfit(voltages,log(currents),1)
'
will give you the appropriate curve fit. This will give you the slope ($ ) and the y intercept
(& "! ). Since the y intercept returned is the ln() of the y-intercept, you need to take the
exponential of what polyfit tells you is the y-intercept in order to get the actual y-intercept... However,
'
the slope is already in linear space… so whatever it returns to you is simply .
F.
Forward Bias Experiment
1.
Introduction
For the setups in this lab, you will use the PCI-7831 and similar code to what you used in Part 2 of Lab 1.
You will also be using the same HOST_V_ISweep_HP34401 VI that you used last week.
A
B
Figure II-4 (A) PCI-7831R and the (B) SCB-68 Breakout Box
Ethan Farquhar, Ph.D. | Diode Characterization
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ECE 335: Laboratory Notebook -Diode Characterization
2.
Current Issues
One other issue with the D/A converters is that sometimes they cannot supply enough current to
adequately supply a particular circuit. The D/As on PCI-7831 can only supply around ±2.5mA each. Your
diodes will need much more than this! Therefore we are going to build a voltage buffer circuit. This
circuit will take a voltage in and output the same voltage, however, the Op-Amp can usually supply large
currents. This will be very useful! (Note that it will be important to make sure that at the end of your
sweep, you reset the output voltage on the D/A to 0V. If you don't the diode will constantly have a lot
of current running through it, and the Op-Amp will have to supply it. This will make them get HOT!)
To build the buffer circuit, hook your commercial Op-Amp in the configuration shown in Figure II-5. You
will now have a buffer. Remember this circuit for the future, as it will be very useful.
Figure II-5 Standard Voltage buffer circuit.
3.
Experiment
You are going to perform a characterization of a diode similar to what you did with the resistor in last
lab. You are going to curve fit this diode in order to determine various parameters which you will use to
simulate your diode in SPICE. The following is a block diagram of the setup for this week.
Figure 6 Block diagram of setup.
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Diode Characterization | Ethan Farquhar, Ph.D.
ECE 335: Laboratory Notebook -Diode Characterization
1. Using LabView and code (similar to last week), hook D/A channel 0 to the input of your buffer
circuit.
2. Hook the output of your buffer circuit to the appropriate diode terminal, and the other diode
terminal to the 34401.
3. Sweep the voltage from 0V to 0.8V (remember to set the voltage back to 0V at the end of the
loop), and gather the current data.
4. Use the data to calculate IS and n.
5. Using these parameters, simulate this diode using SPICE under forward bias conditions.
6. Plot a single linear-linear graph showing both the real data, and the SPICE data.
7. Plot a single logy-linear graph showing both the real data, and the SPICE data.
G.
Reverse Bias
You are now going to use the same code, but sweep the diode with negative voltages.
Obviously, the current should not change much until you get to the breakdown voltage of your
diode. You don’t need collect a lot of data between 0V and the breakdown voltage of your
diode, but near breakdown, you should collect a significant number.
1. Using Labview and from above, collect reverse bias data from your Zener Diode. Make sure that
the voltage you go to is large enough to break down the Zener. However, you don’t need to
drive it much more than 1-2V past where it breaks down.
2. Using Matlab, determine the parameters VZ and RZ (These are not exponential… so no need for
all of the explanation above for this part).
3. Use VZ and RZ to simulate the reverse bias I-V characteristics of the zener diode. (Hint: VZ
corresponds to BV in SPICE, and RZ will be the Ohmic resistance RS.)
4. Turn in a single graph with both the data and the simulation plotted on a single axis. Explain any
deviations.
H.
Diode Picture
Your diode will look something like that shown on the left of Figure II-7. It corresponds to the more
familiar symbol on the right of the same figure.
Figure II-7 The line on the packaging of real diodes corresponds to the line in the circuit schematic. These diodes are pointing
in the same direction
Ethan Farquhar, Ph.D. | Diode Characterization
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ECE 335: Laboratory Notebook - Diode Characterization
Lab
2
TA Check Off Sheet
What is your ?
______________________________________
What is your n?
______________________________________
Good forward bias curve?
______________________________________
What is your () ?
______________________________________
What is your *) ?
______________________________________
Good reverse bias curve?
______________________________________
TA Signature
______________________________________
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Diode Characterization | Ethan Farquhar, Ph.D.