Laboratory 3
Diode Characteristics
BACKGROUND
A diode is a non-linear, two terminal semiconductor device. The two terminals are the
anode and the cathode. The circuit symbol of a diode is depicted in Fig. 3-1. The diode
is said to be forward-biased if the voltage v across it is positive. The diode is reversebiased if v is negative. An ideal diode behaves as follows: It acts as an open-circuit (i =
0) when it is reverse-biased. It acts as a short circuit (v = 0) if a positive current i is
passed through it. The diode is said to be cut off when reverse-biased, and said to be
turned on when forward-biased.
Fig. 3-1: Circuit symbol of a diode
Diode ratings
The forward current rating and the peak reverse voltage (PRV) rating are two important
diode ratings. The forward current rating is the maximum average current that can be
passed continuously through the diode. The PRV is the maximum reverse voltage that
can be applied to the voltage. The diode can suffer reverse breakdown if this rating is
exceeded. The 1N4001 diode designed for rectifier applications has an average forward
current rating of 1A and a PRV of 50V.
Diode characteristics
The ideal diode behavior described above gives one a basic understanding of how a diode
functions. The i-v characteristics of a real diode are however not as simple as described
above. In the forward-bias region the i-v relationship is described as follows:
i = I s ( e v / nVT − 1) Eq. 3-1
In the above equation, i is the forward current, v is the forward voltage, Is is the saturation
current, and VT = kT/q is the thermal voltage (k = 1.38×10-23 is Botzmann's constant, T is
the absolute temperature in Kelvins, and q = 1.6×10-19 is the electronic charge). The
value of VT at room temperature (20o C) is 25.2mV. The constant n in the diode equation
above has a value between 1 and 2 depending on the material and physical structure of
the diode.
The saturation current Is is small - it is of the order of 10-15A for small signal diodes.
When appreciable current i flows in the forward direction (i>> Is), Eq. 3-1 is
approximated by the following exponential relationship:
i = I s e v / nVT
Eq. 3-2
In logarithmic form the above equation becomes a linear relationship between ln(i) and v.
We have
ln(i ) =
v
+ ln( I s ) Eq. 3-3
nVT
Thus the semilog plot of the i-v characteristics of a diode (with the log scale used for i) is
linear. The value of the diode parameter n can be determined from the slope of this line,
as described in the section `Semi-log graphical considerations'.
In the reverse-bias region (v<0), the diode current is ideally zero. In reality there is a
small negative diode current (of the order of nanoamps for small signal diodes). The data
sheet of a 1N4001 diode shows that its maximum reverse current is 5μA at a temperature
of 25oC. If the reverse voltage exceeds the PRV rating of the diode, the diode
breaksdown and the reverse current becomes large. The diode will be destroyed in the
reverse-breakdown region if the power dissipation rating of the device is exceeded.
Zener diodes
A zener diode (unlike a normal diode) is designed for operation in the reverse breakdown
region. The circuit symbol of a zener diode is depicted in Fig. 3-2. The relationship
between Vz and Iz (in the reverse-breakdown region) is almost linear. The following
equation is used to describe the relationship:
Vz = Vzo + I z rz
Eq. 3-4
Here, rz is the dynamic resistance of the zener at the operating point (Vz, Iz). Vzo is the
voltage at which the straight-line approximation of the i-v characteristic intersects the
horizontal axis. Zener diode specifications include a test point (Vz, Iz) in the breakdown
region, and a dynamic resistance rz at this point. A 1N4739A zener diode has a power
rating of 1W, Vz = 9.1V, Iz = 28mA, and rz = 5Ω.
Fig. 3-2: Circuit symbol of a zener diode
Using an oscilloscope to obtain i-v characteristics of a two terminal device
The circuit depicted in Fig. 3-3 can be used to display the I-V characteristic of a twoterminal device on the oscilloscope screen. The device under test is connected in series
with a resistor R which serves as a current sensing as well as a current limiting resistor.
A sine wave of suitable amplitude and frequency of several hundred Hertz (from a signal
generator) is connected across the series combination. The oscilloscope is set to X-Y
mode. The voltage across the current sensing resistor is fed to the vertical scope input
and the voltage across the device under test is fed to the horizontal scope input. The
oscilloscope settings can be adjusted to obtain a display of the I-V characteristics of the
device under test on the screen. Note that a vertical scale setting of V volts/div
corresponds to a current scale of V/R amp/div. For example, if the vertical scale setting
is 5V/div, and the value of R is 1kΩ, then the current scale is 5mA/div.
Note that a single ended scope probe can be used across R since the lower terminal of R
is connected to ground (the - terminal of the signal generator). However, neither terminal
of the device under test is at ground potential. Therefore, a differential scope probe must
be used across the device under test.
Fig. 3-3: Circuit used to obtain device I-V characteristics
Let us investigate component value selection for the case where the device under test is a
diode whose forward characteristic is required over the current range 0-10mA. Let us set
R=1k. There is a 10V drop across the resistor when the maximum current of 10mA flows
through it. There is also roughly a 0.7V drop across the diode. The signal generator
should thus be set to an amplitude (zero to peak) of 10.7V. If this voltage setting is not
possible with the equipment at hand, choose a lower value for R.
PRELAB
1. A PSpice schematic diagram for obtaining the I-V characteristics of a diode is
depicted in Fig. 3-4. Perform a PSpice simulation of this circuit (sweep the voltage
source over the voltage range 0 to 15V). Use Probe to obtain a plot of diode current i
versus diode voltage v. At roughly what forward voltage does the diode begin
conducting?
2. Use Probe to obtain a plot of ln(i) versus v. Use the slope of the graph (beyond the
cut-in region) to determine the value of the diode parameter n.
3. Fig. 3-4 is essentially a half-wave rectifier circuit. Simulate the circuit using a
sinusoidal voltage source (VSIN) of amplitude 10V and frequency 60Hz. Use Probe
to obtain a plot containing 3 subplots, namely, the source voltage waveform, the load
voltage waveform (voltage across R), and the diode voltage v.
4. From your plots in step 3 above, determine the voltage across the diode when it is
conducting. What is the peak value of the load voltage waveform? Explain why the
peak value is what it is. What is the maximum reverse voltage appearing across the
diode?
Fig. 3-4: PSpice schematic for obtaining diode I-V characteristics
IN LAB
1. Your instructor will demonstrate how to use the curve tracer to obtain a diode i-v plot.
Use the curve tracer to obtain the i-v characteristics of the 1N4001 diode in your lab
kit. Obtain a printout of the characteristics for your lab notebook. Use cursors to
obtain data points on the curve (beyond cut-in). Tabulate this data in your lab
notebook. Obtain a semi-log plot of i versus v (fit a straight line to the data points).
Determine the value of the diode parameter n from the slope of the line (see the
section below on semi-log graphical considerations). Assume that the temperature in
lab is 70oF i.e., 21.1oC. Also use your graph to determine the value of the diode
saturation current Is.
2. Construct the diode rectifier circuit. Use the Wavetek signal generator to power the
circuit with a 60Hz sinusoidal signal. Use the oscilloscope to measure the amplitude
of the sinusoid and adjust it to 10V. Obtain a plot of the load voltage. Determine the
average value of the load voltage. Also obtain a plot of the diode voltage (think about
how you could do this). Label your plots appropriately and paste them in your lab
notebook.
3. The instructor will demonstrate how the i/v characteristics of a zener diode may be
obtained on an oscilloscope screen. Obtain a printout of the i/v characteristics (in the
reverse-breakdown region) of the 1N4739 zener diode in your lab kit. Check the
zener test current and voltage specifications against your measurements.
Semi-log Graphical Considerations
As is clear from Eq. 3-3, there is a linear relationship between ln(i) and v when the diode
current i >> Is. Measured data points (Vd, Id) on the diode characteristic beyond cut-in
can be plotted on semi-log paper using the log scale for i. This semi-log plot is expected
to be linear, and a line of “best fit" for these data points can be drawn. Due to
measurement errors this line will not pass through all the data points.
Let (Vd1, Id1) and (Vd2, Id2) be two points on the line of best fit. From Eq. 3-3 we have
ln( I D1 ) =
V D1
V
+ ln( I s ) and ln( I D 2 ) = D 2 + ln( I s ) Eq. 3-5
nVT
nVT
Subtraction of the second expression above from the first yields
nVT =
V D1 − V D 2
ln( I D1 ) − ln( I D 2 )
Eq. 3-6
The denominator of the above equation involves natural logs while the log axis of a semilog plot is base 10. The following is the relationship between the natural log and log base
10 of a number x:
ln( x ) =
log10 ( x )
Eq. 3-7
log10 ( e)
Substituting this into Eq. 3-6 yields the following equation:
⎡
⎤
V D1 − V D 2
nVT = log10 ( e) ⎢
⎥ Eq. 3-8
⎣ log10 ( I D1 ) − log10 ( I D 2 ) ⎦
The denominator of the above equation reduces to 1 if the two data points on the line of
best fit are selected to be a decade apart (i,e., Id1 = 10 Id2) . The value of the diode
parameter n can thus be easily determined using Eq. 3-8.
The diode saturation current Is can be determined using the value of nVT determined
above and any point on the line of best fit (see Eq. 3-5).