Download semiconductor diodes

SEMICONDUCTOR DIODES
I. OBJECTIVES
a. The determination of the current-voltage characteristics for the rectifying diodes and for the Zener
diodes;
b. The determination of some static and differential models;
c. Understanding the regulation properties of a parametric voltage regulator with Zener diode.
II. COMPONENTS AND INSTRUMENTATION
For the experiments you will use Si diodes: a 1N400x rectifying diode and a PL5V1 Zener diode; 4
resistances with different values, a line transformer and a decade resistance box. The dc voltage is obtained
from a double regulated power supply and the sinusoidal voltage (with variable amplitude and frequency) is
obtained from a signal generator. To visualize the variable voltages and the diodes’ characteristics you will
use a dual channel oscilloscope. You will also use a multimeter or even two if necessary.
III. PREPARATION
1. P. THE DIODES TESTING
The ohmmeter is quite useful for the quick analysis of diodes with junctions because the ohmmeter’s
equivalent circuit contains a voltage source and a resistance (as you can see from the marked area in Fig.
5.2).

Why does the ohmmeter shows a lower value of the resistance when a good diode is connected with
the anode to the positive lead and with the cathode to the negative lead (of the ohmmeter) comparing
with the situation when you have the anode to the negative lead and the cathode to the positive one?
2. P. RECTIFYING DIODES
2.1. P. CURRENT- VOLTAGE CHARACTERISTIC iD(vD)

How does iD(vD) characteristic of a diode looks like?
iD – the current through the, from anode to cathode;
vD – the voltage drop across the diode, from anode to cathode.
The IN400x diodes are characterised by the equation:
v
i D  Is exp( D )
nVT
Is = 2.3 * 10-9 A
n2
VT = 25 mV, at 270 C

What are the values of the static resistance?
V
rD1  D1
and
I D1
rD1 
VD 2
I D2
in the points from the diode’s characteristics in which ID1 = 30 mA and ID2 = 200 mA?
 What are the values of the differential resistance (rd1 and rd2) in the same points as before?
VD
rd 
I D
1
For each of two above operating points you will consider another one in its neighbourhood. For example: for
D1 you can consider D1’ in which: ID1 = 33 mA, then: ID1 = ID1 – ID1’
2.1.1. P. Point by point method
If you have nothing better to do, go ahead.
2.1.2. P. The characteristic on the oscilloscope
In order to see the characteristic on the oscilloscope you can use the assembly from Fig. 5.4 or Fig. 5.5.
The resistance, RT, has the role of a current - voltage traducer, necessary to visualize the currrent through the
diode.



Why the ground (GND) of the oscilloscope cannot be connected between D and R if v I is a earthed
(non floating) source (figure 5.4)?
What quantities will appear on the two axes of the oscilloscope (X and Y), for each assembly (Fig
5.4 and Fig 5.5)?
In which quadrant will be the oscillograms obtained in each situation?
3. P. ZENER DIODE
3.1. P. iZ(vZ) CHARACTERISTIC
3.1.1. P. Point by point method



Which is the Zener voltage of the PL5V1 Zener diode?
How does iZ(vZ) characteristic looks like, with the senses of iZ and vZ as in Fig. 5.6?
If you know the operating points from the Zener diode’s characteristic
{ IV
= 100 mA
Z1 = 5.1 V
Z1
and
{ IV
= 90 mA
Z2 = 5 V
Z2
What are the values of the static resistance rZ and the differential resistance rz in the point (IZ1, VZ1)?
3.1.2. P. The characteristic on the oscilloscope
You will consider the requests from 2.1.2. P.
IV. EXPLORATIONS AND RESULTS
1. THE DIODES TESTING
Exploration
With the digital ohmmeter you will check the status of the junctions of the rectifying diode and the Zener
diode (Fig. 5.2).
 If the ohmmeter has drawn the symbol of the diodes on one of its domains, you will do the measurement
within that domain, otherwise you will use any domain you want;
 Connect the rectifying diode with the anode of the (+) lead of the ohmmeter (forward bias of the diode)
and then read the value;
 Reverse the direction of the diode’s connection (reverse bias) and read this value too;
 Repeat the measurements for the Zener diodes:
Results
 The values obtained after the 4 measurements (2 for each diode in forward and reverse bias)
 If the diode is good, the results should be found in table 5.1:
2
RO
+ (red wire)
D
Figure 5.2. Diode testing
with the ohmmeter
D
VI +
- (ground)
The ohmmeter
The ohmmeter
shows a low
value of the
resistance
The ohmmeter
shows a high
value of the
resistance
Table 5.1
D – forward bias
D – reverse bias
Observations
The ohmmeter’s domain
With D’s symbol
Without D’s symbol
0.7 – 0.9
Low resistance
> 2 or over the scale
High resistance
The voltage across the
There is at least one
diode is shown
magnitude order as difference
If you obtain other situations the diode is damaged (short circuit or open circuit).
2. RECTIFYING DIODES
For the experiments you will use a 1N400x semiconductor diode, where you can take any value between 1
and 7.
2.1. CURRENT-VOLTAGE CHARACTERISTIC iD(vD)
2.1.1. The point by point method
Exploration
You will build the assembly from Fig. 5.3.
R
-
+
mA
VI
+
+
100/5W
V
D
-
Figure 5.3 Arrangement for
plotting the diode terminal
characteristic
D – forward bias
 VI – use a dc adjustable voltage source
 The miliammeter shows the current iD and the voltmeter shows the voltage vD
 Modifying the voltage VI in the domain [0, 12] [V] you will measure some pairs of (iD, vD).
D – reverse bias
 You will replace the positive voltage source VI from the schematic in Fig. 5.3 with a negative voltage
source (-40, 0) [V]. To obtain a voltage bigger than 20V (absolute value) you will connect in series the
two voltage sources from the double dc regulated power supply.
 Modifying the voltage VI in the domain [0, -40] [V] you will measure some pairs of (iD, vD).
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The temperature’s influence
 Using the circuit from Fig. 5.3 you will find a value for VI in order to have iD = 50 mA. You will
measure vD with a dc voltmeter;
 You will warm up the diode’s leads;
 Keep iD = 50 mA by adjusting a proper value for VI. Measure vD.
Results.
 Table with the values of iD, vD for all the points measured in E.2.1 (both forward and reverse bias);
 Graphic iD (vD);
 Which is the diode’s threshold value?
 You will choose for D two operating points (ID1, VD1) and (ID2, VD2) at ID1  30 mA and ID2  100 mA.
 You will determine the static resistances rD1 and rD2 in these points. In which point the static resistance
has the higher value?
 What elements constitue the static model of the diode D in the two operating points? (See 2.1. P.)
 You will determine the differential resistances rD1 and rD2 in the diode’s operating points (see 2.1. P.).
For the calculation you will use the values from the operating point and a point in its neighbourhood
(according to the table with iD, vD)
 Compare the values rD1, rD2 that determine the differential model of the diode in the two operating
points.
The temperature’s influence
 The vD’s values, before and after the warming up, at the same current iD = 30 mA. Which is the effect
of temperature growth on the voltage across the diode?
2.1.2. The characteristic on the oscilloscope
Exploration.
a) With an earthed source (signal generator), you will build the assembly from Fig 5.4.
R
X
D
470
~
Y
vI
RT
G



Fig. 5.4 Arrangement for displaying the
iD-vD characteristic using an earthed
source
vI – sinusoidal voltage with 10V amplitude and 100Hz frequency, obtained from the signal
generator;
With the oscilloscope on the Y – X mode, with the input X (Y) of the oscilloscope in the point X
(Y) and ground of the oscilloscope in the point G, you will visualise the diode’s characteristic;
You will also visualise the diode’s characteristic for a 2 kHz frequency of the input signal.
b) With a floating source, build the assembly from Fig. 5.5.
R
Tr
220V ~
X
470
vI
D
G
10
RT
Figure 5.5 Arrangement for
displaying the iD-vD characteristic
using a floating source
Y
4


vI – sinusoidal voltage with 50Hz frequency obtained from a line transformer;
In the same way as to the point a.) you visualise the diode’s characteristic.
Results.
a) With an earthed source
 Draw the characteristics that you have obtained on the oscilloscope for the frequencies of 100 Hz
and 2 kHz of the input sinusoidal signal.
 What is the difference between the diode characteristic from the oscilloscope with the frequency
of 100 Hz and the one drawn with the point-by-point method?
b) With a floating source
 Draw the obtained characteristic from the oscilloscope
 What is the value of the current trough the diode at VD = 0.65 V?
3. ZENER DIODE
3.1. iZ(vZ) CHARACTERISTIC
3.1.1. Point by point method
Exploration
Forward bias
You will use the assembly from Fig. 5.3, but the diode is replaced with a PL5V1 Zener diode.
 Changing the source’s voltage, VI, in the domain [0, 12] [V] you will measure some pairs of (iZ, vZ)
Reverse biasing

Because of the small variations of the voltage at the Zener diode’s leads in the regulation area, the
connection of a voltmeter in parallel with the Zener diode will hardly catch up these variations. In
order to obtain the static characteristic in the regulation area you may use the opposition method.
Vmeas
R
+
mA
+
-
V
-
100/5W
VI
vZ
ZD
VREF
iZ
Fig. 5.6 Arrangement
for Zener diode
characteristic in the
reverse biasing regime.
The voltage on the diode is: vZ = Vmeas + VREF
You will use the experimental assembly from the Fig. 5.6.
VREF – is a regulated dc voltage, with its value equal to the regulation voltage of the Zener diode (in this case
5.1 V). VI – is an adjusted dc voltage source
 Changing the voltage VI in the domain [0, 12] [V] you will measure some pairs of (iZ, vZ)
Results
 Table with the values of iZ, vZ both for forward and reverse biasing of the Zener diode;
 Graphic iZ,(vZ)
 What is the value of the regulated voltage of ZD (read from the characteristic)?
 Which are the values of the static (rZ) and differential (rz) resistance for IZ = -100 mA
 Which are the values of the components that form the static and differential models of ZD (at low
frequencies)?
3.1.2. The characteristic on the oscilloscope
Exploration
You will do the same things as in 2.1.2, but replace the diode with a Zener diode.
Results
Draw the obtained characteristic from the oscilloscope.
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