V-I characteristics of Silicon and Germanium diodes and measurement of static and dynamic resistances
The terminal K is called the cathode or the negative
electrode. The terminal A is called the anode or the positive terminal. We also
refer the terminals as p-side and n-side terminals.
A)
FORWARD BIAS: When the terminal ‘K’ is connected to the
negative terminal of the supply and the terminal ‘A’ is connected to the
positive of the power supply the diode is said to be “forward biased”. In other
words when p-side of the junction diode is connected to the positive and n-side
is connected to the negative of the power supply the diode is connected in the
“forward” direction. The diode gets
forward biased only when Vf > Vr. The forward biased
diode is shown in figure (3). The atomic voltage drop across the body of the
device is zero under ideal conditions. In the forward biased diode the height
of the “potential energy barrier “ at the junction gets lowered by the
magnitude of the forward bias VF. This disturbs the initial
equilibrium between the forces tending to cause diffusion of “majority
carriers” across the junction and the opposing influence of the potential
energy barrier at the junction. Now “holes” cross the junction from the
p-region to the n-region while the electrons cross the unction from the
n-region to the p-region. Flow of both types of carriers causes conventional
electric current from p-region to n-region and these components get added.
Hence under forward biased condition (i.e Vf > vr) the
following occur.
1.
Resistance offered by the junction is low.
2.
P-N junction acts as a closed switch.
3.
Width of the depletion region is reduced.
4.
Drift current increases with increase in bias.
Cut-in Voltage Vg: Cut-in voltage is defined as the voltage at which 1%
of the rated current flows. In practical terms, this is the voltage at which
the diode may be considered to start the conduction.
For Ge, Vg = 0.2v.
For Si, Vg= 0.6v.
B)
REVERSE BIAS CONDITION: a P-N diode with reverse bias
condition i.e., with positive terminal of the battery Vr connected
to the n-side and negative terminal connected to the p-side. This reverse bias
causes both holes in the p-region and electrons in the n-region to move away
from the junction. Hence the region of negative charge density on the p-side
and region of positive charge density on the n-side become wider i.e., the
width of the depletion region increases. Further the height of the potential
energy barrier increases with increase in Vr, the applied voltage.
This increased barrier height serves to reduce the flow of majority carriers to
the other side i.e, holes from p-side to n-side and electrons from n-side to
p-side. However the flow of minority carriers remains uninfluenced by the
increased barrier height. Since these minority carriers fall down the potential
energy barrier, nominally zero current flows under reverse bias condition as there
are a few number of minority carriers. However a small current does flow in the
reverse direction i.e., from n-region to p-region across the junction. This
extremely small reverse current is called reverse saturation current (Io)
The magnitude of Io for Ge is about few mA and for silicon it is a
few nano amperes(nA).
Hence under reverse biased condition.
1.
The resistance of the diode increases
2.
The width of the depletion region increases.
3.
The current is extremely low.
5. PROCEDURE:
FORWARD CHARACTERISTICS:
1. Connect
the circuit as shown in the corresponding circuit diagram.
2. Increase
the supply voltage from zero volts. Observe the corresponding value of current.
3. For
every value of forward voltage across the diode, observe the value of current
and record it.
NOTE: The graph should be drawn showing that the voltage Vf
is an Independent parameter hence the supply voltage must be varied and
corresponding value of current must be noted.
4. Observe
the voltage across the diode where current If just starts flowing
through the diode. Now record the values of voltage and current.
5. The
cutin voltage should be clearly observed and noted.
6. Increase
the diode voltage in suitable steps without exceeding the maximum values
indicated after the cutin voltage to obtain a smooth curve.
7. Repeat
the above procedure for Si diode also observing some precautions.
8. Plot
the graphs and obtain the dynamic and static resistance from the V-I characteristics. Compare them with the
expected values.
REVERSE BIAS CHARACTERISTICS:
1. Connections
are to be made as per the corresponding circuit diagram.
2. The
independent parameter i.e., the diode voltage VR is varied from zero
volts and corresponding values of the reverse current IR is
observed.
3. Vary the supply voltage from zero volts. Note
the values of VR and IR.
4. Tabulate
all the observations.
5. Repeat
the above procedure for Si diode.
6. Find
the dynamic and static resistance from the graph.
OBSERVATIONS:
For Ge diode
10. VIVA QUESTIONS:
1. What do you understand by a junction diode?
2. Is the P-N junction diode a passive element
or an active element?
3. What is the importance of the type number
given to the various diodes?
4. What is meant by potential barrier across a
P-N junction?
5. What is the significance of a diode as a
device?
6. What is cut in voltage? What is the value
of cutin voltage for Ge
and Si
diodes. What is the reason for the difference in cutin?
Voltage of Ge
and Si.
7. Explain physically how a P-N junction
functions as a rectifier.
8. What is the expression for the total
current in a P-N junction? How
does it vary
with the applied voltage?
9. What do you understand by a reverse
saturation current? What
are the
typical values?
10. Why is the magnitude of the current in the
forward biased diode
greater than
that in the reverse biased diode?
11. How does the reverse saturation current vary
with temperature
for Ge and Si
diodes? Is it of significance while the circuit
designer
chooses a particular device in design?
12. What do you understand by dynamic and static
resistance? How
are these
values obtained graphically?
13. Define the terms forward and reverse
resistance of a P-N
Junction
diode.
14. Explain the capacitive effects in a junction.
15. What are the various applications of a P-N
junction diode?