it will help in clearing your concepts about diode.

1. Electronic Devices &
Circuit :
Topic :
Diode Theory

2. D-Group : Dept. : I.C.
Name Enrollment no. Roll no.
Vasani Bhavya V. 130030117058 55
Sinojiya Raj A. 130030117053 49
Sharma Yashraj V. 14003311713 65

3. Basic Ideas :
A diode is non-linear device because the graph
of its current versus voltage is not a straight
line.
The reason is the barrier potential.
When the diode voltage is small than the
barrier potential, the diode current increases
rapidly.
The “P” side is called “cathode” and “N” side is
called “anode”.

4. Diode can be connected in two ways :
(1) Forward Bias : P side is connected to positive
terminal of battery and the N side is connected to
negative terminal of the battery in the circuit.
(2) Reverse Bias: P side is connected to negative
terminal of battery and N side is connected to
positive side of battery in the circuit.

5. (b) (a)

6. Knee Voltage:
In the forward region, the
voltage at which the current
starts to increase rapidly is
called the knee voltage.
Vk ≈ 0.7 V
If the knee voltage is more
than the barrier potential
then the diode conducts
easily and if it is less then it
conducts poorly.

7. Ideal Diode:
An ideal diode works as
perfect conductor when
connected in forward bias and
as a perfect insulator when
connected in reverse bias.
This is the first
approximation taken because
all the diode are having
different Barrier potentials
and we have to study all their
graphs.

8. An ordinary switch acts like a
perfect ideal diode because it
has infinite resistance when
open and zero resistance when
closed.
Therefore the ideal diode acts
like a switch that closes on
forward bias and opens on
reverse bias.

9. The Second Approximation:
In second approximation graph, it shows that no
current exist until 0.7V appears across the diode.
Then after, only 0.7V appears across the diode, no
matter what is the current.
Fig. shows the circuit of silicon diode.
If the thevenin’s voltage will be higher then the barrier
potential the switch will close.
On the other hand, if it is less then it will be open, so
there will be no current.

11. The Third Approximation
The sum of the “Ohmic
Resistance” of a diode is
called Bulk Resistance.
RB = RP + RN .
In the third approximation,
we have included bulk
resistance.
The equivalent circuit for it is
a switch in series with the
barrier potential of 0.7V and
a resistance of RB.

12. When the diode voltage
is larger than 0.7V , the
diode starts conducting
current.
During conduction, the
total voltage across the
diode is :
VD = 0.7V + ID RB
Ignore bulk :
RB < 0.01RTh

13. How to calculate Bulk Resistance?
RB = V2 – V1
 I2 – I1
 where V1 and I1 are the voltage and current at some
point at or above the knee voltage; V2 and I2 are the
voltage and current at some higher point on the diode
curve.
The bulk resistance is the inverse of the slope above
the knee.
The greater the slope the smaller the bulk resistance.

14. DC Resistance of Diode:
If we take the ratio of total diode voltage to total
diode current, we get the dc resistance of the diode.
Rf for forward dc resistance and Rv for reverse dc
resistance.
Dc resistance varies with the current passing
through it.
The dc resistance decreases as the current increases.
The dc resistance of diode equals to the bulk
resistance plus the barrier potential of the diode.

15. Load Lines:
It is a tool used to find exact value of diode current
and voltage.
Load Lines are useful with the transistors.
Equation : ID = (VS – VD) / (Rs).
The Q Point: The point of the intersection of the load
line and the diode curve is called the Q point.
Q = quiescent = at rest.