Covers all concepts of p-n Junction .

4.  A p-n junction forms a popular semiconductor device called p-n junction diode.
 Schematically it is represented as shown fig 1.1. while symbol which is Commonly
used for diode is shown in fig 1.2.

5. In the Symbol of diode, an arrowhead indicates the Conventional Current Direction
When it is Forward biased. The biasing of diode is explained in the next section.
The P-side of the diode is always connected to the positive terminal of supply for
forward bias and is also termed as anode.
 The N-side of the diode is always connected to the negative terminal for forward
bias and is also termed as Cathode.

6. P-N Junction.
Forward Bias Reverse Bias
 In this case, Positive terminal of the
Voltage Source is Connected to the P-
side and negative terminal to the N-
side as shown fig 1.3(a)
 A large amount of current flows
throught the junction under this
Condition.
 In this case, positive terminal of the voltage
source is connected to the N-side and
negative terminal of the voltage source to the
P-side as shown in Fig. 1.3(b).
 Practically, no current flows through the
junction under this condition.

8. REVERSE
BIAS
• Turn the dc source around and you
get above Fig. This time, the
negative battery terminal is
connected to the p side and the
positive battery terminal to the n
side. This connection produces
what is called reverse bias.

9. DEPLETION LAYER
WIDENS
The negative battery terminal attracts the holes, and the positive
battery terminal attracts the free electrons. Because of this, holes
and free electrons flow away from the junction. Therefore, the
depletion layer gets wider.
How wide does the depletion layer get in above
Fig. a?

13. DIODE CURVE (V-I CHARACTERISTIC OF DIODE)
Fig. A : Forward
Bias
Fig. B : Diode
Curve

14. DIODE CURVE (V-I CHARACTERISTIC OF DIODE)
 The curve OAB is called the forward characteristic of a silicon PN junction diode
 A careful study of the forward characteristics indicates that there is no diode current till the point A is
reached
 It is because of the fact, that the external applied voltage is being opposed by the junction voltage or
potential barrier whose value is 0.7 V for silicon and 0.3 V for germanium.
 However, as the voltage is increased above that of the point A, the diode curent increases rapidly. It has been
observed that a voltage of about 1 V produces a forward current of about 20 to 50 mA.
 The applied voltage should not be increased beyond a certain safe limit, otherwise the diode is likely to burn
out.
 The voltage at which the diode starts conducting is called a knee voltage cut-in voltage or threshold voltage.
The knee voltage is designated by Vk.
 Its value is equal to 0.7 V for silicon and 0.3 V for germanium. The knee voltage may be obtained from the
forward characteristic by extending the curve AB backwards, till it meets the horizontal axis. The value on
the horizontal axis is equal to the knee voltage.

15. A : Reverse Bias.

16.  The circuit diagram for obtaining the reverse bias characteristic of a diode is as shown in Fig. 1.9(a). The
circuit is similar to that shown in Fig. 1.8 except two changes namely the diode terminals are reversed
and the milliameter is replaced by a microammeter.
 It may be noted that negative terminal of the voltage source is connected to the anode of a diode and
positive terminal to the cathode
 Hence, the diode is reverse based. The applied reverse voltage is gradually increased above zero in
suitable steps and the values of diode current are recorded at each step.
 Now, if we plot a graph with reverse voltage along the horizontal axis and the diode current along the
vertical axis, we shall obtain a curve marked OCD as shown in Fig. 1.9(b).
 The curve OCD is called reverse bias characteristic of the diode.
 A careful study of the reverse bias characteristic indicates that when the applied reverse voltage is
below the breakdown voltage (VBR), the diode current is small and remains constant.
 This value of current is called reverse saturation current (I0). It is of the order of nanoamperes for silicon
diode and micro amperes for germanium diode. When the reverse voltage is increased to a sufficiently
large value, the diode reverse current increases as rapidly as shown by the curve CD in the figure.
 The applied reverse voltage at which this happens, is known as breakdown voltage (VBR) of a diode. The
reverse current is called reverse saturation current because by increasing the reverse voltage, the
reverse Current remains Constant and is independent of the Magnitude of the reverse voltage.

17.  In silicon atom the distance between the valence band and the conduction band is called the energy gap. The
larger the energy gap, the more difficult it is for thermal energy to produce electron-hole pairs.
 Fortunately, silicon has a large energy gap, this means that thermal energy does not produce many electron
hole pairs at normal temperature.
 In a germanium atom the valence band is much closer to the conduction band. In other words, germanium has
a much smaller energy gap than silicon has.
 For this reason, thermal energy produces many more electron-hole pairs in the germanium device. This is the
fatal flow.
 The excessive reverse current of germanium device precludes their widespread use in modem computer,
consumer electronics and communication circuits.
 The magnitude of reverse saturation current mainly depends upon the junction temperature because the major
source of minority carriers is thermally broken covalent bonds.
 For large applied reverse bias voltage, the free electrons from the N-type moving towards the positive terminal
of the battery acquire sufficient energy to move with high velocity to dislodge valence electrons from the
semiconductor atoms in the crystal.
 These newly liberated electrons, in turn, acquire sufficient energy to dislodge other parent electrons. Thus a
large number of free electrons are formed which is commonly called as an avalanche of free electrons.
 This leads to the break down of the junction leading to very large reverse current. The reverse voltage at which
the junction break down occurs is known as breakdown voltage.
 The PN junction diode will perform satisfactorily, only if it is operated within certain limiting values.

18. Maximum Forward Current.
 The maximum value of the forward current that a PN junction diode Can carry Without damaging it
under forward bias is called its maximum forward current.
Peak Inverse Voltage (P.I.V):
 The maximum value of the reverse voltage that a PN junction diode can withstand without
damagns Caned its peak inverse voltage (PLV ) If the voltage across the junction exceeds PIV
under revene bias condition, the junction gets damaged
Maximum Power Rating:
 It is the maximum power that can be dissipated at the junction without damaging the junction.
Fower dissipation is the product of voltage across the junction and current through the junction.
 Usually diodes are specified with above ratings by the manufactures.

19. Approximations For PN-junction Diode:
 When silicon diode is used in the circuit and when the circuit analysis has to be done, it becomes
easy if some appropriate approximation are made.
 These approximations are used depending upon the circuitary and the accuracy with which the
results are required.
 Three types of the approximations are made
The First
Approximation
The Second
Approximation The Third Approximation

20. The First Approximation
Ideal Diode
Ideal Diode Symbol
Forward & Reverse
Characteristics Of ideal
diode

21. Ideal Diode:
The ideal diode or perfect diode is a two terminal device, which completely allows
the electric current without any loss under forward bias and completely blocks the
electric current with infinite loss under reverse bias.
Ideal diodes actually do not exist. However, the V-I characteristics of ideal diodes is
used to study the diode circuits.
In other words, it is used to study the quality of a real diode by comparing it with
the ideal diode

22. Ideal diode Symbol:
 Ideal diode consists of two terminals : positive terminal and negative terminal.
 The positive and or positive terminal of the diode is called anode and the negative end or negative
terminal Of the diode is called Cathode.
 The electric current always flow from anode or positive terminal of the cathode or negative terminal.
 If the positive terminal of the battery is connected to the p-type semiconductor and the negative
terminal of the battery is connect to the n-type semiconductor,the diode is said to be forward
biased.
 On the other hand, if the positive terminal of the battery is connected to n type semiconductor and
negative terminal is connected to p type semiconductor,the diode is said to be reverse biased.
 Symbol of forward biased and reverse biased ideal diode is shown in the figure below
 Under forward biased Condition,ideal diode acts as a perfect conductor with zero resistance
whereas under reverse biased condition,it acts as a perfect insulator with infinite resistance.
 In other Word, ideal diodes acts as closed circuit or short circuit under forward biased Condition and
acts as an open circuit or open circuit or open switch under reverse biased Condition.
 Ideal diode does not have depletion region or junction barrier,Which resist the flow of electric
current Hence,ideal diode has no voltage drop or Voltage loss.

23. Ideal diode Symbol

24. Forward and Reverse Characteristics of ideal diode.
The Forward and Reverse Characteristics of ideal diode under forward and reverse
biased condition is shown in the below figure.
If the forward voltage or positive voltage(VF)(p-
terminal connected to p-side n-terminal
connectedto n-side)applied on the diode is equal
to zero or greater than zero, the forward electric
current(IF).in the ideal diode increases infinitely.
On the other hand, if the reverse voltage or
negative voltage(VR)(p-terminal connected to n-
side n-terminal connected to p-side)applied on
the diode is less than zero,no Forward electric
current(IF) and reverse electric current(IR)flows in
the ideal diode.

26. The Second Approxximation Barrier potential
considered.Under this approximation the barrier potential
which was ignored in the first equivalent is
considered in case of silicon diode the barrier
potential is 0.7V.
So to Consider this, a battery of 0.7V emf is
Connected in series with the switch(0.3 V battery
when the diode is germanium diode)
The polarity is such that it opposes the applied emf.
The equivalent circuits for both the conditions are
shown is Fig.
 The VI characteristic for this approximation is Fig.
The approximation is reasonable approximation, it
only ignores the forward resistance offered by
diode as its value is less compared to the load
resistance.

27. The Third Approximation.
In this approximation forward resistance Rf (also called the bulk resistance) of the
diode is considered in addition to the barrier potential.
Also the Forward resistance is Considered to be Constant fig. shows the equivalent
circuit for this approximation.

30. Applications Of PN-junction Diode.
 A PN-junction diode has an important Characteristic that it Conducts Well in the
forward direction and poorly in the Reverse Direction. This Characteristics makes a
diode very useful in a number of Applications given below:-
1. As rectifiers or Power diodes in d.c power supplies.
2. As a Switch in logic Circuit used in Computers.
3. As signal diode in Communication Circuits.
4. As zerner diode in Voltage Stabilizing Circuit.
5. As Varactor diode in radio and T.V receivers.
6. In clipping and Clamping Circuit .
7. In protection circuit of transistors,SCR and operational amplifiers.