1. SOLIDS AND SEMICONDUCTOR DEVICES - II
1. PN Junction Diode
2. Forward Bias of Junction Diode
3. Reverse Bias of Junction Diode
4. Diode Characteristics
5. Static and Dynamic Resistance of a Diode
6. Diode as a Half Wave Rectifier
7. Diode as a Full Wave Rectifier
2. PN Junction Diode:
+
-
Mobile Hole (Majority Carrier)
Immobile Negative Impurity Ion
Mobile Electron (Majority Carrier)
Immobile Positive Impurity Ion
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
When a P-type semiconductor is joined to a N-type semiconductor such
that the crystal structure remains continuous at the boundary, the resulting
arrangement is called a PN junction diode or a semiconductor diode or a
crystal diode.
When a PN junction is formed, the
P region has mobile holes (+) and
immobile negatively charged ions.
N region has mobile electrons (-) and
immobile positively charged ions.
The whole arrangement is electrically neutral.
For simplicity, the minority charge carriers are not shown in the figure.
3. +
+
+
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
-
-
-
-
-
-
+
+
+
+
+
+
PN Junction Diode immediately after it is formed :
After the PN junction diode is formed –
i) Holes from P region diffuse into N region due to difference in concentration.
ii) Free electrons from N region diffuse into P region due to the same reason.
iii) Holes and free electrons combine near the junction.
iv) Each recombination eliminates an electron and a hole.
v) The uncompensated negative immobile ions in the P region do not allow any
more free electrons to diffuse from N region.
vi) The uncompensated positive immobile ions in the N region do not allow any
more holes to diffuse from P region.
Fr
Fr
E
V
Depletion region
4. vii) The positive donor ions in the N region and the negative acceptor ions in
the P region are left uncompensated.
viii) The region containing the uncompensated acceptor and donor ions is
called ‘depletion region’ because this region is devoid of mobile charges.
Since the region is having only immobile charges, therefore, this region
is also called ‘space charge region’.
ix) The N region is having higher potential than P region.
x) So, an electric field is set up as shown in the figure.
xi) The difference in potential between P and N regions across the junction
makes it difficult for the holes and electrons to move across the junction.
This acts as a barrier and hence called ‘potential barrier’ or ‘height of the
barrier’.
xii) The physical distance between one side and the other side of the barrier is
called ‘width of the barrier’.
xiii) Potential barrier for Si is nearly 0.7 V and for Ge is 0.3 V.
xiv) The potential barrier opposes the motion of the majority carriers.
xv) However, a few majority carriers with high kinetic energy manage to
overcome the barrier and cross the junction.
ix) Potential barrier helps the movement of minority carriers.
5. Forward Bias:
When the positive terminal of the battery is connected to P-region and
negative terminal is connected to N-region, then the PN junction diode is said
to be forward-biased.
+
+
+
+
+
+
-
-
-
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
-
-
-
E
Depletion region
E
Ie
Ih
i) Holes in P-region are repelled by +ve terminal of the battery and the free
electrons are repelled by –ve terminal of the battery.
ii) So, some holes and free electrons enter into the depletion region.
iii) The potential barrier and the width of the depletion region decrease.
iv) Therefore, a large number of majority carriers diffuse across the junction.
v) Hole current and electronic current are in the same direction and add up.
+
+
+
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
-
-
-
-
-
-
+
+
+
+
+
+
E
V
6. v) Once they cross the junction, the holes in N-region and the electrons in
P-region become minority carriers of charge and constitute minority
current.
vi) For each electron – hole recombination, an electron from the negative
terminal of the battery enters the N-region and then drifts towards the
junction.
In the P-region, near the positive terminal of the battery, an electron
breaks covalent bond in the crystal and thus a hole is created. The hole
drifts towards the junction and the electron enters the positive terminal of
the battery.
vii) Thus, the current in the external circuit is due to movement of electrons,
current in P-region is due to movement of holes and current in N-region is
due to movement of electrons.
viii) If the applied potential is increased, the potential barrier further
decreases. As a result, a large number of majority carriers diffuse
through the junction and a larger current flows.
7. When the negative terminal of the battery is connected to P-region and
positive terminal is connected to N-region, then the PN junction diode is said
to be reverse-biased.
i) Holes in P-region are attracted by -ve terminal of the battery and the free
electrons are attracted by +ve terminal of the battery.
ii) Thus, the majority carriers are pulled away from the junction.
iii) The potential barrier and the width of the depletion region increase.
iv) Therefore, it becomes more difficult for majority carriers diffuse across
the junction.
Reverse Bias:
V
+
+
+
+
+
+
-
-
-
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
-
-
-
E
Depletion region
E
Ie
Ih
+
+
+
-
-
-
P N
-
-
-
-
-
-
+
+
+
+
+
+
-
-
-
-
-
-
+
+
+
+
+
+
E
V
8. v) But the potential barrier helps the movement of the minority carriers. As
soon as the minority carriers are generated, they are swept away by the
potential barrier.
vi) At a given temperature, the rate of generation of minority carriers is
constant.
vii) So, the resulting current is constant irrespective of the applied voltage.
For this reason, this current is called ‘reverse saturation current’.
viii) Since the number of minority carriers is small, therefore, this current is
small and is in the order of 10-9
A in silicon diode and 10-6
A in germanium
diode.
ix) The reverse – biased PN junction diode has an effective capacitance
called ‘transition or depletion capacitance’. P and N regions act as the
plates of the capacitor and the depletion region acts as a dielectric
medium.
9. Vf (Volt)
If (mA)
Ir (μA)
Vr (Volt) Vk
VB
Diode Characteristics:
mA
+
V
+
Forward Bias:
Reverse Bias:
D
μA
+
V
+
D
Resistance of a Diode:
i) Static or DC Resistance Rd.c = V / I
ii) Dynamic or AC Resistance
Ra.c = ΔV / ΔI
L
i
n
e
a
r
R
e
g
i
o
n
Vk – Knee Voltage
VB – Breakdown Voltage
0
10. PN Junction Diode as a
Half Wave Rectifier: D
RL
●
●
●
●
+
D
RL
●
●
●
●
+
No output
D
RL
●
●
●
●
The process of
converting
alternating
current into
direct current
is called
‘rectification’.
The device
used for
rectification is
called
‘rectifier’.
The PN
junction diode
offers low
resistance in
forward bias
and high
resistance in
reverse bias.
11. D1
RL
●
●
+
PN Junction Diode as a
Full Wave Rectifier:
D2
●
●
A B
+
RL
●
●
●
●
A B
D1
D2
RL
●
●
●
●
A B
D1
D2
When the diode
rectifies whole
of the AC wave,
it is called ‘full
wave rectifier’.
During the
positive half
cycle of the
input ac signal,
the diode D1
conducts and
current is
through BA.
During the
negative half
cycle, the diode
D2 conducts
and current is
through BA.
12. Special Purpose p-n Junction Diodes:
Zener Diode
Vf (Volt)
If (mA)
Ir (μA)
Vr (Volt)
VZ
Vz – Breakdown Voltage
0
Zener Diode as a Voltage Regulator
RL
●
RS
●
●
●
Unregulated
Voltage
V
L
Regulated
Voltage VZ
• Heavily doped
• Depletion Region is < 10-6
m
• Electric Field is very high (5x106
V/m)
•Reverse biased
•Internal Field emission or field
ionisation
End of S & SC - II
