How the p-n junction is formed and how various parameters varies across the p-n junction have been discussed

1. Basics of p-n Junction Diode
Dr Biplab Bag
Assistant Professor, Physics

2. p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
−
+
p n

3. +
-
p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
 The majority carriers at the junction will recombine and hence
at the junction there will be very small number of carriers.
 The Regime close to the junction with very less no of carriers
is known as space charge regime or depletion regime
−
+
Variation of Space Charge Density
p n

4. p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
 The majority carriers at the junction will recombine and hence
at the junction there will be very small number of carriers.
 The Regime close to the junction with very less no of carriers
is known as space charge regime or depletion regime
−
+
Variation of electron concentration
p n

5. p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
 The majority carriers at the junction will recombine and hence
at the junction there will be very small number of carriers.
 The Regime close to the junction with very less no of carriers
is known as space charge regime or depletion regime
−
+
Variation of hole concentration
p n

6. p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
 The majority carriers at the junction will recombine and hence
at the junction there will be very small number of carriers.
 The Regime close to the junction with very less no of carriers
is known as space charge regime or depletion regime
−
+
Variation of Electric field
p n

7. p-n Junction Diode
 Suppose, there is a bar of an intrinsic semiconductor, e.g., Si
 One side of which is doped with donor impurity atoms
(positive ions as they donate an electron): n-side
 And other side is doped with acceptor impurity atoms (as they
accepts one electron, the ions are negative): p-side
 Majority carriers in the n-side: electrons
 Majority carriers in the p-side: holes
 The majority carriers at the junction will recombine and hence
at the junction there will be very small number of carriers.
 The Regime close to the junction with very less no of carriers
is known as space charge regime or depletion regime
 Vb is the intrinsic potential barrier, which oppose the charge
carriers to reach the other end.
 An applied external potential may alter Vb
−
+
Variation of electrostatic potential
p n
Vb

8. Forward Biasing of a Diode
−
+
Variation of electrostatic potential
p n
Vb
Va
Vb-Va
 Forward biasing
enforces the carriers
to cross the potential
barrier and reach
the other end
 As a result the
effective potential
barrier decreases

9. Reverse Biasing of a Diode
−
+
Variation of electrostatic potential
p n
Vb
Va
Vb+Va
 Reverse biasing pulls
the carriers away
from the junction
and as result the
potential barrier
increases

10. Diode Equation
= − 1
where:
I = the net current flowing through the diode;
I0 = "dark saturation current", the diode leakage current density in the absence of light;
V = applied voltage across the terminals of the diode;
q = absolute value of electron charge;
kB = Boltzmann's constant; and
T = absolute temperature (K).

11. Diode Equation
= − 1
• For forward bias, V is positive
• So, the exponential term increases
rapidly with applied voltage
• kBT ≡ VT: Thermal volatge
VTh

12. Diode Equation
= − 1
• For reverse bias, V is negative
• So, the exponential term decreases
rapidly with applied voltage and is
extremely small (<< 1)
• Hence the exponential term can be
ignored
• We should have = −
- Is

13. Zener Diode
Characteristics
ℎ
Breakdown

14. Avalanche Breakdown
−
+
p n
Va
 Reverse biasing in
low doped material
leads to large
depletion regime
 Avalanche
Breakdown is
observed at very
large voltage
 = , where d is the
width of the
depletion regime.
 At large V, E will
also be large

15. Avalanche Breakdown
−
+
p n
Va
 This large E will
knock off the
electrons from
bonds, generating
large number of
electron-hole
 This leads to abrupt
enhancement of
current.

16. Zener Breakdown
−
+
p n
Va
 Reverse biasing in
heavily doped
material leads to
very narrow
depletion regime
 Zener Breakdown is
observed in this
large doping regime
and at low voltages
 = , where d is the
width of the
depletion regime.

17. Zener Breakdown
−
+
p n
Va
 In this narrow
regime, electrons can
tunnel through the
depletion regime
 This leads to abrupt
enhancement of
current.

18. Reverse Biasing of a Diode
−
+
Variation of electrostatic potential
p n
Vb
Va
Vb+Va
 Reverse biasing pulls
the carriers away
from the junction
and as result the
potential barrier
increases
 As a result the
effective potential
barrier decreases