Majority Carrier Diode The document discusses several types of diodes including p-n junction diodes, tunnel diodes, Zener diodes, avalanche diodes, backward diodes, Schottky diodes, heterojunction diodes, and metal-semiconductor junction diodes. It describes the breakdown mechanisms and applications of different diodes. Key topics covered include junction breakdown, tunneling effect, current-voltage characteristics, energy band diagrams, and the formation of heterojunctions and metal-semiconductor junctions.

1. Majority Carrier
Diode
By Dr. Ghanshyam Singh

2. Junction Breakdown
• When a huge reverse voltage is applied to a p-n
junction, the junction breaks down and conducts
a very large current.
• Although, the breakdown process is not
naturally destructive, the maximum current mus
t be limited by an external circuit to avoid exces
sive junction heating.
• There are mechanisms dealing with the
breakdown: zener diode, tunneling diode and av
alanche diode.

3. Tunneling Effect
• If a very high electric
field is applied to a p-n
junction in the reverse
direction, a valence
electron can make a
transition from the
valence band to the
conduction band by
penetrating through the
energy bandgap called
tunneling.
• The typical field for Si
and GaAs is about 106
V/cm or higher.

4. Tunneling Effect
• To achieve such a high field, the doping
concentration for both p- and n-regions must be
very high such as more than 5 x 1017 cm-3.
• The breakdown voltage for Si and GaAs
junctions about 4Eg/e is the result of the tunneli
ng effect. With the breakdown voltage is more t
han 6Eg/e, the breakdown mechanism is the resu
lt of avalanche multiplication.
• As the voltage is in between 4Eg/e and 6Eg/e, the
breakdown is due to a mix of both tunneling
effect and avalanche multiplication.

5. Backward Diode
• a backward diode (also called back diode) is a
variation on a Zener diode or tunnel diode having a
better conduction for small reverse biases (for example
–0.1 to –0.6 V) than for forward bias voltages.
• The schematic symbol for the backward diode,
annotated to show which side is P type and which is N;
current flows most easily from N to P, backward
relative to the arrow.

6. Current–voltage characteristics
• The forward I–V characteristic is the same as that of an
ordinary P–N diode.
• The breakdown starts when reverse voltage is applied. In
the case of Zener breakdown, it starts at a particular
voltage. In this diode the voltage remains relatively
constant (independent of current) when it is connected in
reverse bias.
• The backward diode is a special form of tunnel diode in
which the tunneling phenomenon is only incipient, and
the negative resistance region virtually disappears. The
forward current is very small and becomes equivalent to
the reverse current of a conventional diode.

7. Energy Band diagram
Electron energy is on the vertical axis, position within the
device is on the horizontal axis. The backward diode has the
unusual property that the so-called reverse bias direction
actually has more current flow than the so-called forward
bias.

8. Applications of backward diodes
• Detector
Since it has low capacitance and no charge storage
effect, and a strongly nonlinear small-signal
characteristic, the backward diode can be used as a
detector up to 40 GHz.
• Rectifier
A backward diode can be used for rectifying weak signals
with peak amplitudes of 0.1 to 0.7 V.
• Switch
Backward diode can be used in high speed switching
applications.

9. Schottky diode
• The Schottky diode also known as hot carrier
diode is a semiconductor diode with a low
forward voltage drop and a very fast switching
action.
• When current flows through a diode there is a
small voltage drop across the diode terminals.
A normal silicon diode has a voltage drop
between 0.6–1.7 volts, while a Schottky diode
voltage drop is between approximately 0.15–
0.45 volts. This lower voltage drop can provide
higher switching speed and better system
efficiency.

10. Schottky effect
• In electron emission devices, especially electron guns, the
thermionic electron emitter will be biased negative relative
to its surroundings. This creates an electric field of
magnitude F at the emitter surface. Without the field, the
surface barrier seen by an escaping Fermi-level electron has
height W equal to the local work-function. The electric field
lowers the surface barrier by an amount ΔW, and increases
the emission current. This is known as the Schottky
effect or field enhanced thermionic emission. It can be
modeled by a simple modification of the Richardson
equation, by replacing W by (W − ΔW). This gives the
equation

11. Applications
Voltage clamping
• While standard silicon diodes have a forward voltage drop
of about 0.7 volts and germanium diodes 0.3 volts,
Schottky diodes’ voltage drop at forward biases of around
1 mA is in the range 0.15 V to 0.46 V which makes them
useful in voltage clamping applications and prevention
of transistor saturation. This is due to the higher current
density in the Schottky diode.
Reverse current and discharge protection
• Because of a Schottky diode's low forward voltage drop,
less energy is wasted as heat making them the most
efficient choice for applications sensitive to efficiency.
Power supply
• Schottky diodes can be used in power supply circuits in
products that have both an internal battery and a mains
adapter input, or similar.

12. Heterojunction
A heterojunction is
defined as a junction
formed by two semicondu
ctors with different energ
y bandgaps Eg, different di
electric permittivities εs,
different work function e
φs, and different electron
affinities eχ.

13. Heterojunction
• The difference energy
between two conduction band
edges and between two valen
ce band edges are represente
d by ∆EC and ∆EV, respectively
, as
∆EC = e ( χ 2 − χ1 )
∆EV = Eg1 + eχ1 − ( E g 2 + eχ 2 ) = ∆E g − ∆EC
where ∆Eg is the difference
energy bandgap of two
semiconductors.

14. Heterojunction
• Generally, heterojunction has to be formed
between semiconductors with closely matched
lattice constants.
• For example, the AlxGa1-xAs material is the most
important material for heterojunction.
• When x = 0, the bandgap of GaAs is 1.42 eV with
a lattice constant of 5.6533 Å at 300 K.
• When x = 1, the bandgap of AlAs is 2.17 eV with
a lattice constant of 5.6605 Å.

15. Heterojunction
• We clearly see that the lattice constant is
almost constant as x increased. The total built-
in potential Vbi can be expressed by
Vbi = Vb1 + Vb 2
ε 2 N 2 ( Vbi − V )
Vb1 =
ε1 N1 + ε 2 N 2
ε1 N1 ( Vbi − V )
Vb 2 =
ε1 N1 + ε 2 N 2
where N1 and N2 are the doping concentrations in
semiconductor 1 and 2, respectively.

16. Heterojunction
• The depletion widths x1 and x2 can be found by
2ε1ε 2 N 2 ( Vbi − V )
x1 =
eN1 ( ε1 N1 + ε 2 N 2 )
2ε1ε 2 N1 ( Vbi − V )
x2 =
eN 2 ( ε1 N1 + ε 2 N 2 )

17. Heterojunction
Ex. Consider an ideal abrupt heterojunction with a
built-in potential of 1.6 V. The impurity concent
rations in semiconductor 1 and 2 are 1 x 1016 don
ors/cm3 and 3 x 1019 acceptors/cm3, and the diele
ctric constants are 12 and 13, respectively. Find
the electrostatic potential and depletion width i
n each material at thermal equilibrium.

18. Heterojunction
13 × ( 3 × 1019 ) × 1.6
Soln Vb1 =
12 × ( 1 × 10
= 1.6 V
16
) + 13 × ( 3 ×10 )19
12 × ( 1× 10 ) × 1.6
16
−4
Vb 2 = = 4.9 × 10 V
12 × ( 1 × 10 ) + 13 × ( 3 × 10 )
16 19
2 × 12 × 13 × ( 8.85 × 10 ) × 3 × 10 × 1.6
−14 19
−5
x1 = = 4.608 × 10 cm
1.6 × 10 × 1× 10 × ( 12 × 10 + 13 × 3 × 10 )
−19 16 16 19
2 × 12 × 13 × ( 8.85 × 10 ) × 1× 10 × 1.6
−14 16
−8
x2 = = 1.536 × 10 cm
1.6 × 10 × 3 × 10 × ( 12 × 10 + 13 × 3 × 10 )
−19 19 16 19
Note: Most of the built-in potential is in the
semiconductor with a lower doping concentration and
also its depletion width is much wider.

19. Metal-Semiconductor Junctions
• The MS junction is more
likely known as the Schott
ky-barrier diode.
• Let’s consider metal band
and semiconductor band
diagram before the contac
t.

20. Metal-Semiconductor Junctions
• When the metal and
semiconductor are joined,
electrons from the semico
nductor cross over to the
metal until the Fermi leve
l is aligned (Thermal equil
ibrium condition).
• This leaves ionized donors
as fixed positive charges
that produce an internal e
lectric field as the case of
one-sided p-n junction.

21. Metal-Semiconductor Junctions
• At equilibrium, equal number of electrons across the
interface in opposite directions.
• Hence, no net transport of charge, electron current Ie
equals to zero. The built-in voltage Vbi = φm - φs.
• The barrier for electrons to flow from the metal to
semiconductor is given by eφb = e(φm - χs) or it is called the
barrier height of MS contact.

22. Metal-Semiconductor Junctions
• When a voltage is applied, the barrier height
remains fixed but the built-in voltage changes as
increasing when reverse biased and decreasing
when forward biased.

23. Metal-Semiconductor Junctions
• Reverse bias
• Few electrons move across the interface from metal to
semiconductor due to a barrier, but it is harder for elect
rons in the semiconductor to move to the metal.
• Hence, net electron transport is caused by electrons
moving from metal to semiconductor. Electron current fl
ows from right to left which is a small value.

24. Metal-Semiconductor Junctions
• Forward bias
• Few electrons move across the interface from metal to
semiconductor, but many electrons move across the inter
face from semiconductor to metal due to the reduced bar
rier.
• Therefore, net transport of charge flows from
semiconductor to metal and electron current flows from l
eft to right.

25. Metal-Semiconductor Junctions
• Under forward bias, the electrons emitted to
the metal have greater energy than that of the
metal electrons by about e(φm - χs).
• These electrons are called hot-carrier since
their equivalent temperature is higher than that
of electrons in the metal.
• Therefore, sometimes, Schottky-diode is called
“hot-carrier diode”.

26. Metal-Semiconductor Junctions
• This leads to the thermionic emission with
thermionic current density under forward bias as
− e( φm − χ s −VF ) / kT 
J F = A**T 2e  
= J s e eVF / kT
− e( φm − χ s ) / kT 
where J s = A**T 2e  
= saturation current
m*
A =A.
**
= effective Richardson's constant
*
m0

27. Metal-Semiconductor Junctions
• This behavior is referred to a rectification and
can be described by an ideal diode equation of
J = J s ( eeV / kT − 1)
where V positive for forward bias and negative
for reverse bias.

28. Metal-Semiconductor Junctions
• The space-charge region width of Schottky diode is
identical to that of a one-sided p-n junction.
• Therefore, under reverse bias, they can contain the
charges in their depletion region and this is called Schott
ky diode capacitance.

29. Metal-Semiconductor Junctions
Ex. A Schottky junction is formed between Au and
n-type semiconductor of ND = 1016 cm-3. Area of ju
nction = 10-3 cm2 and me*= 0.92 m0. Work function
of gold is 4.77 eV and eχs = 4.05 eV. Find current
at VF = 0.3 volts.

30. Metal-Semiconductor Junctions
Soln
*
me
A** = A* = 120 × 0.92 = 110 A/ ( cm 2 .K )
m0
− e( φm − χ s ) / kT
J s = A**T 2e
J = J s ( eeV / kT − 1)
= ( 110 × 3002 ) × e { − ( 4.77 − 4.05) / 0.0259 
 
} . ( e0.3 / 0.0259 − 1)
= 0.897 A/(cm 2 )
I = A × J = ( 10−3 ) ( 0.897 ) = 0.897 mA

31. Metal-Semiconductor Junctions
Ex. Si-Schottky diode of 100 μm diameter has (1/
C2) v.s. VR slope of 3 x 1019 F-2V-1. Given εr = 11.9 fo
r Si. Find NB for this semiconductor.

32. Metal-Semiconductor Junctions
Soln
1 2 ( Vbi + VR ) C
= ; Cj = [F/cm 2 ]
2
Cj eε s N B Area
2
slope = [cm 4 F-2 V -1 ]
eε s N B
2
NB =
slope × Area 2 × eε s
2
NB =
 −4 2 
2

19    100 × 10   
3 × 10 π  −19 −12
÷  × 1.6 × 10 × 8.85 × 10 × 11.9 
  2  
   
= 6.414 × 1019 cm -3

33. Ohmic contact
• This contact is defined as a junction that will
not add a significant parasitic impedance to the
structure on which it is used and will not
sufficiently change the equilibrium-carrier
densities within the semiconductor to affect the
device characteristics.
• The I-V characteristic of ohmic contact is linear
for an ideal case.

34. Ohmic contact
• A specific contact resistance RC is given by
1 ∂J −1
= Ω.cm 2 
 
RC ∂V V =0
• A good ohmic contact should have a small
specific contact resistance about 10-6 Ω.cm2.

35. Ohmic contact
• When the semiconductor
is heavily doped with an
impurity density of 1019 c
m-3 or higher, the depleti
on layer of the junction
becomes very thin so tha
t carriers can tunnel inst
ead of going over the pot
ential barrier.

36. THANK YOU