Semiconductor Diodes

Electron Vs Hole Flow
 If a valence Electron acquires sufficient
kinetic energy to break its covalent bond
and fills the void created by a hole then a
vacancy, or hole will be created in the
covalent bond that released the electron
 Hence there is a transfer of holes to the
left and electrons to the right

Majority and Minority Carriers
 In the intrinsic state, the number of free electrons in Ge or Si is due only to those few electrons
in the valence band that have acquired sufficient energy from thermal or light sources to break
the covalent bond or to the few impurities that could not be removed.
 The vacancies left behind in the covalent bonding structure represent very limited supply of
holes.
 In an n-type material, the number of holes has not changed significantly from this intrinsic
level.
 Hence, the number of electrons are much more than the number of holes
 Therefore, In an n-type material the electron is called the majority carrier and the hole the
minority carrier
 Similarly, For p-type material the number of holes are more than the number of electrons.
 Therefore, In a p-type material the hole is the majority carrier and the electron is the minority
carrier.

Diode
 This is the simplest semiconductor
device
 Created by joining an n-type and a p-
type material together- pn Junction
 Joining of one material with a majority
carrier of electrons to one with a
majority carrier of holes

Diode
• A combination of a piece of intrinsic silicon which is doped with trivalent impurities
and the other block with a pentavalent impurities
• Normally, p–n junctions are manufactured from a single crystal with different
dopant concentrations diffused across it. Creating a semiconductor from two
separate pieces of material would introduce a grain boundary between the
semiconductors which severely inhibits its utility by scattering the electrons and
holes
P-N junction
Diode

Diode
 Diffusion: movement due to difference in concentration, from higher to lower
concentration
 In absence of electric field across the junction, holes “diffuse” towards and across
boundary into n-type and capture electrons
 Electrons diffuse across boundary, fall into holes (“ recombination of majority
carriers”)
 charged ions are left behind (cannot move):
 negative ions left on p-side: net negative charge on p-side of the junction
 positive ions left on n-side: net positive charge on n-side of the junction
 electric field across junction prevents further diffusion
 formation of a “depletion region” (region without free charge carriers) around
the boundary

Diode p-n junction
 Before forming p-n junction, n-type and p-type are neutral in terms of protons
and electrons amount
 Diffusions makes the depletion region exist where the region near to the pn
junction is depleted of charge carriers

Basic operation
Ideally it conducts current in only one direction
and acts like an open in the opposite direction
Forward diode
Reverse diode

Characteristics of an ideal diode:
Conduction Region
Look at the vertical line!
In the conduction region, ideally
• the voltage across the diode is 0V,
• the current is ,
• the forward resistance (RF) is defined as RF = VF/IF,
• the diode acts like a short.

Characteristics of an ideal diode:
Non-Conduction Region
Look at the horizontal line!
In the non-conduction region, ideally
• all of the voltage is across the diode,
• the current is 0A,
• the reverse resistance (RR) is defined as RR = VR/IR,
• the diode acts like open.

Operating Conditions
Bias: Application of External voltage across the
two terminals of the device to extract a
response
 No Bias
 Forward Bias
 Reverse Bias

No Bias Condition
No external voltage is applied: VD = 0V , so no current is
flowing: ID = 0A.
In the absence of an applied bias across a semiconductor diode, the net flow of
charge in one direction is zero

Forward Bias
 A forward bias or ‘on’ potential is
established by applying the positive
potential to the p-type and negative
potential to the n-type material.
 The Forward bias potential VD will
pressurize electrons in the n-type and
holes in p-type to recombine with the
ions near the boundary
 Hence the width of the depletion
region is reduced
 This reduction in width of the
depletion region results in a heavy
majority flow across the junction

Continued...
 Now an electron of the n-type material ‘sees’ a reduced
barrier at the junction and feels an strong attraction for
the positive potential applied to the p-type material.
 If the applied bias increases, the depletion region will
continue to decrease until a flood of electrons can pass
through the junction
 This results in an exponential rise in current !!!

Reverse Bias
 A reverse bias potential is established by
applying a positive potential to the n-type
and negative potential to the p-type
material
 The reverse bias potential VD will
pressurize a large number of free electrons
to be drawn towards the applied positive
voltage
 As a result, the positive ions in the
depletion region of n-type material will
increase
 Similarly the number of negative ions will
increase in the p-type material and Hence
the depletion region will be widened.

Continued...
 The widening of depletion region establish a great barrier for
the majority carriers to overcome and hence reduced the
majority carrier flow to zero.
 Yet the number of minority carriers will continue to enter the
depletion region and a little current will exist
 This current under reverse bias condition is called the
reverse saturation current , Is
 The term “Saturation” comes from the fact that the reverse
saturation current reaches its maximum value quickly and
does not change signifcantly with increase in the reverse bias
potential

Silicon semiconductor diode characteristics.
( 1)
D
T
V
nV
D SI I e 
Semiconductor Diode Characteristics
where Is = reverse saturation current
VD = Applied forward bias voltage
across the diode
Vt = Thermal Voltage
n= 11,600/ with 1 for Ge and 2 for Si for
relatively low levels of diode current (at
or below the knee of the curve) and 1
for Ge and Si for higher levels of diode
current (in the rapidly increasing
section of the curve)
TK =TC +273°

Zener Region
 If the Diode is reverse biased and the voltage is
increased, a point will be reached when the diode
enters reverse breakdown and current will flow
with a very rapid rate
 The direction of this current will be opposite to
that of the positive voltage region
 The reverse bias potential that results in this
dramatic change in characteristics is called the
Zener Potential /Voltage and is given by the symbol
Vz
 The reverse or Avalanche breakdown is often
termed as Zener Breakdown (at very low levels)
 The region where this sharp change in
characteristics occur is known as Zener or
Avalanche breakdown region and the current is
termed as avalanche current

Continued...
 The maximum reverse-bias potential that can be applied before
entering the Zener Region is called the peak inverse voltage (referred to
simply as the PIV rating) or the peak reverse voltage (denoted by PRV
rating)

• Diodes that employ the dramatic
characteristics of a p-n junction are
called Zener Diodes
• Therefore, a Zener diode operates in
reverse bias
• Common Zener Voltages: 1.8V to
200V
Zener Diode