Transcript ee201 semiconductor device chp1

EE201
SEMICONDUCTOR DEVICES
MOHD ZULHILMI JAAFAR
JABATAN KEJURUTERAAN ELEKTRIK
SEMICONDUCTOR MATERIAL
 • The term conductor is applied to any material that will
support a generous flow of charge when a voltage source of
limited magnitude is applied across its terminals.
 • An insulator is a material that offers a very low level of
 conductivity under pressure from an applied voltage
 source.
 • A semiconductor (SC) therefore, is a material that has a
conductivity level somewhere between the extremes of an
insulator an a conductor.
ATOM STRUCTURE
 • Bohr Atom Structure was first introduced by Niels Bohr (1913).
 • Examine the structure of an atom; atom is composed of 3 basic
particles: the electron, the proton, and the neutron.
 • In the atomic lattice, the neutrons and protons form the nucleus,
while the electrons revolve around the nucleus in a fixed orbit.
 • The Bohr models of the 2 most commonly used semiconductor
(SC): Ge and Si are shown as followed:
Silicon
Germanium
Figure 1.1
COVALENT BONDING
 Covalent bonds are formed bythe sharing of valence
electrons with neighboring atoms.
 Fig. 1.1 shows how each Si atom with four adjacent atoms
to form a Si crystal. A Si atom with its 4 valence e- shares
an e- with each of its 4 neighbour.
 This effectively creates 8 valence e- for each atom &
produces a state of chemical stability.
Ge atom has 32 orbiting e-, while Si
has 14 orbiting e-.
• In each case, there are 4 e- in the
outermost shell.
• The potential required to remove any
one of these
4 valence e- is lower than that required
for any other e- in the structure.
• In a pure Ge & Si crystal
these 4 valence e- are bonded to 4
adjoining atoms as shown in the
following figure.

Figure 1.2
CONDUCTION ELECTRONS & HOLES
 When e- jumps to the CB, a vacancy is left in the valence band within
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the crystal. This vacancy is called a hole.
Hole also referred as positive charge carrier.
Foe every e- raised to the CB by external energy, there is 1 hole left in
the valence band, creating what is called electron–hole pair (EHP).
Recombination occurs when a CB e- loses energy & falls back into a
hole in the valence band.
To summarize, a peace of intrinsic Si at room temperature has, at any
instant a no. of CB (free) e- that are unattached to any atom & are
essentially drifting randomly throughout the material. Also an equal
no. of holes are created in the valence band when these electrons
jump into the CB
ENERGY LEVEL
 In the atomic structure there are
discrete energy levels associated
with each orbitting e-.
 The more distant the e- from the
nucleus, the higher the energy
state and any e- that has left its
parent atom has a higher energy
state than any e- in the atomic
structure.
 Between the discrete energy
levels are gaps in which no e- in
the isolated atomic structure
can appear.
Figure 1.3
 Note that there are boundary levels & max. energy states in which
any e- in the atomic lattice can find itself, and there remains a
forbidden region between the valence band & the ionization level.
 Recall that ionization is the mechanism whereby an e- can absorb
sufficient energy to break away from atomic structure & enter the
conduction band (CB).
 Energy associated with each e- is measured in eV 4
 Since energy is also measured in joules & the charge of one
e- =1.6 x 10-19 coulomb,
W = QV = (1.6 x 10-19 C) (1 V)
Q= charge associated with single current
* 1eV = 1.6 x 10-19 J
ENERGY LEVEL COMPARISON
Figure 1.4
INSULATOR
 The energy diagram for the insulator shows the insulator with
a very wide energy gap. The wider this gap, the greater the
amount of energy required to move the electron from the
valence band to the conduction band.
 Therefore, an insulator requires a large amount of energy to
obtain a small amount of current. The insulator "insulates”
because of the wide forbidden band or energy gap.
SEMICONDUCTOR
 The semiconductor, on the other hand, has a smaller forbidden
band and requires less energy to move an electron from the
valence band to the conduction band.
 Therefore, for acertain amount of applied voltage, more
current will flow in the semiconductor than in the insulator.
CONDUCTOR/METAL
 The last energy level is for a conductor.
 Notice, there is no forbidden band or energy gap and the
valence and conduction bands overlap.
 With no energy gap, it takes a small amount or energy gap and
the valence and conduction bands overlap.
 With no energy gap, it takes a small amount easily.
INTRINSIC MATERIAL
 Intrinsic material is the pure semiconductor that has no
any additional elements. Examples : Silicon &
Germanium.
 Intrinsic Semiconductor has no advantages.
EXTRINSIC MATERIAL
 Because Si is the material used most frequently as the base material
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in construction of solid state electronic device, the discussion to
follow in this & the next few sections deals solely with Si SC.
Extrinsic Material: material that has been subjected to the doping
process.
There are 2 types of Extrinsic Material: n-type & p-type.
Both materials are formed by adding a predetermined no. of
impurity atoms to a Si base.
n-type: created by introducing impurity elements that have 5
valence e- (eg. As & P).
p-type: formed by doping a pure Ge or Si crystal with impurity
atoms having 3 valence e- (eg. B, Al, In & Ga).
N-TYPE MATERIAL
 Happens when pure semiconductor (exp:
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Silicon) doped with pentavalent atom (has 5
e.v)(refer Fig. 1.3):
Four out of five electron valence from
pentavalent atom formed covalent bond with
silicon atoms but there is an electron that has
no pair
This electron will escape from its orbit and
become free electron, negative current carrier.
More pentavalent atoms more free electrons,
thus more negative current carriers.Therefore
this material is called N-type material.
At very low temperature, majority current
carriers are free electrons.
At room temperature, a few holes produced
and called minority current carriers
Si
Si
Si
Si
Si
Ar
Si
Si
Si
Si
Free Electron
from arsenic
atom that has
no pair
Figure 1.5
Silicon doped with Arsenic
P-TYPE MATERIAL
 P- type material formed when pure
semiconductor doped with trivalent atom.
(Figure 1.4)
 Three electron valences form covalent bond
but another one has no pair. This situation
creates hole, positive current carrier.
 More trivalent atoms, more holes, thus more
negative current carriers, therefore this
material is called P-type material.
 At very low temperature, majority current
carriers are holes and at very high
temperature, minority current carriers are
electrons
Si
Si
Si
Si
Si
In
Si
Si
Si
Si
Figure 1.6
Silicon doped with Indium
hole
PN JUNCTION
 One of the crucial keys to solid state electronics is the nature
of the P-N junction. When p-type and n-type materials are
placed in contact with each other, the junction behaves very
differently than either type of material alone.
 Specifically, current will flow readily in one direction
(forward biased) but not in the other (reverse biased), creating
the basic diode. This non-reversing behavior arises from the
nature of the charge transport process in the two types of
materials.
Figure 1.7
 The open circles on the left side of the junction above
represent "holes" or deficiencies of electrons in the lattice
which can act like positive charge carriers.
 The solid circles on the right of the junction represent the
available electrons from the n-type dopant.
 Near the junction, electrons diffuse across to combine with
holes, creating a "depletion region".
DEPLETION REGION
 When a p-n junction is formed, some of the free electrons in the n-region
diffuse across the junction and combine with holes to form negative ions.
In so doing they leave behind positive ions at the donor impurity sites.
Figure 1.8
THRESHOLD VOLTAGE
The process of combination electrons and holes continues until
threshold voltage produced, 0.7 V for silicon and 0.3 V for
germanium,
P
-
+
-
+
+
-
+
0.7 V
Figure 1.9
N
PN JUNCTION BIAS
 Voltage is supplied to both ends of the junction to give
bias.
 There are two types of bias, forward bias and reverse
bias
FORWARD BIAS
 Forward bias is shown in Figure 1.10 where negative probe power supply is
connected to n-type material and positive probe power supply is connected
to p-type material.
 This situation will decrease the size of depletion region. This occurs because
negative probe power supply will push the electrons in n-type material and
the positive probe power supply will push holes in p –type material.
 The current produced is called forward current.
0.7 V
P
-
+
+
+
+
N
Figure 1.10
 Figure 1.11 shows both current carries move and finally surpassing
the voltage limit (threshold voltage) at the depletion region.
 Power supply has to surpass the voltage limit, 0.7V for silicon and
0.3V for germanium before flowing
Electron movements
P
N
Holes movement
Attract Electrons
Attract holes
Figure 1.11
REVERSE BIAS
 Figure 1.12 shows PN junction is reverse-biased where negative probe power supply is
connected to p-type material and positive probe power supply is connected to n-type material.
 This situation will increase the size of depletion region. This occurs because negative probe
power supply will attract the electrons in n-type material and the positive probe power supply
will attract holes in p –type material.
 There is a very small current produced which is called reverse current but can be ignored.
 When the power supply is increased instantly, it will break the junction and will blow the
component. The voltage is called breakdown voltage.
=V
-
P
-
+
+
+
-
-
+
-
-
+
+
-
-
+
+
N
V
Figure 1.12