Semiconductor devices

Semiconductor Devices
Dr G.S. Virdi
Director R & D Former Director Grade Scientist
GGS College of Modern Technology CSIR- Central Electronic Engineering
Kharar Mohali Research Institute- Pilani

In the modern world no other technology
permeates every nook and cranny of our
existence as does electronics.
Application of electronics are : Televisions,
radios, stereo equipment, computers,
scanners, electronic control systems (in cars
for example) etc.
Introduction to Electronic Devices

Outline Presentation
 Introduction
 Diodes
 Electrical Properties of Solids
 Semiconductors
 P-n Junctions
 Semiconductor Diodes
 Diode Circuits

Introduction
 This course adopts a top-down approach to the
subject and so far we have taken a ‘black-box’ view
of active components
 It is now time to look ‘inside the box’
 we will start by looking at diodes and
semiconductors
 then progress to transistors
 later we will look at more detailed aspects of
circuit design

The p-n junction is at the heart of electronics
technology. Most electronics is silicon based,
that is, the devices are made of silicon. Silicon
wafers are subjected to special procedures
which result in what is called p-type silicon
material and n-type silicon material. Where
these two types of materials meet we have a
p-n junction.
p-n junction

Diodes
An ideal diode passing electricity in one
direction but not the other

Diodes
One application of diodes is in rectification
 the example below shows a half-wave rectifier
 Inpractice, no real diode has ideal characteristics but
semiconductor p-n junctions make good diodes
 To understand such devices we need to look at some
properties of materials

Semiconductors are materials whose electrical
conductivities are higher than those of insulators
but lower that those of conductors.
Silicon, Germanium, Gallium, Arsenide, Indium,
Antimonide and cadmium sulphide are some
commonly used semiconductors.
Semiconductors have negative temperature
coefficients of resistance, i.e. as temperature
increases resistivity deceases
Electrical Properties of Solids

Energy Band Diagram
Conduction band Ec
Ev
Eg
Band gap
Valence band
• Energy band diagram shows the bottom edge of conduction
band, Ec , and top edge of valence band, Ev .
•
Ec and Ev are separated by the band gap energy, Eg .

Energy Bands in Insulators and Conductors

Energy Bands in Semiconductors

Forbidden band small for
semiconductors.
● Less energy required
for electron to move from
valence to conduction
band.
● A vacancy (hole)
remains when an electron
leaves the valence band.
● Hole acts as a positive
charge carrier to
conduction band

Energy gap in a conductor, semi conductor, and insulator?.
Conductor - no energy gap
Semi Conductor - 1. 1 ev.
Insulator 6 -9 ev.
Energy Bands in Insulators & Semiconductors
and Metals

 Conductors
 e.g. copper or aluminium
 have a cloud of free electrons (at all
temperatures above absolute zero). If an electric
field is applied electrons will flow causing an
electric current
 Insulators
 e.g. polythene
 electrons are tightly bound to atoms so few can
break free to conduct electricity

Semiconductors
 e.g. silicon or germanium
 at very low temperatures these have the properties
of insulators
 as the material warms up some electrons break free
and can move about, and it takes on the properties
of a conductor - albeit a poor one
 however, semiconductors have several properties
that make them distinct from conductors and
insulators

Silicon Crystal Structure
Unit cell of silicon crystal
is cubic.
Each Si atom has 4
nearest neighbors.

Pure semiconductors
 thermal vibration results in some bonds being
broken generating free electrons which move
about
 these leave behind holes which accept electrons
from adjacent atoms and therefore also move about
 electrons are negative charge carriers
 holes are positive charge carriers
 At room temperatures there are few charge carriers
 pure semiconductors are poor conductors
 this is intrinsic conduction

Doping
 the addition of small amounts of impurities
drastically affects its properties
 some materials form an excess of electrons
and produce an n-type semiconductor
 some materials form an excess of holes and
produce a p-type semiconductor
 both n-type and p-type materials have much
greater conductivity than pure semiconductors
 this is extrinsic conduction

Extrinsic Semiconductors / Doping
 The electron or hole concentration can be
greatly increased by adding controlled
amounts of certain impurities
 For silicon, it is desirable to use impurities
from the group III and V.
 N-type Semiconductor can be created by
adding phosphorus or arsenic.
 P -type Semiconductor can be created by
adding Boron or Gallium

Donors: P, As, Sb Acceptors: B, Al, Ga, In
By substituting a Si atom with a special impurity atom (Column V
or Column III element), a conduction electron or hole is created.
Doping

 The dominant charge carriers in a doped semiconductor
(e.g. electrons in n-type material) are called majority
charge carriers. Other type are minority charge carriers
 The overall doped material is electrically neutral

p-type material
Semiconductor material doped with
acceptors.
Material has high hole concentration
Concentration of free electrons in p-
type material is very low.
n-type material
Semiconductor material doped with
donors.
Material has high concentration of
free electrons.
Concentration of holes in n-type
material is very low.
Extrinsic Semiconductors

p-type material
Contains NEGATIVELY charged
acceptors (immovable) and
POSITIVELY charged holes (free).
Total charge = 0
n-type material
Contains POSITIVELY charged
donors (immovable) and
NEGATIVELY charged free
electrons.
Total charge = 0
Extrinsic Semiconductors

donor: impurity atom that increases n
acceptor: impurity atom that increases p
n-type material: contains more electrons than
holes
p-type material: contains more holes than
electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n = p = ni
extrinsic semiconductor: doped semiconductor
Semiconductor Terminology

The p-n junction is at the heart of electronics
technology. Most electronics is silicon based, that is,
the devices are made of silicon. Silicon wafers are
subjected to special procedures which result in what
is called p-type silicon material and n-type silicon
material. Where these two types of materials meet
we have a p-n junction.

On its own a p-type or n-type semiconductor is
not very useful. However when combined very
useful devices can be made.
The formation of p-n junction

The p-n junction is the basic element of all bipolar
devices. Its main electrical property is that it rectifies
(allow current to flow easily in one direction only).The
p-n junction is often just called a DIODE. Applications;
>photodiode, light sensitive diode,
>LED- ligth emitting diode,
>varactor diode-variable capacitance
diode
 transistors and integrated circuits

 The p-n junction can be formed by pushing a piece of p-
type silicon into close contact with a piecce of n-type
silicon. But forming a p-n junction is not so simply.
Because;
 There will only be very few points of contact and any
current flow would be restricted to these few points
instead of the whole surface area of the junction.
 Silicon that has been exposed to the air always has a
thin oxide coating on its surface called the “native
oxide”. This oxide is a very good insulator and will
prevent current flow.
 Bonding arrangement is interrupted at the surface;
dangling bonds.

 A P-N Junction cannot be produced by simply
pushing two pieces together or by welding
etc…..Because it gives rise to discontinuities
across the crystal structure.
 Special fabrication techniques are adopted to
form P-N Junction, e.g. Crystal Preparation ,
Masking, Photolithographic Process ,
Deposition ,Implantation , Diffusion
,Oxidation ,Epitaxy ,Contacts, Interconnects
,Metallization and Planarization.

To overcome these surface states
problems
p-n junction can be formed in the bulk of
the semiconductor, away from the
surface as much as possible.
Surface states

PN junction is present in perhaps every semiconductor devi
diode
symbol
N P
V
I
– +
Building Blocks of the PN Junction Theory
V
I
Reverse bias Forward bias
Donor ions
N-type
P-type

When p-type and n-type materials are
joined this forms a pn junction
 majority charge carriers on each side diffuse
across the junction where they combine with
(and remove) charge carriers of the opposite
polarity
 hence around the junction there are few free
charge carriers and we have a depletion layer
(also called a space-charge layer)

DEPLETION REGION
Free electrons on the n-side and free holes on the p-side can initially
diffuse across the junction. Uncovered charges are left in the
neighborhood of the junction.
This region is depleted of mobile carriers and is called the
DEPLETION REGION (thickness 0.5 – 1.0 µm).

The diffusion of positive
charge in one direction and
negative charge in the
other produces a charge
imbalance
 this results in a
potential barrier
across the junction

++++++
++++++
++++++
++++++
++++++
++++++
++++
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - - - -
- - - -
Hole
Movement
Electron
Movement
++++
++++
++++
Fixed positive
space-charge
- - - -
- - - -
- - - -
Fixed negative
space-charge
Ohmic
end-contact
n-type p-type
Metallurgical
junction

Potential barrier
 the barrier opposes the flow of majority charge
carriers and only a small number have enough
energy to surmount it
 this generates a small diffusion current
 the barrier encourages the flow of minority carriers
and any that come close to it will be swept over
 this generates a small drift current
 for an isolated junction these two currents must
balance each other and the net current is zero

Forward bias to p-n junction

When an external voltage is applied
to the P-N junction making the P side
positive with respect to the N side the
diode is said to be forward biased
(F.B).
The barrier p.d. is decreased by the
external applied voltage. The depletion
band narrows which urges majority
carriers to flow across the junction.
A F.B. diode has a very low resistance.

Forward bias to p-n junction
Forward bias
 if the p-type side is made positive with respect to
the
n-type side the height of the barrier is reduced
 more majority charge carriers have sufficient
energy to surmount it
 the diffusion current therefore increases while the
drift current remains the same
 there is thus a net current flow across the junction
which increases with the applied voltage

When an external voltage is applied to the
PN junction making the P side negative
with respect to the N side the diode is said
to be Reverse Biased (R.B.).
The barrier p.d. increases. The depletion
band widens preventing the movement of
majority carriers across the junction.
A R.B. diode has a very high resistance.
Reversed bias to p-n junction

Reversed bias to p-n junction
Reverse bias
 if the p-type side is made negative with respect to
the
n-type side the height of the barrier is increased
 the number of majority charge carriers that have
sufficient energy to surmount it rapidly decreases
 the diffusion current therefore vanishes while the
drift current remains the same
 thus the only current is a small leakage current
caused by the (approximately constant) drift current
 the leakage current is usually negligible (a few nA)

Currents in a pn junction
Currents in a pn junction

Forward and reverse currents
 pn junction current is given approximately by
 where I is the current, e is the electronic charge, V
is the applied voltage, k is Boltzmann’s constant, T
is the absolute temperature and η (Greek letter
eta) is a constant in the range 1 to 2 determined
by the junction material
 for most purposes we can assume η = 1






−= 1exp
ηkT
eV
II s

Semiconductor Diodes
 Thus
at room temperature e/kT ~ 40 V-1
 If V > +0.1 V
 If V < -0.1 V
 IS is the reverse saturation current






−≈ 1exp
kT
eV
II s
( )VI
kT
eV
II ss 40expexp =





≈
( ) ss III −=−≈ 10

Semiconductor Diodes
19.6

Silicon diodes
Silicon diodes
 generally have a turn-on voltage of about 0.5 V
 generally have a conduction voltage of about 0.7 V
 have a breakdown voltage that depends on their
construction
 perhaps 75 V for a small-signal diode
 perhaps 400 V for a power device
 have a maximum current that depends on their
construction
 perhaps 100 mA for a small-signal diode
 perhaps many amps for a power device

I-V characteristics of electronic components.
Resistor
The I-V plot represents is the dependence of the current I
through the component on the voltage V across it.
V
R
IRIV ×





==>×=
1
;I = V /
R;R = V/I
V
I
R
∆V
∆I
α
tg(α) = 1/R
The I-V characteristic of the resistor

Silicon diodes
Turn-on and breakdown voltages for a silicon
device

 There is no turn-on voltage because current flows in any case.
However , the turn-on voltage can be defined as the forward
bias required to produce a given amount of forward current.
 If 1 m A is required for the circuit to work, 0.7 volt can be
called as turn-on voltage.
VVbb
II00
VVbb ; Breakdown voltage
II0 ;0 ; Reverse saturation current
Forward BiasForward BiasReverse BiasReverse Bias
I(current)
V(voltage)
Ge ~ 0.2 – 0.4 V
Si ~ 0.6 – 0.8 V
Applying bias to p-n junction

The experimental I-V characteristic of a
Si diode
Small leakage
current

Temperature dependence of the volt
ampere of a p-n diode

Diode Circuits
Half-wave rectifier
 peak output voltage
is equal to the peak
input voltage minus
the conduction
voltage of the diode
 reservoir capacitor
used to produce a
steadier output

Diode Circuits
Full-wave rectifier
 use of a diode
bridge reduces
the time for which
the capacitor has
to maintain the
output voltage
and thus reduced
the ripple voltage

Diode Circuits
Signal rectifier
 used to demodulate
full amplitude
modulated signals
(full-AM)
 also known as an
envelope detector
 found in a wide range
of radio receivers from
crystal sets to
super heterodynes

Diode Circuits
Signal clamping
 a simple form of
signal conditioning
 circuits limit the
excursion of the
voltage waveform
 can use a
combination of
signal and Zener
diodes

Key Points of Diode
 Diodes allow current to flow in only one direction
 At low temperatures semiconductors act like insulators
 At higher temperatures they begin to conduct
 Doping of semiconductors leads to the production of p-
type and n-type materials
 A junction between p-type and n-type semiconductors
has the properties of a diode
 Silicon semiconductor diodes approximate the behavior
of ideal diodes but have a conduction voltage of about
0.7 V
 There are also a wide range of special purpose diodes
 Diodes are used in a range of applications

Parameter Germanium Silicon Comments
Depletion layer p.d. 0.15V 0.6V
Germanium can be useful for low
voltage applications.
Forward current
A few milli-
Amperes
Tens of
Amperes
Silicon much better for high
current applications.
Reverse leakage current
A few micro-
amperes
A few nano-
amperes
Germanium 1000 times more
leaky than silicon.
Max. reverse voltage Volts
Hundreds of
volts
Silicon the only real choice for
high voltage applications.
Temperature stability Poor Good
Germanium more sensitive to
temperature. Can be a problem or
can be useful.
Junction capacitance
Very low (point
contact)
Comparativel
y high
This is a useful feature for high
frequency use. Note: low
capacitance silicon diodes are also
available but their capacitance is
still higher than point contact type.
Silicon & Germanium diode Comparison

G.S.VIRDI
• Crystal Preparation
• Masking
• Photolithographic Process
• Deposition
• Implantation
• Diffusion
• Oxidation
• Epitaxy
• Contacts, Interconnect ,Metalization and
Planarizatiomn
IC Fabrication Techniques

IC Fabrication Laboratory
G.S.VIRDI

Design of an Integrated Circuit
Thorough Inspection of Design
Fully Processed Wafer
STAGES OF IC FABRICATION
G.S.VIRDI

60
Micro-Fabrication Facilities, CEERI
Fabricated 4” Silicon Wafer – MEMS Acoustic Sensor
MEMS Devices
G.S.VIRDI

G.S.VIRDI

Semiconductor devices

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