Review of Semiconductor Physics, PN Junction Diodes and Resistors

woundcallousSemiconductor

Nov 1, 2013 (3 years and 9 months ago)

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Review of Semiconductor
Physics, PN Junction Diodes
and Resistors


Semiconductor fundamentals


Doping


Pn

junction


The Diode Equation


Zener diode


LED


Resistors

What Is a Semiconductor?















Many materials, such as most metals, allow electrical current to
flow through them


These are known as conductors


Materials that do not allow electrical current to flow through
them are called insulators


Pure silicon, the base material of most transistors, is considered
a semiconductor because its conductivity can be modulated by
the introduction of impurities


Semiconductors


A material whose properties are such that it is not quite a
conductor, not quite an insulator


Some common semiconductors


elemental

»
Si
-

Silicon (most common)

»
Ge
-

Germanium


compound

»
GaAs
-

Gallium arsenide

»
GaP
-

Gallium phosphide

»
AlAs
-

Aluminum arsenide

»
AlP
-

Aluminum phosphide

»
InP
-

Indium Phosphide

Crystalline Solids


In a crystalline solid, the periodic arrangement of atoms

is
repeated over the entire crystal


Silicon crystal
has a
diamond lattice

Crystalline Nature of Silicon


Silicon as utilized in integrated circuits is crystalline in nature


As with all crystalline material, silicon consists of a repeating
basic unit structure called a
unit cell


For silicon, the unit cell consists of an atom surrounded by four
equidistant nearest
neighbors

which lie at the corners of the
tetrahedron

What’s so special about Silicon?


Cheap and abundant


Amazing mechanical, chemical and
electronic properties


The material is very well
-
known to
mankind


SiO
2
: sand, glass

Si is column IV of the
periodic table

Similar to the carbon
(C) and the
germanium (Ge)

Has 3s
² and 3p²
valence electrons

Nature of Intrinsic Silicon


Silicon that is free of doping impurities is called
intrinsic


Silicon has a valence of 4 and forms covalent
bonds with four other

neighboring
silicon atoms

Semiconductor Crystalline Structure


Semiconductors have a regular
crystalline structure


for monocrystal, extends
through entire structure


for polycrystal, structure is
interrupted at irregular
boundaries


Monocrystal has uniform 3
-
dimensional structure


Atoms occupy fixed positions
relative to one another, but

are in constant vibration about
equilibrium

Semiconductor Crystalline Structure


Silicon atoms have 4
electrons in outer shell


inner electrons are very
closely bound to atom


These electrons are shared
with neighbor atoms on
both sides to “fill” the shell


resulting structure is
very stable


electrons are fairly
tightly bound

»
no “loose” electrons


at room temperature, if
battery applied, very
little electric current
flows

Conduction in Crystal Lattices


Semiconductors (Si and Ge) have 4 electrons in their outer shell


2 in the s subshell


2 in the p subshell


As the distance between atoms decreases the discrete subshells
spread out into bands


As the distance decreases further, the bands overlap and then
separate


the subshell model doesn’t hold anymore, and the electrons
can be thought of as being part of the crystal, not part of the
atom


4 possible electrons in the lower band (
valence band
)


4 possible electrons in the upper band (
conduction band
)

Energy Bands in Semiconductors


The space
between the
bands is the
energy gap
, or
forbidden band

Insulators, Semiconductors
,

and Metals


This separation of the valence and conduction bands determines
the electrical properties of the material


Insulators

have a large energy gap


electrons can’t jump from valence to conduction bands


no current flows


Conductors

(metals) have a very small (or nonexistent) energy gap


electrons easily jump to conduction bands due to thermal
excitation


current flows easily


Semiconductors

have a moderate energy gap


only a few electrons can jump to the conduction band

»
leaving “
holes



only a little current can flow

Insulators, Semiconductors, and Metals
(continued)

Conduction
Band

Valence
Band

Conductor

Semiconductor

Insulator

Hole
-

Electron Pairs


Sometimes thermal energy is enough to cause an electron to
jump from the valence band to the conduction band


produces a hole
-

electron pair


Electrons also “fall” back out of the conduction band into the
valence band, combining with a hole

pair elimination

hole

electron

pair creation

Improving Conduction by Doping


To make semiconductors better conductors, add impurities
(dopants) to contribute extra electrons or extra holes


elements with 5 outer electrons contribute an extra electron to
the lattice (
donor

dopant)


elements with 3 outer electrons accept an electron from the
silicon (
acceptor

dopant)

Improving Conduction by Doping
(cont.)


Phosphorus and arsenic are
donor dopants


if phosphorus is
introduced into the silicon
lattice, there is an extra
electron “free” to move
around and contribute to
electric current

»
very loosely bound to
atom and can easily jump
to conduction band


produces
n type
silicon

»
sometimes use + symbol
to indicate heavier
doping, so n+ silicon


phosphorus becomes
positive ion after giving up
electron

Improving Conduction by Doping
(cont.)


Boron has 3 electrons in its outer
shell, so it contributes a hole if it
displaces a silicon atom


boron is an
acceptor

dopant


yields
p type
silicon


boron becomes negative ion
after accepting an electron


Epitaxial
Growth of
Silicon


Epitaxy

grows silicon on top of
existing silicon


uses chemical vapor
deposition


new silicon has same
crystal structure as
original


Silicon is placed in chamber at
high temperature


1200
o

C (2150
o

F)


Appropriate gases are fed into
the chamber


other gases add
impurities to the mix


Can grow n type, then switch to
p type very quickly

Diffusion of Dopants


It is also possible to introduce
dopants into silicon by heating
them so they
diffuse

into the
silicon


no new silicon is added


high heat causes diffusion


Can be done with constant
concentration in atmosphere


close to straight line
concentration gradient


Or with constant number of atoms
per unit area


predeposition


bell
-
shaped gradient


Diffusion causes spreading of
doped areas


top

side

Diffusion of Dopants (continued)

Concentration of dopant in
surrounding atmosphere kept
constant per unit volume

Dopant deposited on
surface
-

constant
amount per unit area

Ion Implantation of Dopants


One way to reduce the spreading found with diffusion is to use ion
implantation


also gives better uniformity of dopant


yields faster devices


lower temperature process


Ions are accelerated from 5 Kev to 10 Mev and directed at silicon


higher energy gives greater depth penetration


total dose is measured by flux

»
number of ions per cm
2

»
typically 10
12

per cm
2

-

10
16

per cm
2


Flux is over entire surface of silicon


use masks to cover areas where implantation is not wanted


Heat afterward to work into crystal lattice

Hole and Electron Concentrations


To produce reasonable levels of conduction doesn’t
require much doping


silicon has about 5 x 10
22

atoms/cm
3


typical dopant levels are about 10
15

atoms/cm
3


In undoped (intrinsic) silicon, the number of holes and
number of free electrons is equal, and their product
equals a constant


actually, n
i

increases with increasing temperature


This equation holds true for doped silicon as well, so
increasing the number of free electrons decreases the
number of holes

np = n
i
2

INTRINSIC (PURE) SILICON


At 0 Kelvin Silicon
density is 5*10
²³

particles/cm
³


Silicon has 4 valence
electrons, it covalently bonds
with four adjacent atoms in
the crystal lattice



Higher temperatures create
free charge carriers.


A “hole” is created in the
absence of an electron.


At 23C there are 10
¹º
particles/cm
³

of free carriers

DOPING


The N in N
-
type stands for negative.


A column V ion is inserted.


The extra valence electron is free to
move about the lattice

There are two types of doping

N
-
type and P
-
type.


The P in P
-
type stands for positive.


A column III ion is inserted.


Electrons from the surrounding
Silicon move to fill the “hole.”

Energy
-
band Diagram


A very important concept in the study of semiconductors is the
energy
-
band diagram


It is used to represent the range of energy a valence electron can
have


For semiconductors the electrons can have any one value of a
continuous range of energy levels while they occupy the valence
shell of the atom


That band of energy levels is called the
valence band


Within the same valence shell, but at a slightly higher energy
level, is yet another band of continuously variable, allowed energy
levels


This is the
conduction band

Band Gap


Between the valence and the conduction band is a range of energy
levels where there are no allowed states for an electron


This is the band gap



In silicon at room temperature [in electron volts]:


Electron volt
is an atomic measurement unit, 1 eV energy is
necessary to decrease of the potential of the electron with 1 V.

Impurities


Silicon crystal in pure form is
good insulator
-

all electrons are
bonded to silicon atom


Replacement of Si atoms can alter
electrical properties of
semiconductor


Group number
-

indicates number
of electrons in valence level (Si
-

Group IV)

Impurities


Replace Si atom in crystal with Group V atom


substitution of 5 electrons for 4 electrons in outer shell


extra electron not needed for crystal bonding structure

»
can move to other areas of semiconductor

»
current flows more easily
-

resistivity decreases

»
many extra electrons
--
>
“donor” or n
-
type material


Replace Si atom with Group III atom


substitution of 3 electrons for 4 electrons


extra electron now needed for crystal bonding structure

»
“hole” created (missing electron)

»
hole can move to other areas of semiconductor if electrons continually
fill holes

»
again, current flows more easily
-

resistivity decreases

»
electrons needed
--
>
“acceptor” or p
-
type material

COUNTER DOPING


Insert more than one
type of Ion


The extra electron and
the extra hole cancel out

A LITTLE MATH

n= number of free electrons

p=number of holes

n
i
=number of electrons in intrinsic silicon=10
¹º/cm³

p
i
-
number of holes in intrinsic silicon=
10
¹º/cm³

Mobile negative charge =
-
1.6*10
-
19

Coulombs

Mobile positive charge = 1.6*10
-
19

Coulombs

At thermal equilibrium (no applied voltage) n*p=
(
n
i
)
2

(room temperature approximation)

The substrate is called n
-
type when it has more than 10¹º free
electrons (similar for p
-
type)

P
-
N Junction


Also known as a diode


One of the basics of semiconductor technology
-


Created by placing n
-
type and p
-
type material in close
contact


Diffusion
-

mobile charges (holes) in p
-
type combine with
mobile charges (electrons) in n
-
type

P
-
N Junction


Region of charges left behind (dopants fixed in crystal
lattice)


Group III in p
-
type (one less proton than Si
-

negative
charge)


Group IV in n
-
type (one more proton than Si
-

positive
charge)


Region is totally depleted of mobile charges
-

“depletion
region”


Electric field forms due to fixed charges in the depletion
region


Depletion region has high resistance due to lack of mobile
charges

THE P
-
N JUNCTION

The Junction



The “potential” or voltage across
the silicon changes in the depletion
region and goes from + in the n
region to


in the p region

Biasing the P
-
N Diode

Forward Bias

Applies
-

voltage
to the n region
and + voltage to
the p region

CURRENT!

Reverse Bias

Applies + voltage to
n region and


voltage to p region

NO CURRENT

THINK OF THE
DIODE AS A
SWITCH

P
-
N Junction



Reverse Bias


positive voltage placed on n
-
type material


electrons in n
-
type move closer to positive terminal, holes
in p
-
type move closer to negative terminal


width of depletion region increases


allowed current is essentially zero (small “drift” current)

P
-
N Junction



Forward Bias


positive voltage placed on p
-
type material


holes in p
-
type move away from positive terminal, electrons in n
-
type move further from negative terminal


depletion region becomes smaller
-

resistance of device decreases


voltage increased until critical voltage is reached, depletion region
disappears, current can flow freely

P
-
N Junction
-

V
-
I characteristics

Voltage
-
Current relationship for a p
-
n junction (diode)


Current
-
Voltage Characteristics

THE IDEAL DIODE

Positive voltage yields
finite current

Negative voltage yields
zero current

REAL DIODE

The Ideal Diode Equation

Semiconductor diode
-

opened region


The p
-
side is the cathode, the n
-
side is the anode


The dropped voltage, V
D

is measured from the cathode
to the anode



Opened: V
D



V
F
:



V
D

=

V
F



I
D

= circuit limited, in our model the V
D

cannot exceed V
F

Semiconductor diode
-

cut
-
off region


Cut
-
off: 0

<

V
D

<

V
F
:



I
D



0

mA

Semiconductor diode
-

closed region


Closed: V
F

<

V
D



0:


V
D

is determined by the circuit, I
D

=

0

mA


Typical values of V
F
: 0.5 ¸ 0.7 V

Zener Effect


Zener break down: V
D

<= V
Z
:



V
D

= V
Z
, I
D

is determined by the circuit.


In case of standard diode the typical values of the break
down voltage V
Z

of the Zener effect
-
20 ...
-
100 V


Zener diode


Utilization of the Zener effect


Typical break down values of V
Z

:
-
4.5 ...
-
15 V

LED


Light emitting diode, made from GaAs



V
F
=1.6 V



I
F

>= 6 mA

Resistor in an Integrated Circuit