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

�SiO2: sand, glass

Si is column IV of the

periodic table

Similar to the carbon

(C) and the

germanium (Ge)

Has 3s² 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 cm2

» typically 1012 per cm2 - 1016 per cm2

� 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 1022 atoms/cm3

– typical dopant levels are about 1015 atoms/cm3

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

– actually, ni 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 = ni2

INTRINSIC (PURE) SILICON

�At 0 Kelvin Silicon

density is 5*10²³ particles/cm³

�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 10¹º

particles/cm³ 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.

EG

E eVG ==== 11.

1eV 1.602 10 joule19==== ×××× −−−−

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

ni=number of electrons in intrinsic silicon=10¹º/cm³

pi-number of holes in intrinsic silicon= 10¹º/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=(ni)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 BiasForward BiasForward BiasForward Bias

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

CURRENT!

Reverse BiasReverse BiasReverse BiasReverse 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

I IqV

kT

where

I diode current with reverse bias

q coulomb the electronic ch e

keV

KBoltzmann s cons t

====

−−−−

====

==== ××××

==== ××××

−−−−

−−−−

0

0

19

5

1

1602 10

8 62 10

exp ,

. , arg

. , ' tan

Semiconductor diode - opened region

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

� The dropped voltage, VD is measured from the cathode

to the anode

� Opened: VD ≥ VF:

VD = VF

ID = circuit limited, in our model the VD cannot exceed VF

Semiconductor diode - cut-off region

� Cut-off: 0 < VD < VF:

ID ≅ 0 mA

Semiconductor diode - closed region

� Closed: VF < VD≤ 0:

– VD is determined by the circuit, ID = 0 mA

� Typical values of VF: 0.5 ̧0.7 V

Zener Effect

� Zener break down: VD <= VZ:

VD = VZ, ID is determined by the circuit.

� In case of standard diode the typical values of the break

down voltage VZ of the Zener effect -20 ... -100 V

� Zener diode

– Utilization of the Zener effect

– Typical break down values of VZ : -4.5 ... -15 V

LED

� Light emitting diode, made from GaAs

– VF=1.6 V

– IF >= 6 mA

Resistor in an Integrated Circuit