Semiconductor FundamentalsDr. M. Yousuf Soomro

Semiconductors Fundamentals

Course Outline

To understand the semiconductor materials that are suitable for electronic devices

To study the properties of materials for electronic devices

Semiconductor devices are fabricated using specific materials has the desired physical properties

Solid State Materials

Metals (conductor)InsulatorsSemiconductorsSuperconductors

Metals

Materials with zero bandgap are metals

A metal has a partially filled conduction band, so there is no energy gap between filled and unfilled regions.

A significant number of electrons can be excited by heat into empty energy levels and move easily throughout the material, allowing the material to conduct electricity

Insulators

Materials with an energy gap larger than 3 eV

An insulator possesses a considerable energy gap between the valence band and the conduction band

It is difficult to excite electrons from the valence band to the conduction band. As a result an insulator does not conduct electricity

Semiconductor

Special class of materials having conductivity b/w that of a good conductor and that of an insulator

A material with electrical resistivity lying in the range of 10-2 – 109 Ω.cm

Material whose energy gap for electronic excitations lies between zero and about 4 electron volts (eV).

a small number of electrons from the valence band can be promoted to the conduction band by an energy input (e.g. thermal energy from heat)

SemiconductorClassification of Semiconductor materials

oElemental semiconductoroCompound semiconductoroNarrow band-gap semiconductoroWide band-gap semiconductoroOxide semiconductoroMagnetic semiconductoroOrganic semiconductoroLow dimension semiconductor

Semiconductor

Elemental semiconductorSilicon (Si)Germanium (Ge)o These are important group IV elemental semiconductorso All of them have diamond crystal structureBoron (B)o It belongs to group III o It has rhombohedral crystal structurePhosphorus (P)o It belongs to group VSulphur (S)Selenium (Se)Tellurium (Te)o These belong to group VI

Semiconductor Elemental semiconductorCurrently silicon is the most important semiconductor material

used in electronic devicesAdvantages of Si over other semiconductors are:

A relative ease of passivating the surface by oxidizing in a controlled manner forming a layer of stable native oxide that substantially reduces the surface recombination velocity

Its hardness that large wafers to be handled safely without damaging it

It is thermally stable up to 11000C that allows high-temperature processes like diffusion, oxidation, and annealing

It is relatively low cost due to established processes

Semiconductor Elemental semiconductorlimitations of silicon

Its energy band-gap is 1.12eVIt is a direct semiconductor that limits the application in

optoelectronicsIt has relatively low carrier mobility as compared to other

semiconductor such as gallium arsenide GaAs

Semiconductor

Compound SemiconductorsThey are usually formed from o III-V groupo II-VIo IV-VIIII-V group semiconductors are GaAs, GaP, GaN, A1As, InSb,

InAs, InP etcIn general, these crystallized materials

Semiconductor

Compound SemiconductorsGaAs, InAs, InP, InSb have direct energy band-gaps and high

carrier mobilities Common applications of these semiconductors: used to design a variety of optoelectronic devices for1. detection and generation of electromagnetic radiation2. in high-speed electronic devicesThe energy band-gaps of these compounds are useful for

optoelectronic applicationsThe energy bandgap ranges from 0.17eV for InSb to 3.44eV for

GaN

Semiconductor

Compound SemiconductorsII-VI compound semiconductorII-VI compound semiconductor such as Zn and compounds with

oxygen O, S, Se These cover a wide range of electronic and optical properties

due to the wide variations in their energy bandgapThese are typically n-type as grown, except ZnTe, which is p-

typeAll the II-VI compound semiconductors have direct energy

bandgaps

Semiconductor

Compound SemiconductorsIV-VI compound semiconductorPbS, PbSe, and PbTecharacterized by narrow energy gaps, high carrier mobilities, and

high dielectric constantsThe unique feature of the direct energy gap in these compounds

is that its energy band-gap increases with increasing temperature, which means the energy gap has a positive temperature coefficient

Main applications of these compounds are in light emitting devices and detectors in the infrared spectral region

Semiconductor

Narrow Band-gap Semiconductor InSb InAs PbSe PbTe PbSThey have the energy band-gap below about 0.5eV they are direct semiconductor materialsextensively employed in: infrared optoelectronic device applications as detectors and

diode lasers

Semiconductor

Wide Band-gap SemiconductorSiCII-V nitrides high thermal conductivityhigh saturation electron drift velocityhigh breakdown electric fieldsuperior chemical stabilityphysical stabilityIt has wide band-gap that enables detection and emission of light

in short-wavelength region likes blue and ultraviolet

SemiconductorOxide SemiconductorCu2OBi2OZnO These are also referred as semiconductor ceramicsThey are used in electronic devices and sensors

SemiconductorMagnetic SemiconductorSemiconductor compound that contains magnetic ions such as

Cr, Mn, Fe, Co, Ni, may exhibit magnetic properties

Some oxides such as FeO and NiO exhibit antiferromagnetic properties and oxide such as europium oxide EuO is ferromagnetic properties

The semiconductor exhibits large magneto-optical effect that can be used to design optical modulators

SemiconductorOrganic SemiconductorAnthracene CH14H10

Polyacetylene (CH)n

advantages of organic semiconductors 1. Diversity2. relative ease of changing their properties to specific

application

SemiconductorOrganic Semiconductor

One of the promising applications of organi semiconductors is in less iexpensive light emitting diode, covering whole the spectrum of colors

The main advantages of organic materials in such applications include

I. low operating voltagesII. color tunabilityIII. relative simplicity of device fabrication