Electronics Chapter One : Conduction Mechanisms in Semiconductors

Electronics Chapter One : Conduction Mechanisms in Semiconductors
Third ..fourh weeks 6 - 19/ 1/ 1439 هـ أ / سمر السلمي

Time of Periodic Exams The First Homework
you can put any paper or homework in my mailbox in Faculty of Physics Department I will put any announcement or apology in my website ( , so please check it my is for any question. Time of Periodic Exams The first periodic exam in / 2 / 1439 هـ The First Homework I put the first homework in my website in the university homework Due Thursday / 1/ 1439 هـ 22 in my mailbox in Faculty of Physics Department , I will not accept any homework after that , but if you could not come to university you should sent it to me by in the same day

Chapter One : Conduction Mechanisms in Semiconductors
Electronics Concepts Electronics is the science of how to control electric energy, energy in which the electrons have a fundamental role. Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits. In the first Chapter, we will study conduction mechanisms in semiconductors, which is one of the main material types as division in terms of different conduction

Atomic Structure An atom is the smallest particle of an element that retains the characteristics of that element. Each of the known 109 elements has atoms that are different from the atoms of all other elements. This gives each element a unique atomic structure. According to classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons, as in figure. The electrons has negative charge. The nucleus consists of positively charged called protons and uncharged particles called neutrons. Every atom has atomic number, which is the same number of electrons and protons. The atomic number for any element is different from other elements .

Atomic Structure Each discrete distance (orbit) from the nucleus correspond to a certain energy level. These orbits are grouped into energy bands known as shells. Each sell has maximum number of electrons at permissible energy levels(orbits), as shown in the left column of table. The differences in energy levels within a shell are much smaller than the difference in energy between shells. The shells are designated 1, 2, 3 and so on, as its distance from the nucleus. In addition, some references designate shells by the letters K, L , M, as shown in the table and figure.

Atomic Structure Valence electrons are electrons that are in orbits farther from the nucleus have higher energy and less tightly bound to the atom. Their shell is not complete their electrons (expect 8th group in Periodic Table) as shown in figure for Si atom, which consists 4 valence electrons and its electron configuration so electrons and shells divided in to inter and outer (or valence).These valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties. As in the Periodic Table

Conduction Mechanisms in Semiconductors Energy Band Theory
when we study material types, we know that the main material as division in terms of different conduction are three types which are: Conductors, Insulators, and Semiconductors. how we compare between those materials? What are their properties ? of course, you study some of those properties over the previous years in different courses as: Electricity physics, Modern physics, and Solid State Physics 1 in Solid State Physics 1, you study Energy Band Theory. We will begin from this theory as a review.

The electron configuration for Oxygen atom O8 then to two atoms
Source: Dr. Abdul Aziz Kutub

What about the electron configuration for N atoms ( thus, according to the number of atoms, the number of energy levels is consisted ) Source: Dr. Abdul Aziz Kutub

The electron configuration for N atoms (those energy levels consists energy band)
Source: Dr. Abdul Aziz Kutub

Explain Energy Band Theory
At crystal solid state consists energy levels (as of the case of isolated atoms ) . But when there are N atoms consists energy bands (which are infinitesimal numbers of energy levels that are near from each others , so it is difficult to designated between them. every level represents energy level occupied with electrons. in the next example in next slide, we will focus at outer shell or valence levels. Here the bonding orbital band is consisted in down part of valence levels and the bonding antiorbital band is consisted in upper part of its. Between those two bands is the energy gap, which consists Fermi level. also we can notice that valence band is the top band from the bonding orbital band ( which is filled with electrons) and conduction band is the bottom band from the antibonding orbital band ( which is different according to material types which either empty of or partially filled with electrons).

Another example Si, its electron configuration is
We will focus at valence shells only (from here materials classifies conductors, insulators, and semiconductors according to Energy Band Theory)

bonding antiorbital band
Energy levels at outer orbitals : The red box in the last slide bonding orbital band bonding antiorbital band

Conduction and valence bands in solid:

Comparison of three materials
From last slide, we notes in terms of energy band theory and electrical conductivity that : Conductors Insulators Semiconductors the energy gap is overlap that means the part of conduction band inside valence band . Because of that, we find conduction electrons in conduction band. Thus, it is a good electronic conductor. There are some conductors have very narrow energy gap and we find conduction electron in conduction band. There is a wide gap between the conduction band and valence band . This gap is forbidden area for the existence of electrons. Thus, the conduction band is empty of electrons. This is what makes it insulated on the electrical conduction the gap between the conduction band and valence band is narrower; but at small degrees of heat, electrons move across the gap to the conduction band and become conductive material but at a temperature of absolute zero, its behave as insulator

Comparison of three materials
Semiconductors Insulators Conductors Example Germanium . Silicon. Lead sulfide PdS. Cadmium sulfide CdS Glass. Quartz. Porcelain. Alaunit. Amber. Ceramic Metals (silver. Copper. Iron. Lead .. bonds Covalent or mixed bonds Ionic bond Metal bond Resistivity Medium at normal temperature in the range of from 10-5 Ω.m to 106 Ω.m very large at room temperature in the range of 106 Ω.m to 1016 Ω.m From 10-5 Ω.m to Ω.m Valence band Filled with electrons Connection band Completely empty of electrons in the degree of absolute zero, but it contains a number of electrons in the normal temperature or greater than absolute zero Empty of free electrons at normal temperatures Partially filled with electrons at normal temperatures Energy gap Medium from 0.7 eV to 2 eV very large 5 eV Very small 0.01 eV The effect of raising the temperature on the resistivity and conductivity Resistance decreases dramatically. it has a moderate connectivity between metals and insulators. conductivity increases with raising the temperature Resistance decreases but remains so large that the material fused before it becomes conductive Increasing resistance and conductivity decreases with increasing temperature

Composition of Semiconductors
From those three materials, we will focus at studying about semiconductor because it is the main elementary in making vacuum tubes, transistors, ..etc. Thus, semiconductor is the main forming in electronic science. Composition of Semiconductors elements: from the 4th group in Periodic Table (IV) as Si and Ge binary compounds : from the 3th and 5th group (III- V) as GaAs :also from the 2th and 6th group (II- VI) as ZnS , 2th column here is Zn and Cd.

Structure and Lattices of Semiconductors
in this course, we will focus at most important structures for semiconductors. diamond structure : it has a cube shape. in which has eight atoms at corners (eighth ), six atoms at center face of the cube (half), and four atoms inside the cube (complete). It almost as two face-centered lattices (fcc) direction (111). The elements semiconductors from the 4th group take this structure Zincblende structure : it is similar to diamond lattice. The different is arranged of atoms, in a why one element in one of fcc lattice and the other element in the other fcc lattice) . Compounds semiconductors from the III-V , IV-IV , and some of II-VI take this structure. diamond structure zinc blende structure

Chemical Bonds of Semiconductors
As we mention at the comparison that semiconductors consist covalent or mixed bonds Covalent bond : its for elements the 4th group as Si and Ge . it is exhibited by the diamond structure. In these crystal, each atom shares its valence electrons with its four neighbors. Mixed bond : its for compounds as III-V and II-VI such as GaAs and ZnS. it is exhibited by the Zincblende structure. The mixed bond consist of covalent bond and ionic bond). In which the ionic bond is between the two ions in compounds , however covalent bond is between one compound and the others. Covalent bond in Si mixed bond in GaAs

Charge Carriers and Conduction Mechanisms in Semiconductors
In conductors : to understand the conduction mechanism in conductors, we can image that metal atoms immersed in sea of electrons. This electrons can move as group under the effect of electric energy. For insulators: there is no conduction electrons in insulators because of the lack of electrons. So there are no swimming electrons in model of ionic compound ( as NaCl) ; also, because of strong of ionic bond in insulators. negative Ion Positive Ion

there is no conduction electrons in semiconductor at normal condition at absolute zero temperature similar to insulators because there are no swimming electrons in model . As in covalent bond for Silicon, all atoms share with four valence electrons to form the bond. however, this situation changes ,and there will be conduction electrons in semiconductor according to two main factors: rising temperature higher than absolute zero adding impurity to intrinsic semiconductors

At rising temperature higher than absolute zero in semiconductors We know that covalent bond is a weak bond. Thus, when the temperature raised, the bond breaks. As a result, a electron generates. This electron is the charge carrier in semiconductors similar to conductors . However, this electron leaves behind a hole .This hole has a positive charge, and it moves opposite direction than electron. Thus, by increasing rising temperatures, more electron – hole pair generate. This process happens only in semiconductors, not in conductors.

At rising temperature higher than absolute zero in semiconductors:
The last explanation was about the models and chemical bonds . We can also explain by energy bands. Thus, when the temperature raised, a electron receive enough thermal, energy to excited from filled valence band to empty conduction band leaving behind a hole (another charge carrier). Thus, we will notice electron – hole pair (EHP) appearance as we explain in last slide. Those charge carriers are effect factors to conduction electron in semiconductors.

The different between electron and hole
We study about electrons but what about holes? Electrons flow from minus to plus, however, holes flow from plus to minus. Therefore, they are equal in magnitude and opposite in the direction. Because of that, electron has negative charge and hole has positive charge . Mobile hole has the same direction of current and electric field Note (hole movement is virtual but electron movement is real) holes electrons positive charge negative charge Energy increases going down Energy increases going up The same direction of electric field The opposite direction of electric field

At adding impurity to intrinsic semiconductors
Another method to get charges carriers in semiconductors instead of giving excitation energy (thermal energy) is by adding impurity or doping. (what is the different between them?) impurity is adding different atoms from the original atoms , and happened automatically in natural. However, doping is similar but we add intentional impurities in order to change its electronic properties as in increasing electron or hole numbers. in the case of elemental semiconductors as Si, impurities either be from 3ed group (such as B) which called acceptor meaning it gives extra hole and accept extra electron, or from 5th group (such as P) which called donor meaning it gives extra electron

Also, acceptor is called extrinsic semiconductor p-type which has higher holes concentration than electrons concentration , and Fermi level is near valence band. Donor is called extrinsic semiconductor n-type which has higher electrons concentration than holes concentration , and Fermi level is near conduction band. The extra electron or hole associates with positive or negative ion by a weak bond. Thus, it can swim freely in crystal in case presence thermal energy.

In case of no presence thermal energy in both n-type and p-type, the presence of impurities contribute in moving Fermi level near to two bands. As a result, possibility of electron or hole existence in it. In the presence of a small thermal energy in extrinsic semiconductor p-type or n-type has effect in transport electron from valence band to connection band Therefore, the presence of both impurities and thermal energy together effect more conduction mechanisms in semiconductors than the presence of one of them.

the figure below illustrates in details the role of acceptor and donor levels in case of no presence thermal energy both n-type (above) and p-type (below), and the case of presence of small thermal energy. In room temperature , we obtains complete ionization acceptor and donor atoms.

We studied in the case of elemental semiconductors such as Si and Ge impurities either be from 3ed group which called acceptor and extrinsic semiconductor p-type or from 5th group which called donor and extrinsic semiconductor n-type As in the case of binary compound semiconductors such as GaAs and ZnS which consist mixed bond (ionic bond association between two elements of one compound and the covalent bond between compounds) in the case of (III- V) compound such as GaAs impurities either be from 2nd group (such as Mg) which called acceptor and extrinsic semiconductor p-type if it substitutes for column III (Ga) of GaAs. or from 6th group (such as S) which called donor and extrinsic semiconductor n-type if it substitutes for column V (As) of GaAs.

in the case of (III- V) compound such as GaAs impurities either be from 2nd group (such as Mg) which called acceptor and extrinsic semiconductor p-type if it substitutes for column III (Ga) of GaAs. or from 5th group (such as S) which called donor and extrinsic semiconductor n-type if it substitutes for column V (As) of GaAs. Mg S

Conduction Mechanisms in Semiconductors
Concentration Laws in Intrinsic and Extrinsic Semiconductors we study last lecture about intrinsic and extrinsic semiconductors in general. The general law (charge conservation law) in semiconductors in terms of concentration is n electron concentration & p hole concentration charge carrier concentration in intrinsic semiconductors or we can say ni is intrinsic charge carrier concentration (whether electrons or holes)

Concentration Laws in Intrinsic and Extrinsic Semiconductors
where nn is electron concentration in n-type which is the majority carrier{ the most number carrier in a semiconductor) & pn is hole concentration in n-type which is the minority carrier{ the fewest number carrier in a semiconductor) the opposite , np is electron concentration in p-type which is the minority carrier pp is hole concentration in p-type which is the majority carrier . (concentration is a number per volume unit) (what is unit of concentration) =

ND+ is positive ionized donor concentration ( such as Sb +) NA _ is negative ionized acceptor concentration ( such as B - ) ND is donor atoms concentration NA is acceptor atoms concentration In room temperature, we obtains complete ionization acceptor and donor atoms. (concentration is a number per volume unit) (what is unit of concentration) =

Here also, we applies general law of concentration in extrinsic semiconductors (charge conservation law) charge carrier concentration in n-type It will be also charge carrier concentration in p-type =

Energy laws and Fermi level in intrinsic and extrinsic semiconductors
intrinsic n-type p-type acceptor level (EA) & donor level (ED) & intrinsic level (Ei) Ec is for conduction band & Ev is for valence band Fermi level (Ef) location depend on type of semiconductors is intrinsic or extrinsic (n-type or p-type ) = C.B V.B Ec Ev EA C.B V.B Ec Ev Ei C.B V.B Ec Ev ED

There also equations connect carrier concentration by Fermi level at thermal equilibrium condition From them, we obtains Therefore in n-type & in p-type = C.B V.B Ec Ev Ei Ef C.B V.B Ec Ev Ei Ef

From last equations, we note how location of Fermi level dependence on carrier concentrations n-type p-type This equations applied only for condition of Fermi level is away from conduction and valence band with 3KT

Temperature dependence of carrier concentrations :
Also from last equations about concentration, we note not only Fermi level location changes when carrier concentration changes, but also temperature changes when carrier concentration changes whether intrinsic or extrinsic semiconductors. The figure is example of dependence carrier concentration on temperature in intrinsic semiconductors. The relation here is proportional exponential .

Temperature dependence of carrier concentrations :
Example in n-type (semiconductors Si doping with P) T=0K low T Room T high T C.B V.B Ec Ev ED

Non – equilibrium and Excess Carriers
Our discussion until now is about equilibrium condition, we supposed to symbolize the electron and hole concentration respectively no & Po . What about non-equilibrium condition?? The simple type is low- level injection which happened when the disorder occurs in electrons concentrations at conduction band or disorder occurs in holes concentrations at valence band less than the concentrations of majority carriers. N-type P-type n, p are carrier concentrations in any circumstance Express of disorder of concentrations from equilibrium values. If there was a negative decrease and if there is a positive increase =

Non – equilibrium and Quasi – Fermi levels
At equilibrium condition, we can locate Fermi level from equilibrium carrier concentration. At non equilibrium condition, we can locate Fermi level from non equilibrium carrier concentration Ei C.B V.B Ec Ev Ef Ei C.B V.B Ec Ev Fn Fp

The transfer of current in semiconductors
The current transfer in the extrinsic semiconductor depends on two important factors, namely drift and diffusion Drift: movement of charge carriers, which is due to the applied electric field, This movement is regular which opposite of random Brownian motion. As we know, the holes always move in the direction of the electric field opposite of electrons movement. This movement is called drift. This movement represent by drift velocity vd .Drift coefficient is called mobility µ . Mobility for electron and hole respectively µn & µp . Also, drift velocity for electron and hole respectively vn & vp . Those coefficients connect by electric field. =

The transfer of current in semiconductors Drift:
What matters to us in this factor is its effect at current transfer and therefore the current density Current density of drift: Electrons Holes from the relation between the current density, electric field and resistivity or conductivity The resistivity and conductivity in the semiconductor express the following equations respectively =

Diffusion: The migration of carriers (electrons or holes) from the most concentration to fewest concentration Diffusion depends on the concentrations and also on random motion(thermal velocity ) Diffusion coefficient for electron and hole, respectively Dn & Dp Similar to drift, we need to know current density of diffusion Current density of diffusion: Electrons Holes =

Total current density in the semiconductor: is the sum of drift and diffusion current density Electrons Holes Einstein Relation We can connect drift and diffusion with another relation. Which is a Einstein Relation =

http://www. ioffe. ru/SVA/NSM/Semicond/index
This link is important to know properties of some semiconductors.

The process that electron recombining with
There are two other processes than drift and diffusion occur in the semiconductor. Which is recombination and generation processes: Recombination: The process that electron recombining with hole looses energy and moves from conduction band to valence band Generation: The process that acquiring energy to generation electron which moves from valence band to conduction band leaving free hole in the valance band; formation of electron-hole pair there are number types of this two processes according to given energy (optically, thermally, and kinetically) depending on the circumstances (such as the presence of an electric field) and so on =

Direct and indirect semiconductors of conduction and valence bands in energy-momentum space
Because we dealt with the subject of recombination and generation, we should clarify that there are two types of semiconductor directly and indirectly. If the top of valence band and the bottom of conduction band have the same wave number, we called direct semiconductor. However, if they have different wave number, we called indirect semiconductor.

recombination or generation direct (band to band) occur in direct semiconductor GaAs, which is famous in the optical diode (LED). The opposite in case of recombination or generation indirect ( by recombination and generation centers) occur in indirect semiconductor Si and Ge. However, the Si more widespread in manufacture diode and transistor than Ge. This is because Ge less stability at temperature, because the valence electrons in it are far from the nucleus and thus their ability to escape easily. Electrons occupy in most decline valley in conduction band such as ball into the hole. While holes occupy most high peak in valence band such as balloon in a ceiling of a room. =

This link is important to know properties of some semiconductors. Example of direct and indirect semiconductors

Continuity Equations drift, diffusion, recombination and generation are processes that occur in semiconductor at the same time and change or remain constant on order of given condition Therefore, there are equations called continuity equations combining all those processes in the semiconductor. We will focus in one important equations called minority carrier diffusion Equations =

Continuity Equations We will test a sample of semiconductor has a volume A Δx . Therefore, the inside current density at the point x is different from the outside current density at the point x+ Δx whether in the case of holes or electrons. Therefore, there are equations called continuity equations combining all those processes in the semiconductor. We will focus in one important equations called minority carrier diffusion Equations =

Minority carrier diffusion Equations
We will focus in one important equations called minority carrier diffusion Equations In the case of stability and the absence of light energy Note that it quadratic equation, hence the solution are We definite constant Ln & Lp as minority carrier diffusion length, also, τn & τp minority carrier lifetimes both for electron and hole respectively. =

Different materials adjacent
So far, everything we have studied before for one material or one type in term of properties each separately In the first lecture, we discussed the differences between the three materials: conductors, insulators and semiconductors, also, mention a bit of properties of each material. The second and third lectures focused on semiconductor, also mentioned two types of this material n-type and p-type and differences between them, and then the processes that occur in general in the semiconductor as drift, diffusion, recombination and generation. what about if two different materials next to each other (as conductor and semiconductor)? or different types from the same material (as n-type and p-type)? What happens in the process of transmission of electrons or holes ? and how current flows between them ? How is the Fermi level between the two? And how drift, diffusion, recombination and generation processes occur? All of that, we will study them in detail in the second chapter and then complete it in the third and fourth chapters? From those chapters, we will understand the concept of electronics science.

Electronics Chapter One : Conduction Mechanisms in Semiconductors

Similar presentations

Presentation on theme: "Electronics Chapter One : Conduction Mechanisms in Semiconductors"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Electronics Chapter One : Conduction Mechanisms in Semiconductors

Similar presentations

Presentation on theme: "Electronics Chapter One : Conduction Mechanisms in Semiconductors"— Presentation transcript:

Similar presentations

About project

Feedback