Chapter 1
Atoms? All matters on earth made of atoms (made up of elements or combination of elements).
Atomic Structure Atom is the smallest particle of an element that can exist in a stable or independent state. There are three important particles in an atom, which are proton, electron and neutron. Bohr s model (by Niels Bohr, 1913) Nucleus [Protons (Positive) & Neutron (Neutral)] -planetary type of structure that consists of a central nucleus surrounded by orbiting electrons - as seen in this model, electrons circle the nucleus
Atomic Structure Proton is a particle with a positive charge. Electron is a particle with a negative charge. Neutron is a particle that does not have an electric charge. The nucleus is the center part of an atom, made up of the proton and neutron. Proton and neutron are the nucleons. Component Electron Proton Neutron Mass 9.109534 x 10-28 g 1.6726485 x 10-24 g 1.6726485 x 10-24 g Electric charge -1.60219 x 10-19 C +1.60219 x 10-19 C 0 each type of atom has a certain number of electrons and protons that distinguishes it from atoms to other elements
Atomic Number The atomic number, Z of an element is the number of protons in its nucleus atom. The nuclide is represented by a symbol of, where X is the given element. A X Z Example The atomic notation for carbon-12 6 protons + 6 neutrons 6 protons 12 C 6 The nuclide is specified by mass number, A and atomic number, Z.
Electron Shells and Orbits Electrons orbit the nucleus of an atom at certain distances from the nucleus. Each discrete distance (orbit) from the nucleus corresponds to a certain energy level. Atoms have has a fixed number of shells that electrons can be in. The shells is the orbits are grouped into energy bands. Electrons cannot orbit the nucleus in the space that exists between any two orbital shells
Electron Configuration Chemical behavior of an element depends on the electron configuration Electron configuration - describes the "seating arrangement" of the electrons in (the orbitals of) an atom.
ENERGY LEVELS AND SUBLEVELS 1. Principal energy level (n =1, 2, 3, ) could be divided into energy sublevels (s, p, d, f). 2. Each n level has n sublevels. The sublevels for the first four energy levels are provided below: Example n = 1 has one sublevel 1s n = 2 has two sublevels 2s 2p n = 3 has three sublevels 3s 3p 3d n = 4 has four sublevels 4s 4p 4d 4f
The Energy Band Concept The number of electrons in shell 1-4 can be calculated as: N e = 2n 2 Electron orbits the nucleus at certain distances. The outermost shell is called the valence shell. Electrons on this shell are called valence electrons. Valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties. Maximum number of valence electron is 8. In general, the number of valence electrons of a representative element is equal to the group number
Materials Classification Materials Insulators Conductors Semiconductors Insulators Conductors Semiconductors Material that does not conduct electrical current under normal conditions Valence electrons are tightly bound to the atoms Very few free electrons in an insulator Material that easily conducts electrical current The free electrons are valence electrons Material that is between conductors and insulators in its ability to conduct electrical current Atoms have four valence electrons
Think Quiz pair share 1. What is the basic difference between conductors and insulators? 2. How do semiconductors differ from conductors and insulators? 3. How many valence electrons does a semiconductor have? 4. Name three of the best conductive materials?
What is the basic difference between conductors and semiconductors?? Semiconductor: Atoms have four valence electrons Conductor: The free electrons are valence electrons
What is the basic difference between insulator, conductors and semiconductors?? Energy gap: The difference in energy between the valence band and the conduction band. Insulator: Very large gap between the valence band and the conduction band. Semiconductor: Exist an energy gap between the valence band and the conduction band. Conductor: Valence band and the conduction band are overlap.
Explain how a semiconductor (i.e silicon) crystal is formed
Crystalline Structure When atoms combine to form a solid, they arrange themselves in a symmetrical pattern Atoms of most semiconductors are arranged in the form of crystal lattice structure In crystalline structure, each atom shares its four valence electrons with its four neighboring atoms The atoms within the crystal structure are held together by covalent bonds
Covalent Bond A means of holding atoms together by sharing valence electrons The adjacent atoms pull on the shared electrons The atoms are all electrically stable because their valence shell are complete The completed valence shell cause the semiconductor to act as an insulator
Example Electron sharing process in intrinsic (pure) silicon crystal. Semiconductor atom has 4 valence electrons. The covalent sharing satisfies the rule of eight.
How current is produced in a semiconductor
Conduction in Semiconductor The covalent bonds can only be broken by the expenditure of some amount of energy An electron in the valence band of silicon (Si) must absorb more energy than germanium (Ge) to enter the conduction band (become a free carrier). An electron in the valence band of gallium arsenide (GaAs) must absorb more energy than silicon (Si) or germanium (Ge) to enter the conduction band. The unit of measurement for energy absorption are electron volt. Energy, E = Charge, Q x Voltage, V Substituting the charge of 1 electron and potential difference of 1 V results in an energy level referred to as 1 electron volt. Example Energy gap, Eg = 0.67 ev for germanium (Ge) Eg = 1.1 ev for silicon (Si) Eg = 1.43 ev for gallium arsenide (GaAs)
Principle to create conduction electrons Mechanism for electron and hole current
Example Covalent bonds can be broken by heating a silicon crystal. Hole Free electron
Conduction vs Temperature When there is no thermal energy (0 K = -273 C) to agitate the electrons, a semiconductor is acting like an insulator As the temperature is raised from absolute zero, more and more heat energy becomes available An occasional covalent bond will be broken At room temperature (25 C) there is enough heat energy Many electrons are set free from their covalent bonds by thermal ionization
Conduction vs Temperature (cont.) When more valence electrons jump to the conduction band, the semiconductor material conductivity increases Electrons that get off from a covalent bond are known as free electrons Conductivity in a semiconductor is directly proportional to temperature
Summary...
Glossary Free Electron An electron that has acquired enough energy from an outside source to break away from the valence band of the parent atom Conduction Electrons The valence electrons within the crystal structure that manage to escape (becoming free electrons) from their parent atom Hole A vacant position in the structure of interlocking valence electrons in a crystal The absence of an electron in a covalent bond Electron-hole Pair A free electron and its matching valence band hole Recombination When a free electron returns to the valence shell Lifetime The time between electron-hole pair generation and recombination
Question Describe briefly mechanism for electron and hole current in semiconductors. Answer - When a voltage is applied across a piece of semiconductor, the thermally generated free electron in the conduction band and attracted toward the positive end (from left to right). - The movement of free electrons is called electron current. - Then, a holes created by the free electrons exist in valence band. - When the nearby valence electron moves left to right to fill the hole, while leaving another hole behind, the hole has moved from right to left. - The movement of holes is called hole current.
Flow of electrons & holes Valence Band, Conduction Band and Forbidden Energy Gap Insulators, semiconductors and conductors How Fast is an Electron and Electricity
Doping and Intrinsic & Extrinsic Semiconductor
Doping A process of adding other materials called impurities to the semiconductor crystal to change its electrical characteristics The added impurities increase the number of current carriers (elelctrons or holes) and change (increase) the conduction capabilities of semiconductor The conductivity of semiconductor can be increased by the controlled addition of impurities to the pure semiconductor material Adding impure substance to intrinsic semiconductor material in a controlled manner so that the crystal form does not changed
Intrinsic Semiconductor Also known as pure semiconductor Eg: pure Si, Ge. Electron and hole concentrations are relatively small. Hence, very small current are possible.
Intrinsic Semiconductor (cont.) Semiconductor materials do not conduct current well in their intrinsic state Intrinsic semiconductor contains very few free electrons to support the flow of current Therefore, pure semiconductor crystals are very poor conductors
Intrinsic Semiconductor (cont.) High temperature can make it semiconduct because thermal carriers (electrons and holes) are produced However, there is a better way to its conductivity and make it useful in electronic devices This is done by adding impurities to the pure semiconductor
Extrinsic Semiconductor An impure semiconductor An extrinsic semiconductor is obtained when an intrinsic semiconductor is doped with impure atom Even a ratio of 1:10 million of impure substance to intrinsic material, could change the electrical characteristics of a substance. Almost all semiconductor components are composed from extrinsic semiconductor material.
Example Other atoms with 5 electrons such as Antimony are added to Silicon to increase the free electrons. Other atoms with 3 electrons such as Boron are added to Silicon to create a hole charges. n-type p-type
Pentavalent Atom Pentavalent impurity atoms are added to intrinsic semiconductor Pentavalent atoms are atoms with 5 valence electrons Arsenic (As) Bismuth (Bi) Phosphorus (P) Antimony (Sb)
Pentavalent impurity atom in a silicon crystal structure The covalent bonds with neighboring Si atoms will capture 4 of the Sb atom s valence electrons No hole is created from this process The valence electrons enters the conduction band as free electron The 5 th valence electron which cannot form a bond does not alter the structure of the basic crystal
Donor Atom Pentavalent atom is often called a donor atom A conduction electron created by this doping process does not leave a hole in the valence band This doping process lowers the resistance of the Si crystal Excess electron Crystal structure consisting donor atom
Donor Ion Holes are not produced by the addition of the pentavalent atoms but are created when electron-hole pairs are thermally generated When the pentavalent impure atom donate its excess electron, it looses one electron This process caused the atom becomes unstable because the number of electron is no longer the same as the proton This process has changed the donor atom to positive ion or donor ion
N-type Material Semiconductor doped with pentavalent atoms is known as N-type material; since most of the current carriers are electrons N stands for the negative charge (negative carrier) on an electron Electrons in N-type material are called the majority carriers Holes in N-type material are called the minority carriers
Trivalent Atom Trivalent impurity atoms are added to intrinsic semiconductor Trivalent atoms are atoms with 3 valence electrons Boron (B) Gallium (Ga) Indium (In)
Trivalent impurity atom in a silicon crystal structure. Each trivalent atom forms covalent bonds with 4 adjacent Si atoms Since 4 electrons are required, a hole results when each trivalent atom is added A hole created by this doping process is not accompanied by a free electron
Acceptor Atom Trivalent atom is called an acceptor atom By adding the trivalent impurity atoms, free electrons are not produced A few free electrons are produced when electron-hole pairs are thermally generated
Acceptor Ion When the trivalent impure atom received an electron, it becomes unstable The number of electron is more than the number of proton This process has changed the acceptor atom to negative ion or acceptor ion
P-type Material Semiconductor doped with acceptor atoms is called P-type semiconductor Holes are the majority carriers in P-type material Electrons in P-type material are the minority carriers
Effect of heat At low temperature, N-type extrinsic semiconductor has many free electrons as its current carrier, while p-type has many holes. These current carriers are the majority carriers of the material. Heat produces carrier in pairs As additional heat energy enters the crystal, more and more electrons will gain enough energy to break their bonds Each broken bond produces both a free electron and a hole
Effect of heat (cont.) If the crystal was manufactured to be N-type material Every thermal hole becomes a minority carrier Minority carriers increase as the temperature increases Carrier production by heat decreases the crystal s resistance Heat and the resulting minority carriers can have an adverse effect on the way semiconductor devices work
Summary Semiconductor Intrinsic & Extrinsic Semiconductor