1. المملكة العربية السعودية وزارة التعليم العالي - جامعة أم القرى كلية الهندسة و العمارة الإسلامية قسم الهندسة الكهربائية 802311-4 ELECTRONIC DEVICES K INGDOM OF S AUDI A RABIA Ministry of Higher Education Umm Al-Qura University College of Engineering and Islamic Architecture Electrical Engineering Department Lecture 1 By: Dr Tarek Abdolkader

2. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader2 Types of solids (Amorphous, crystalline, and polycrystalline solids) Atomic bonding (ionic, metallic, and covalent bonding) Energy bands Formation of bands Valence and conduction bands, energy gap Metals, semiconductors, and insulators Electrical conduction in semiconductors Bonding and Energy band models Concept of a hole Effective mass

3. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader3 MATTER GASESLIQUIDSSOLIDS

4. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader4 Amorphous Crystalline Polycrystalline

5. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader5 Atoms in a solid are bonded together with a certain force. There are three main types of bonding of atoms in solids: 1.Ionic bonding 2.Metallic bonding 3.Covalent bonding 1. Ionic bonding Ionic bonding occurs between two different atoms due to permanent transfer of valence electrons from one atom to another. It is a very strong bonding. Example: sodium chloride (NaCl) Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). Chlorine (2,8,7) has 1 electron short of a stable noble gas structure (2,8,8). If a sodium atom gives an electron to a chlorine atom, both become more stable.

6. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader6 2. Metallic bonding Metallic bonding occurs due to the electrostatic force between free electrons in metals and positive ions. This occurs in metals which have plenty of free electrons. Examples: Sodium (Na), Copper (Cu) 3. Covalent bonding A covalent bond is formed when electrons are shared between atoms Example: Silicon Examples: Silicon (Si), Germanium (Ge)

7. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader7 In a solid, due to large number of atoms, the split energy levels form essentially continuous bands of energy. energy levels energy bands Formation of bands Electrons in an isolated atom takes some definite values of energy (isolated energy levels) As isolated atoms are brought together to form a solid, these levels are split to form a group of levels of energy.

8. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader8 In an energy band, the allowed energies are almost continuous. These energy bands are separated by regions, which have no allowed energy levels. These regions are known as “ forbidden bands ” or “ energy gaps ”. Formation of bands http://www.rpi.edu/dept/phys/ScIT/InformationProcessing/semicond/sc_content/semi_25.html

9. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader9 The band of highest energy that is completely filled with electrons is called the valence band. The top of the valence band corresponds to energy E v. ENERGY GAP E g POSITION x E Bot EvEv EcEc E Top CONDUCTION BAND VALENCE BAND The separation in energy between E c and E v is the width of the energy gap E g The next higher band must not be completely filled with electrons and is called the conduction band. The conduction band corresponds to the lowest unfilled energy band. The bottom of the conduction band is E c.

10. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader10 Metals, insulators and semiconductors (I)Conductors (Metals): As shown in figure, for a conductor, the conduction band is partially filled at absolute zero of temperature. Electrons in the conduction band easily acquire additional energy to move to the higher unoccupied energy levels within the same band without crossing any energy gap. The additional energy is in the form of kinetic energy, and the moving electrons constitute an electric current. Conductors are, therefore, good conductors of electricity. Whether a solid is an insulator, conductor or semiconductor, it is determined by the energy band structure, the arrangement of electrons with respect to these bands and the width of the forbidden band (Energy gap). Conduction band is partially filled

11. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader11 (II) Insulators: The conduction band is completely empty at the absolute zero of temperature. There exists a large energy gap E g ( 5 eV or higher) between the valence and conduction bands. The electrons in the valence band would not move within the band because there are no unoccupied levels in this band. They can, however, move to the higher empty band provided that they get the required energy E g to cross the forbidden band. An electric field cannot give this amount of energy to an electron in the solid. Thus, no electron acquires additional kinetic energy in the electric field. Therefore, there will be no electric current and the solid will be an insulator.

12. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader12 (III) Semiconductors: Thus a semiconductor has an electrical resistivity lying between that of conductors and insulators. Hence it is termed a semiconductor. Semiconductors such as silicon and germanium are conductors at higher temperatures even though they are insulators at very low temperatures. Semiconductors have an energy band structure resembling that of insulators but with E g relatively smaller (1 – 1.5 eV). At ordinary room temperature, since E g is small, a small proportion of electrons in the valence band have sufficient kinetic energy of thermal motion to cross the narrow energy gap and enter the conduction band above it. These electrons are sufficient to permit a limited amount of current to flow when an electric field is applied. At zero temperature, conduction band is empty and semiconductor behavior is the same as an insulator.

13. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader13 MetalsInsulatorsSemiconductors EgEg noneLarge (> 5 eV)Small (< 2 eV) Valence band at 0 K completely filled Conduction band at 0 K Partially filled Completely empty Conductivity at 0 K Very highzero With increasing temp above 0 K conductivity decreases conductivity is still nearly zero conductivity increases Conductivity at room temperature Very highVery lowintermediate Comparison between metals, insulators, and semiconductors:

14. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader14 For electrons to move under an applied electric field, there must be states available to them. A completely filled band cannot contribute to current transport; neither can a completely empty band. In metals, conduction band is partially filled, thus, electrons and empty states coexist leading to great electrical conductivity. At temperatures other than 0 K, the magnitude of the band gap separates an insulator from a semiconductor, e.g., at 300 K, (diamond) = 5 eV (insulator), and (Silicon) = 1.12 eV (semiconductor). Semiconductors at 0 K are perfect insulators. With thermal or optical excitation of semiconductors, some electrons can be excited from the valence band to the conduction band, and then they can contribute to the current transport process. Number of electrons available for conduction can be increased greatly in semiconductors by reasonable amount of thermal or optical energy. Remember that:

15. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader15 at zero temperature BOND PICTURE ALL ELECTRONS ARE INVOLVED IN BONDING BAND PICTURE THE VALENCE BAND IS FILLED WITH ELECTRONS E Bot EvEv EcEc E Top

16. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader16 at finite temperatures BOND PICTURE Some of the bonds in the crystal is broken by the aid of thermal agitation. Free electrons are created that may carry electrical current. BAND PICTURE Thermal energy is EXCITING some of the electrons from the valence band into the conduction band. - E Bot EvEv EcEc E Top

17. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader17 Concept of a hole Excitation of an electron into the conduction band leaves a void or HOLE in the valence band. In the BONDING model we picture the hole as carrying an electrical current when an electron HOPS from a nearby bond to FILL the original hole. By completing the initially broken bond in this manner ANOTHER bond is broken and we can think that the POSITIVE hole has MOVED to a new position 1 2 1 2 INITIALLY A BOND IS BROKEN IN REGION 1. THEN, AN ELECTRON HOPS FROM REGION 2 TO 1... THIS COMPLETES THE ORIGINALLY BROKEN BOND BUT AT THE SAME TIME BREAKS A BOND IN REGION 2

18. 28/10/1433Electronic devices (802311) Lecture 1 Dr Tarek Abdolkader18 Concept of a hole In the BAND model we picture the hole as the EMPTY state that is left behind in the VALENCE band when an electron is excited into the conduction band. An electron can move from an occupied state in the valence band to fill the initial hole. E Bot EvEv EcEc E Top This process leaves behind ANOTHER empty state in the valence band so that the hole appears to change position. The hole carries CURRENT through the lattice. E Bot EvEv EcEc E Top