Electronics1

Semiconductor Materials

Lecture1

Mr. Farman Ullah

06/02/2012

f.ullah99@yahoo.co.uk

Office: Electronics Lab

Electronics1

Electronics1 EEE231• 1st Sessional: Basic Electronics• 2nd Sessional: Responses• Terminal Exam: Applications• Quizes• Assignments

Course Contents

Introduction to Electronics

• First IC is developed by Jack Kilby in 1958• The Intel Pantium 4 Processor (IC) has more than 42 millions transistors and host of other components. This number could increase to 1 billions in the nearest future and could be even more than that. However, there are some factors that limit further miniaturization e.g. quality of semiconductor, network design technique and limit of the manufacturing and process equipment.

Semiconductor Materials

• Semiconductor devices having conductivity between good conductor and insulator.• Semiconductor material is the basic unit of almost all the ICs or electronic devices.• Semiconductor materials fall into one of two classes• Single crystals i.e. Germenium (Ge) and Silicon (Si) with repetitive crystal structure.• Compound i.e. GaAs and CdS etc of different atomic structures.• Ga, Si and GaAs are the most frequently used in the construction of electronic devices.

History of Semiconductor Materials

•Ge was the most easily found and most abundant element, after the discovery of diode (1939) and transistor (1947).• Characteristics: easy to refine and to obtain high levels of purity.• Low level of reliability.• Worse temperature sensitivity.

• Si was found the most abundant material on earth and was quickly considered the semiconductor material of choice after the first silicon transistor was discovered in 1954.• Although it is difficult to obtain a high level of purity, but having• Improve temperature sensitivity.

• As the communication systems speeds progressed, GaAs transistor is developed to meet current needs.• Speed of operation is 5 times that of Si.• Cheaper to manufacture• But difficult to manufacture at high levels of purity, more expensive and design

constraints etc.

Electron Shells, Orbits and Energy Level

• Orbits: Discrete distance from the nucleus• Shell: Orbits are grouped into energy level. Each

shell have different energy level. Shells are represented by either 1,2,3...or K,L,M...

• Number of electrons in each shell=2n^2• Valance Shell and Valence Electrons• Ionization: positive & negative ions

Conductor, Semiconductor and Insulator

• Conductor: Conductive materials have many free electrons and are capable to conduct electric current easily.

• Insulator: Insulator materials do not conduct electrical current under normal conduction. It is difficult to make the free electrons into conduction band while the physical characteristic of the material remain unchanged.

• Semiconductor: Conductivity of Semiconductors is in between conductor and insulator.

Energy Bands

Atomic Structure of Si and Ge and reason for the choice of selection

Atomic Structure of Semiconductor Materials and Intrinsic Carrier

• Tri, Tetra and Pentavalent.• Sharing of electrons, called covalent bond.• Valence electrons can absorb sufficient energy to make them free/free carrier/intrinsic carrier.

•Fig

•Fig1 Fig2 Fig3

Intrinsic Carrier and Relative Mobility

Intrinsic Carrier: Free electrons

Relative Mobility Factor: Ability of free carriers to move throughout the material

Semiconductor Intrinsic Carrier

GaAs 1.7x10^6

Si 1.5x10^10

Ge 2.5x10^13

Semiconductor Relative Mobility

Si 1500

Ge 3900

GaAs 8500 (5xSi)

Positive & Negative Temperature Coefficients

• Positive Temperature Coefficients: e.g. Conductors. The resistance increases with increase in temperature because of the carriers vibration pattern• Negative Temperature Coefficients: e.g. Semiconductors. The conductivity increases with increase in temperature by absorbing energy to break the covalent bond and contribute to free electrons.