Engineering Module - Semester I - Electronic & Electrical Engineering PDF

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Part 7 Introduction to Semiconductors Dr Nevan Bermingham Electronic Materials  The goal of electronic materials is to gener...

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Part 7 Introduction to Semiconductors Dr Nevan Bermingham Electronic Materials  The goal of electronic materials is to generate and control the flow of an electrical current.  Electronic materials include: 1. Conductors: have low resistance which allows electrical current flow 2. Insulators: have high resistance which suppresses electrical current flow 3. Semiconductors: can allow or Conductors Good conductors have low resistance so electrons flow through them with ease. Best element conductors include: Copper, silver, gold, aluminum, & nickel Alloys are also good conductors: Brass & steel Good conductors can also be liquid: Salt water Conductor Atomic Structure The atomic structure of good conductors usually includes only one electron in their outer shell. It is called a valence electron. It is easily striped from Copper the atom, producing current flow. Atom Insulators Insulators have a high resistance so current does not flow in them. Good insulators include: Glass, ceramic, plastics, & wood Most insulators are compounds of several elements. The atoms are tightly bound to one another so electrons are difficult to strip away for current flow. Semiconductors Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between. Common elements such as carbon, silicon, and germanium are semiconductors. Silicon is the best and most widely used semiconductor. Semiconductor materials Semiconductor Valence Orbit The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit. Crystal Lattice Structure The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice. The atoms link together with one 2D Crystal Lattice another sharing Structure their outer electrons. 3D Crystal Lattice Structure Semiconductor Crystalline Structure Semiconductors have a regular crystalline structure for monocrystal, extends through entire structure for polycrystal, structure is interrupted at irregular boundaries Monocrystal has uniform 3-dimensional structure Atoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium Semiconductors can be Insulators If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow. Good insulating semiconductor material is referred to as intrinsic. Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow. Silicon in this form is a great insulator. Semiconductor material is often used as an Intrinsic Material A perfect semiconductor crystal with no impurities or lattice defects is called an intrinsic semiconductor. Valence band is filled with electrons Conduction band is empty Electron-hole pairs in the covalent bonding model in the Si crystal. Doping To make the semiconductor conduct electricity, other atoms called impurities must be added. “Impurities” are different elements. This process is called doping. By doping, a crystal can be altered so that it has a predominance of either electrons or holes.  Thus there are two types of doped semiconductors, n-type (mostly electrons) and p-type (mostly holes). When a crystal is doped the material is Semiconductors can be Conductors An impurity, or element like arsenic, has 5 valence electrons. Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms. The one electron left over for each arsenic atom becomes available to conduct current flow. Improving Conduction by Doping To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes elements with 5 outer electrons contribute an extra electron to the lattice (donor dopant) elements with 3 outer electrons accept an electron from the silicon (acceptor dopant) Improving Conduction by Doping Phosphorus and arsenic are donor dopants if phosphorus is introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current  very loosely bound to atom and can easily jump to conduction band produces n type silicon  sometimes use + symbol to indicate heavier doping, so n+ silicon phosphorus becomes positive ion after giving up electron Improving Conduction by Doping Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom boron is an acceptor dopant yields p type silicon boron becomes negative ion after accepting an electron Resistance Effects of Doping If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely. If you use only a few boron atoms, there will be fewer free electrons so the resistance will be high and less current will flow. By controlling the doping amount, virtually any resistance can be achieved. Another Way to Dope You can also dope a semiconductor material with an atom such as boron that has only 3 valence electrons. The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond. This place where a fourth electron should be is referred to as a hole. The hole assumes a positive charge so it can attract electrons from some other source. Holes become a type of current carrier like the electron to support current flow. Types of Semiconductor Materials The silicon doped with extra electrons is called an “N type” semiconductor. “N” is for negative, which is the charge of an electron. Silicon doped with material missing electrons that produce locations called holes is called “P type” semiconductor. “P” is for positive, which is the charge of a hole. Current Flow in Semiconductors The DC voltage source Electrons from the has a positive terminal that attracts the free negative supply electrons in the terminal are attracted semiconductor and to the positive holes pulls them away from and fill them. their atoms leaving the The positive terminal of atoms charged the supply pulls the positively. electrons from the Electrons from the holes leaving the holes negative terminal of the to attract more supply enter the electrons. semiconductor material Electrons flow from the and are attracted by negative terminal to the the positive charge of positive terminal. the atoms missing one Inside the of their electrons. semiconductor Current (conventional) conventional current flows from the positive flow is actually the terminal to the negative movement of the holes Current Flow in Semiconductors In Summary In its pure state, semiconductor material is an excellent insulator. The commonly used semiconductor material is silicon. Semiconductor materials can be doped with other atoms to add or subtract electrons. An N-type semiconductor material has extra electrons. A P-type semiconductor material has a shortage of electrons with vacancies called holes. The heavier the doping, the greater the conductivity or the lower the resistance. By controlling the doping of silicon the semiconductor material can be made as https://www.youtube.com/watch?v=5ZNeDxfgYAE Any Questions?