Presentation on semiconductor
- 1. Semi-conductor Material Presenter : Damion Lawrence
- 2. Review of Basic Atomic Model Atoms are comprised of electrons, neutrons, and protons. Electrons are found orbiting the nucleus of an at atom at specific intervals, based upon their energy levels. The outermost orbit is the valence orbit.
- 3. Energy Levels Valence band electrons are the furthest from the nucleus and have higher energy levels than electrons in lower orbits. The region beyond the valence band is called the conduction band. Electrons in the conduction band are easily made to be free electrons.
- 4. Intrinsic Semiconductors Silicon, germanium, and gallium arsenide are the primary materials used in semiconductor devices. Silicon and germanium are elements and are intrinsic semiconductors. In pure form, silicon and germanium do not exhibit the characteristics needed for practical solid-state devices.
- 5. Semiconductor Crystals Tetravalent atoms such as silicon, gallium arsenide, and germanium bond together to form a crystal or crystal lattice. Because of the crystalline structure of semiconductor materials, valence electrons are shared between atoms. This sharing of valence electrons is called covalent bonding. Covalent bonding makes it more difficult for materials to move their electrons into the conduction band.
- 6. Electron Distribution As more energy is applied to a semiconductor, more electrons will move into the conduction band and current will flow more easily through the material. Therefore, the resistance of intrinsic semiconductor materials decreases with increasing temperature. This is a negative temperature coefficient.
- 7. Semiconductor Doping Impurities are added to intrinsic semiconductor materials to improve the electrical properties of the material. This process is referred to as doping and the resulting material is called extrinsic semiconductor. There are two major classifications of doping materials. Trivalent - aluminum, gallium, boron Pentavalent - antimony, arsenic, phosphorous
- 8. Atomic Outer Electron Shells Silicon Tetravalent Boron Trivalent “Acceptor” Phosphorus Pentavalent “Donor”
- 9. Forward Biased Junction An external source can either oppose or aid the barrier potential. If the positive side of the voltage is connected to the p- type material, and the negative side to the n-type material, then the junction is said to be forward biased.
- 10. Forward Biased Junction In a forward biased junction, the following conditions exist: Forward bias overcomes barrier potential. Forward bias narrows the depletion region. There is maximum current flow with forward bias.
- 11. Reverse Biased Junction Reverse bias occurs when the negative source is connected to the p-type material and the positive source is connected to the n- type material. Reverse bias strengthens the barrier potential. Reverse bias widens the depletion region. Current flow is minimum.
- 12. Reverse Biased Junction A reversed biased junction has zero current flow (ideally). Reverse current is temperature dependent. If reverse biased is increased enough, the reverse current increases dramatically. This breakdown is called junction breakdown. The voltage required to reach this point is the reverse breakdown voltage. As the breakdown occurs, avalanche may occur and destroy the device if uncontrolled.
- 13. DIODE diode = “biased p-n junction”, i.e. p-n junction with voltage applied across it “forward biased”: p-side more positive than n- side; “reverse biased”: n-side more positive than p- side;
- 14. forward biased diode: the direction of the electric field is from p-side towards n-side ⇒ p-type charge carriers (positive holes) in p- side are pushed towards and across the p-n boundary, n-type carriers (negative electrons) in n-side are pushed towards and across n-p boundary ⇒ current flows across p-n boundary
- 16. The Diode: a tiny P-N Junction A diode comprises a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction.
- 17. When no voltage is applied, to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow.