ME 140L - Lecture 5 - Diodes Transistors

ME 140L - Lecture 5 - Diodes Transistors - MECHATRONICS LAB...

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1 MECHATRONICS LAB ME 140L CHARACTERIZATION OF SEMICONDUCTOR DEVICES: DIODES AND TRANSISTORS Background: Conventional Current Flow versus Electron Flow From Chemistry : Outer valence electrons essential for materials to conduct electricity. Covalent bond: materials share the electrons in the outermost valence. Absence of an electron in the outermost valence creates a ‘hole’. Current flows because of the movement of electrons (-) and holes (+). Conventional current : Conventional Current really shows the flow of holes from the positive to negative terminal. Electron flow (actual): Electrons flow from negative terminal to the positive terminal of the source. Both Conventional Current and Electron Flow are used in industry. Be careful to note the definitions or descriptions used in textbooks.
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2 I. SEMICONDUCTORS: Characterization Ref: How Semiconductors Work (Marshall Brain) http://www.uoguelph.ca/~antoon/tutorial/xtor/xtor1/xtor1.html http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/lecture_3 .html http://en.wikipedia.org/wiki/Bipolar_junction_transistor Most common semiconductor material is silicon (sand/quartz). Periodic Table: Si next to Al and P, below C, and above Germanium Placement in Periodic Table: B, Al, Ga: 3 electrons in outer valence C, Si, Ge: 4 electrons in outer valence N, P, As: 5 electrons in outer valence
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3 A. Doping From Chemistry: C, Si and Ge—each has four electrons in its outer valance orbit. A lattice is formed when four electrons from adjacent atoms form covalent bonds—no ‘free’ electrons— insulator. Adding a small amount of ‘impurities’ allow free electrons in Si n-type doping Add phosphorus or arsenic: each has five outer electrons Use four to link with silicon—5 th electron is ‘free electron’ 20 micrograms P/1kg Si—very minute additive n for negative charge from electron: electron majority carrier p-type doping Add boron or gallium: each has three outer electrons Use three to link with silicon—missing 4 th electron creates ‘hole’ in the lattice p for positive charge from absence of electron (note holes conduct current): hole is majority carrier Doping turns silicon from an insulator to a conductor (but not great)— therefore name semiconductor.
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4 B. P-N Junctions (basis for diodes/transistors) Semiconductors function when n-type and p-type silicon put together in a
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This note was uploaded on 09/18/2011 for the course ME 140L taught by Professor Staff during the Fall '09 term at University of Texas.

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ME 140L - Lecture 5 - Diodes Transistors - MECHATRONICS LAB...

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