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Computer Science
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Steve Engels

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 p-n junctions  Transistors o Using n-type/p-type doped semiconductors together form the basis o Use the characteristics of p-n junctions to create more interesting of all transistors behavior, 3 main types o for instance, the p-n junction:  Biopolar Junction Transistors (BJTs)  by placing a chuck of p-type material next to a chunk of n-tpe  Metal Oxide Semiconductor Field Effect Transistor (MOSFET) material and applying a voltage from one end to the other, we  Junction Field Effect Transistor (JFET) see different effects on the resulting current o MOSFET & JFET same family o occasionally electron & empty space move around between p-type &  Bipolar Junction Transistors (BJTs) n-type  diffusion of electrons and empty spaces o When a p-n junction is reverse biased, the electric field in the  when diffusion occurs, resultant Potassum now positivly b/c depletion layer prevents the electrons from getting through extra proton & Boron now negatively charged  This could be overcome by artificially injecting more electrons o when left alone, the electrons form the n section of the junction will through the p-region mix with the holes of the p section, cancelling each other out, and  Make p-region narrower creating a particle-free section called the depletion layer  Inject electrons at high velocity, so they don’t have time  made up of positive Potassum and negative Borons to recombine  Solution: use a forward biased p-n junction to inject electrons into the reverse biased p-n junction o once this depletion layer is wide enough, the dopping atoms that remain will create an electric field in that region  an electric field is what occurs in the region between two o A BJT has three contact points surfaces that have a voltage difference between them  The base (middle layer)  The emitter (the source of the injected electrons)  the electric field cause by a depletion layer will cause holes to  The collector (the recipient of the injected electrons) flow back to the p section, and eletroncs to flow back to the n o By increasing the voltage btwn the base and the emitter, can make section  the current caused by this field is called drift (+) the current from the base to the collector go from zero (V =EB) to higher values  the current caused by electron/hole recombination is o Not all the electrons are injected directly from the emitter to the called diffusion (-) collector  at rest, these two currents reach equilibrium  Some are absorbed into the positive voltage terminal at the base, some are absorbed by the p-type section, some stay in  all the electrons entering area one of the n-type sections with empty spaces balanced by  Most make it through, cause the constant current output to electrons escaping that are b/c increase as V rises more negatively charged EB  Metal Oxide Semiconductor Field Effect Transistor (MOSFET)  What happens when a voltage is applied to this junction? o The “field effect” is an electric field that creates a channel between o It depends on the direction in which the voltage is applied two n-type regions for electrons to pass through o Forward bias: o This field is generated by an electrically-charged metal layer that is  When a positive voltage is applied to the p end of this junction, electrons are injected into the n-type section. shieled from the semiconductor material by an insulating oxidized layer (thus the MOS component)  This narrows the depletion layer and increasing electron o The semiconductor sections are two pockets of n-type
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