lec4_transistors0 - Lecture 4: 21st Century Transistors I....

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Lecture 4: 21 st Century Transistors I. Some History bipolar junction transistor : first solid-state amplifier element and started the solid-state electronics revolution. Bardeen, Brattain and Shockley at the Bell Laboratories invented it in 1948 as part of a post-war effort to replace vacuum tubes with solid-state devices. A bipolar junction transistor consists of two back-to-back p-n junctions which share a thin common region. Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. The device is called "bipolar" since its operation involves both types of mobile carriers, electrons and holes which travel in opposite directions. Notice that the region barrer between the emitter and base is forward biased while the region barrier between the base and collector is reversed biased. When the collector fills with electrons, an increased current, and therefore load is produced on the base/collector resistor thereby producing conduction for a switch, or increased power for an amplifier. However, heat dissipation and power loss is a problem. MOSFET : (metal-oxide semiconductor field-effect transistor) special type of field-effect transistor ( FET ) that works by electronically varying the width of a channel along which charge carriers ( electron s or hole s) flow. The wider the channel, the better the device conducts. The charge carriers enter the channel at the source , and exit via the drain . The width of the channel is controlled by the voltage on an electrode called the gate , which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide, in the same way that voltage biased barriers exist between the emitter-base-collector of a NPN bipolar transistor. There are two ways in which a MOSFET can function: depletion mode or enhancement mode. In the former, no voltage on the gate implies maximum conductance and as voltage is applied, the conductance decreases. In the latter, the opposite happens. MOSFETs have certain advantages because the gate is insulated electrically from the channel, and thus no current
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This note was uploaded on 04/07/2010 for the course ELEC ecse 421 taught by Professor Guss during the Winter '10 term at McGill.

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lec4_transistors0 - Lecture 4: 21st Century Transistors I....

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