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3_bjt_slide_achar - Transistor: A three terminal device...

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Unformatted text preview: Transistor: A three terminal device Applications: Amplifiers, Digital Logic Gates, Oscillators, Filters.. In Products: Almost anything you see, that is electronic in nature barb4right TVs, radios, cell-phones, Monitors, computers.. Inventors: When: 16 December 1947 Where: Bell Labs, Murray hill, New Jersey, USA Significance: One of the greatest inventions of the 20th century; it changed the way the mankind lived. barb4right First working Transistor barb4right barb4right Modern ones: Semiconductor type Lecture 9: ELEC 2507: Ram Achar - BJTs: Bipolar Junction Transistors Who: William Shockley, John Bardeen and Walter Brattain BJTs: Bipolar Junction Transistors barb4right 3-Terminal Devices barb4right Two Junctions barb4right Two Types: n-p-n or p-n-p n-p-n Emitter Collector Base E B C n p n C E B B C E p-n-p Emitter Collector Base E B C p n p E B C B C E barb4right Emitter: Highly doped (Medium Area ) barb4right heavy charge carriers barb4right Base: Lightly doped (Very thin, smallest area ) barb4right few charge carriers barb4right Collector: Medium doped (Largest Area ) EBJ: Emitter-Base Junction Major Modes of Operation barb4right Since BJT has 2 junctions, several combinations of biasing is possible. The three commonly used modes are as follows: BJT: Concepts and Related Physics Objective: To understand the basic operations of BJT Emitter Highly doped Moderate area (n) Collector Medium doped Large area (n ) Base Very thin (p) E B C CBJ: Collector-Base Junction FB FB Saturation RB FB Active RB RB Cut-off CBJ status EBJ status MODE ( Note: In case of diodes, there was only one junction and only two modes of operations was possible (FB & RB) ) In a macroscopic view, BJT can be viewed as two back-to- back connected diodes: B C E Operation in the Active Mode npn transistor a, c leads to barb4right Emitter Current I E b, c lead to barb4right Emitter Current I B Case (1): B-E Junction (Forward Biased) a) Note that the EMITTER is heavily doped. Due to the FB, heavy dosage of electrons are now injected across the EB junction i.e., large number of electrons from the emitter region are pumped towards the base region. b) However, since the base is very thin and lightly doped, only few of the electrons have a chance to recombine in the base region, and c) At the same time the holes are injected from the base to the emitter (small numbers) EBJ: FB E B C CBJ: RB n n p +- V BE +- V CB e- Injected electrons Recomb ined e- s Collected electrons Injected holes I B I E I C (small voltage) (large voltage) Operation in the Active Mode npn transistor Case (2): C-B Junction (Reverse Biased) barb4right Collector is more positive than the base, hence CBJ is reverse biased....
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3_bjt_slide_achar - Transistor: A three terminal device...

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