121B_1_EE121B_Slide03_master - BJT1

121B_1_EE121B_Slide03_master - BJT1 - EE 121B Principles of...

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Slide 3-1 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui EE 121B Principles of Semiconductor Device Design Bipolar Junction Transistors - I Professor Chi On Chui Electrical Engineering Department University of California, Los Angeles Email: [email protected] Slide 3-2 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Outline Transistor Operation Fundamentals of BJT Operation Amplification with BJTs Minority Carrier Distributions and Terminal Currents Diffusion equation in the base Evaluation of the terminal currents Emitter injection efficiency Approximations of the terminal currents Base transport factor Current transfer ratio (common base or α current gain)
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Slide 3-3 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Transistor Basics Transistor = Trans + resistor It is a 3-terminal device with the important feature that the current through the 2 terminals can be controlled by small changes (either in the current or voltage) at the 3rd terminal This control feature enables a host of electronic functions by allowing us to: Amplify small ac signals, or Switch the device from an on state to and off state and back v D i D E R L v G + v D + i D ( ) D D G D D v R i E v v f i + = = , v G Slide 3-4 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui External Control of Reversed Biased Junction Current V D I D n p I D R L + V D More EHP generation Reverse saturation current depends on the carrier generation rate in the neighborhood of the junction Electron-Hole Pair (EHP) generation mechanisms: Thermal generation Optical excitation Injection of minority carriers electrically (?)
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Slide 3-5 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Notice that the two back-to-back connected PN junctions are oppositely biased Emitter base junction is forward biased, collector base junction is reverse biased Two coupled junctions: One injects, the other collects V BC I C Desired Characteristic I E p+ n p I E I C I B Holes injected Holes collected Emitter Collector Base n Slide 3-6 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Combine two PN junctions With a long base…where excess hole populations go to their equilibrium values, there is no interaction between the two junctions The result is a constant trickle of current in the output circuit independent of the input p n -x p x n p n p n0 n p n n0 n p0 p p0 n p -x p x n p n n p0 n p p p0 p n0 n n0 Forward Biased Diode Reverse Biased Diode
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Slide 3-7 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Combine two PN junctions With a short base… excess carriers in the base are as shown above to satisfy the diffusion equation and boundary conditions Recall that the slope of the line is proportional to current and notice that the slopes are nearly equal on the two sides of the base
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This note was uploaded on 11/11/2010 for the course EE 121b taught by Professor Bjt-gamma during the Winter '08 term at UCLA.

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121B_1_EE121B_Slide03_master - BJT1 - EE 121B Principles of...

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