EE 330 Lect 20 Spring 2011

EE 330 Lect 20 Spring 2011 - EE 330 Lecture 20 Bipolar...

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EE 330 Lecture 20 Bipolar Device Modeling Bipolar Process
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Basic Devices and Device Models • Resistor • Diode • Capacitor • MOSFET • BJT Review from Last Lecture
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Bipolar Junction Transistors Operation and Modeling Review from Last Lecture
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Bipolar Transistors npn stack pnp stack E E B B C C With proper doping and device sizing these form Bipolar Transistors pnp transistor B C E npn transistor B C E Bipolar Devices Show Basic Symmetry Electrical Properties not Symmetric Designation of C and E critical Review from Last Lecture
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Bipolar Operation npn stack E B C Consider npn transistor F 1 F 2 So, what will happen? Some will recombine with holes and contribute to base current and some will be attracted across BC junction and contribute to collector Size and thickness of base region and relative doping levels will play key role in percent of minority carriers injected into base contributing to collector current Review from Last Lecture
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Bipolar Operation npn stack E B C Under forward BE bias and reverse BC bias current flows into base region Consider npn transistor Efficiency at which minority carriers injected into base region and contribute to collector current is termed α α is always less than 1 but for a good transistor, it is very close to 1 For good transistors .99 < α < .999 Making the base region very thin makes α large Review from Last Lecture
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Bipolar Operation npn stack E B C Consider npn transistor In contrast to MOS devices where current flow in channel is by majority carriers, current flow in the critical base region of bipolar transistors is by minority carriers Review from Last Lecture
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Bipolar Operation I C I E I B E B C B C I I β is typically very large Bipolar transistor can be thought of a current amplifier with a large current gain In contrast, MOS transistor is inherently a tramsconductance amplifier Current flow in base is governed by the diode equation t BE V V B e I S I ~ t V V C e I S I ~ Collector current thus varies exponentially with V BE Review from Last Lecture
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Simple dc model I C C E B I B V BE V CE t BE V V B e I S I ~ t V V C e I S I ~ q kT V t Summary: This has the properties we are looking for but the variables we used in introducing these relationships are not standard S I ~ It can be shown that is proportional to the emitter area A E Define and substitute this into the above equations E S A J 1 ~   S I Review from Last Lecture
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Simple dc model t BE V V B e I S I ~ t V V C e I S I ~
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EE 330 Lect 20 Spring 2011 - EE 330 Lecture 20 Bipolar...

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