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lecture38 - 6.720J/3.43J Integrated Microelectronic Devices...

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6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-1 Lecture 38 - Bipolar Junction Transistor (cont.) May 9, 2007 Contents: 1. Non-ideal effects in BJT in FAR Reading material: del Alamo, Ch. 11, § 11.5 ( §§ 11.5.1, 11.5.3, 11.5.4, 11.5.5 but only qualitatively) Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
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6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-2 Key questions Why are the output characteristics of a BJT in FAR not perfectly flat? What is the maximum voltage that the collector of a BJT can sustain in FAR? What are the key design issues for the break- down voltage? Why does the performance of a BJT degrade at high collector current? How does one model the base resistance in a BJT? Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
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6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-3 1. Non-ideal effects in BJT Base-width modulation V BE and V BC affect x BE and x BC , respectively W B = f ( V BE , V BC ). log N log N N E N E N B N C x BE x BC x thermal equilibrium (V BE =V BC =0) forward active (V BE >0, V BC <0) N B N C x BE x BC Early effect : impact of V BC on W B | V BC | ↑⇒ x BC ↑⇒ W B ↓⇒ I C , I B unchanged Reverse Early effect : impact of V BE on W B V BE ↑⇒ x BE ↓⇒ W B ↑⇒ smaller I C than ideal β F , I B unchanged x Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
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6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-4 Early effect : impact of V BC on W B I C = f ( V BC ) n W B (V BC ) n B (0) n B (x) I C n B (W B )=0 n i 2 N B 0 x V BC more negative W B ( V BC ) ↓⇒ I C ( V BC ) To capture first-order impact, linearize W B ( V BC ): V BC W B ( V BC ) W B ( V BC = 0)(1 + ) V A V A is Early voltage : qN B W Bo V A = C jco Impact on I C : I C ( V BC = 0) V BC I C ( V BC ) I C ( V BC = 0)(1 ) 1 + V BC V A V A Also, since I B unchanged: β F ( V BC = 0) V BC β F ( V BC ) = β F ( V BC = 0)(1 ) 1 + V BC V A V A Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
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6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 38-5 Manifestation of Early effect in output characteristics: V CE -V A I C I B I B =0 0 Main consequence of Early effect: finite slope in output characteris- tics in FAR: output conductance. C jc C B + v be g m v be g π g o - C π + C je E with g o given by: I C g o = V A Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007.
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