MIT6_012F09_lec08 - 6.012 Microelectronic Devices and...

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6.012 - Microelectronic Devices and Circuits Lecture 8 - BJTs Wrap-up, Solar Cells, LEDs - Outline Announcements Exam One - Tomorrow, Wednesday, October 7, 7:30 pm BJT Review Wrapping up BJTs (for now) History - 1948 to Today p-n Diode review Reverse biased junctions - photodiodes and solar cells In the dark: no minority carriers, no current With illumination: superposition, i D (v AB , L); photodiodes The fourth quadrant: optical-to-electrical conversion; solar cells Video: "Solar cell electricity is better electricity - putting 6.012 to work improving our world (a true story)" Forward biased junctions - light emitting diodes, diode lasers Video: "The LEDs Around Us" Diode design for efficient light emission: materials, structure The LED renaissance: red, amber, yellow, green, blue, white Clif Fonstad, 10/6/09 Lecture 8 - Slide 1
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BJT Modeling: FAR models/characteristics B E C v BE + i F I ES ! F i F i " i B E C v BE + i B I BS ! F i B " F # $ i C i E = 1 $ % B ( ) 1 + % E ( ) & 1 1 + % E ( ) " F # i C i B = 1 $ % B ( ) % E + % B ( ) & 1 % E Defects " E = D h D e # N AB N DE # w B , eff w E , eff " B # w B , eff 2 2 L eB 2 Design Clif Fonstad, 10/6/09 Doping : npn with N DE >> N AB w B , eff : very small L eB : very large and >> w B , eff Lecture 8 - Slide 2
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BJT's, cont.: What about the collector doping, N DC ? An effect we didn't put into our large signal model Base width modulation - the Early effect and Early voltage: The width of the depletion region at the B-C junction increases as v CE increases and the effective base width, w B,eff , gets smaller, thereby E Clif Fonstad, 10/6/09 reverse breakdown of the B-C junction. Lecture 8 - Slide 3 Punch through - base width modulation taken to the limit When the depletion region at the B-C junction extends all the way through the base to the emitter, I C increases uncontrolably. Punch through has a similar effect on the characteristics to that of Punch through p n w B,eff C Early effect B n+ increasing β and, in turn, i C . We will take this effect into account in our small signal LEC modeling. To minimize the Early effect we make N DC < N AB
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pnp BJT's: The other "flavor" of bipolar junction transistor pnp Symbol and FAR model: Oriented with emitter down like npn: C Structure: i C + C i C i B B + v BE E v CE B E C v EB + I ES ! F I ES e i B " F i B or q V EB /kT Oriented as found in circuits: + p + N AE n N DB p N AC i B B C E v EB + I ES ! F I ES e i B " F i B or q V EB /kT B E + v EB v BE B i B -i C C i E E Clif Fonstad, 10/6/09 Lecture 8 - Slide 4
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Metallic base contact n -type base wafer Active base region Metallic emitter contact (2.5 mm) Recrystallized p -type regions Metallic collector contact Approx. 0.001" (25 µ m) Approx. 0.1" Figure by MIT OpenCourseWare. Alloy junction BJT - Early1950's Early Bipolar Junction Transistors - the first 10 yrs. Clif Fonstad, 10/6/09 Photograph of grown junction BJT (showing device width of 5 mm) removed due to copyright restrictions.
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