MIT6_012F09_lec08

# MIT6_012F09_lec08 - 6.012 Microelectronic Devices and...

This preview shows pages 1–6. Sign up to view the full content.

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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern