EE210: Microelectronics-I Lecture-7 : Bipolar Junction Transistor-1 B. Mazhari Dept. of EE, IIT Kanpur B. Mazhari, IITK 1G-Number Invention of the Transistor By late November 1947, Bardeen and Brattain managed to make a working transistor. It must be said that it was very crude, but they improved it from late November through the first part of December. By December 16, 1947, they had a working point-contact transistor. They were able to gradually improve it and actually make a circuit to demonstrate to Bell Labs management on Christmas Eve. Eve Of course, course this was a very big event. event During the next six months at Bell Labs, Labs Bardeen and Brattain spent a lot of time making sure they had patents filed and then clearing it for release to the public with the military. It was interesting that at one stage, the military was threatening to classify this discovery as top secret. Fortunately, Bell Labs management worked around that. By June 30, 1948, Bell Labs had a press conference in New York City which was quite elaborate. B. Mazhari, IITK https://www.youtube.com/watch?v=kLBII5x43P0 2G-Number 3Qs What is transistor action and how does it occur in a BJT? Is BJT two back-to-back diodes connected together? g How does a BJT amplify? B. Mazhari, IITK 3G-Number Transistor Trans-resistor IO + + VIN Vo - - Current IO is much more sensitive to VIN than VO “Ideal Transistor” I0 + + gm v i Vi V0 - - I0 I0 Vi3 Vi Vi2 Vi1 V0 Voltage controlled Current Source (VCCS) Transistors can be used for AMPLIFICATION V0 VS vS + vi RL v0 g m vi RL Vo AV g m RL Vs By choosing sufficiently larger load resistance, voltage gain can be obtained Bipolar Junction Transistor (BJT) E N P N C E P P N B B C B E NPN C E: E E Emitter itt B: Base C: Collector B E PNP E: Emitter B: Base C: Collector C More Realistic View LE E WE n+ P N P WB C N DE ~ 1019 cm 3 N AB ~ 1017 cm 3 N DC ~ 1016 cm 3 N - B + W B ~ 2000 A W E ~ 1000 A LE ~ 1m BJT is not symmetric: emitter and collector cannot be simply interchanged E B n+ P N N P - + C Top View N E P P - B C N Background N P I IN IP V I N ; I P exp( p( ) VT If doping in N region is much larger than doping in p region then IN >> IP Basic Transistor Operation N+ P P N C E B B I IN IP We will assume that doping in emitter is much more than base so that electron l current iis much h llarger than h hole current I N I P In the reverse biased junction current is small because there are very few electrons in P and holes in N-region Basic Transistor Operation E I IN IP N+ P N+ P P P B N N C IE N+ P N IC C E IB IE IN IP Current Gain : B IC I N IB IP IC I N 1 IB IP N+ P IC N C E 5V B 0.7V Same operation as before I C I N exp( VBE ) VT T Transistor i t action ti Current is affected by base-emitter voltage and not by collector-base voltage I0 + Vi - B + g m vi V0 - IC + VBE + f(VBE) E C VCE - Alternative representation IB IC B C F.IB IC IB I B I BS V BE ) 1 exp( VT V BE ) 1 exp( VT IC IS IB IC E F Forward Active Mode Transistor Characteristics P N IB N IC IC IB3 IB2 IB1 VBE VBE VCB Output Characteristics of the transistor IC IB3 IB2 IB1 VCB IB4 IB3 IB2 IB1 VCE = VCB + VBE = VBE - VBC VCE 0.7 VBC
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