Week-7-BEE-4AB

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EE215: Electronic Devices and Circuits

Week-7

17th – 21st March 2014

Slide 1 Hammad M. Cheema EE215 Spring 2014

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Week1 – Week5

EE215 Spring 2014 Hammad M. Cheema Slide 2

• Chp-3: Physics of semiconductors

• Chp-4: Diode models and circuits

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This week’s topics


• Chapter 6 – Bipolar Junction transistors (BJTs)

– Device structure and physical operation

– Modes of operation

– Circuit symbols

– Current-Voltage (I-V) characteristics

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A voltage dependent current source as amplifier


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• Voltage between two terminals controls the current in the third terminal • A voltage-dependent current source can act as an amplifier. • If KRL is greater than 1, then the signal is amplified.

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Exponential Voltage-Dependent current source


• A three-terminal exponential voltage-dependent current source is shown above.

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Structure and Symbol of Bipolar Transistor

Hammad M. Cheema Slide 6

• Bipolar transistor can be thought of as a sandwich of three doped Si regions. The outer two regions are doped with the same polarity, while the middle region is doped with opposite polarity.

EE215 Spring 2014

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Bipolar terminals in simple terms

Hammad M. Cheema Slide 7

• The emitter “emits” charge carriers and the collector “collects” them while the base controls the number of carriers that make this journey.

EE215 Spring 2014

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NPN BJT transistor

• Two junctions present • Emitter-base junction (EBJ) • Collector-base junction (CBJ)


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PNP BJT transistor

• PNP transistor having n-type sandwiched between two p-type regions


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Modes of operation

Mode EBJ CBJ

Cutoff Reverse Reverse

Active Forward Reverse

Saturation Forward Forward


• Active mode – For amplifier circuits

• Cutoff & Saturation – For switching circuits

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Active mode


• For Active mode operation: VBE > 0,

VBC < 0 (VCB>0)

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Operation in active mode


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Operation in active mode

• Base Emitter junction is forward biased Electrons flow from Emitter to base & holes in the opposite direction

• Electron flow should be more than hole flow so doping level of Emitter is kept higher than base

• As electrons enter base, because it is thin they reach the collector base depletion region and are swept to the collector region due to high electric field and are absorbed by the +ve battery terminal.

• In active mode, npn BJT carries large number of electrons from the emitter, through the base to the collector


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Carrier concentration in active mode


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BJT Large signal model (active mode)


• A diode is placed between base and emitter and a voltage controlled current source is placed between the collector and emitter.

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I-V curves for BJT


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I-V curves example


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Cross-section of a practical NPN BJT

• Collector surrounds the emitter region, collecting all electrons


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Saturation mode of BJT

• EBJ – Forward biased

• CBJ – Forward biased

• Collector current decreases in Saturation mode


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PNP transistor


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Circuit symbols

• Drawing convention: Current flows from top to bottom

• Therefore Emitter for PNP drawn at the top


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Example 6.2


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Dependence of ic on the collector voltage – The Early voltage

• Changing the collector voltage does change the collector voltage slightly

• This happens due to increase in CBJ depletion region and shrinking of effective Base width thereby increasing Saturation current


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Collector current incorporating Early voltage

• To incorporate increase in collector current an extra term is introduced in the collector current expression


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SEECS, NUST Hammad M. Cheema Slide 25

The End

EE215 Spring 2014

Week-7-BEE-4AB

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