Lecture 27 - BJT Steady-State Response

Lecture 27 - BJT Steady-State Response - BJT Steady-State...

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1 BJT Steady-State Response Objective: To quantitatively describe the BJT’s behavior in steady-state. Questions to be answered: • How does a BJT function? (review) • What parameters describe BJT performance? • How do we determine (quantitatively) the BJTs EE360 – Lecture 27 output characteristics? Device Types pnp npn • Emitter usually heavily doped (i.e. N AE = 10 18 cm -3 ) • Base more lightly doped (i.e. N DB = 10 15 cm -3 ) • Collector even more lightly doped (i.e. N AC = 10 14 cm -3 ) From Pierret, pg. 372 EE360 – Lecture 27 • New notation: N AE = emitter acceptor concentration • Base is “narrow,” or else we would just have two pn junction diodes.
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2 Conceptual Operation: Forward Active Mode 1) As always, few electrons drift from emitter to base or collector to base. 1 4 2 B E 2) CB junction reverse biased so no electrons diffuse from base to collector. 3) Many holes diffuse from emitter to base 4) Fewer electrons diffuse from base to emitter (asymmetrical Forward Active Mode Band Diagram 3 5 5 Base is narrow: Most holes C From Pierret, pg. 380 EE360 – Lecture 27 base to emitter (asymmetrical doping, diffusion current depends exponentially on forward bias). 5) Base is narrow: Most holes injected into the base make it to the CB depletion region without recombining and are swept into the collector. Conceptual Operation: Forward Active Mode •F o r w a r d A c t i v e Mode: EB forward biased, CB reverse biased From Pierret, pg. 381 Schematic illustration of currents present in the pnp BJT Define currents: I Ep , I Ep : hole and electron emitter currents I Cp , I Cp : hole and electron collector currents EE360 – Lecture 27 small base current (electron diffusion into the emitter, RG in the base) controls a large collector current (hole diffusion into and across the base and hole drift into the collector).
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3 Conceptual Operation: Forward Active Mode From Pierret, pg. 373 Current amplification from a BJT Common Emitter Configuration From Pierret, pg. 382 EE360 – Lecture 27 • Current amplification: a small electron current through the EB junction controls a large hole current through the entire device. 2 3 4 5 6 ive Energy (eV) Conceptual Operation: Common Base N AE =10 19 cm -3 N DD =10 17 cm -3 N AC =10 15 cm -3 W B =3 µ m V CB =-5V V CB =-3V -2 0 2 4 6 x 10 -4 -2 -1 0 1 Position (cm) Relat • Emitter current set by V EB , collector current emitter current regardless of V CB . V EB =0.75V From Pierret, pg. 373 EE360 – Lecture 27 current regardless of V CB =-5V, I E I C =4.3mA V CB =-3V, I E I C =4.3mA • Provides large voltage gain. Current gain < 1.
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4 1 2 3 4 5 ve Energy (eV) Conceptual Operation: Common Emitter N AE =10 19 cm -3 N DD =10 17 cm -3 N AC =10 15 cm -3 W B =3 µ m -2 0 2 4 6 x 10 -4 -2 -1 0 Position (cm) Relativ •F i x V CE = -5V and vary V EB V EB =0.75V, V CB =-4.25V, I C =4.3mA, I B =9.7 µ A, lower (blue) line From Pierret, pg. 373 EE360 – Lecture 27 V EB =0.80V, V CB =-4.20V, I C =30mA, I B =67 µ
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Lecture 27 - BJT Steady-State Response - BJT Steady-State...

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