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BJG cơ bản

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05 Bipolar Junction Transistors (BJTs) basics
The first bipolar transistor was realized in 1947 by Brattain, Bardeen and
Shockley. The three of them received the Nobel prize in 1956 for their
invention. The bipolar transistor is composed of two PN junctions and hence is
also called the "Bipolar Junction Transistor" (BJT).

05 Bipolar Junction Transistors (BJTs) basics
There are two types of bipolar transistors: the NPN transistor, in which a
P-type region is sandwiched between two N-type regions, and the PNP
transistor, where N-type silicon is confined between two P-type regions.
Emitter

Base

Collector

05 Bipolar Junction Transistors (BJTs) basics
Long-base device
If the width of the neutral base, is large enough, all the electrons injected by the emitter into
the base recombine in the P-type material, because the base width is larger than the electron
diffusion length in the base. There is no interaction between both junctions and therefore no
current flowing between emitter and collector. Neglecting the small reverse current in the
collector-base junction, the only current flowing through the device is between the base and
the emitter:

05 Bipolar Junction Transistors (BJTs) basics
Short-base device
The term "short base" implies that the neutral base width is smaller than the electron
diffusion length: WB < LnB
Let the emitter-base junction be forward biased VBE = VB – VE > 0
and the collector-base junction be reverse biased VBC = VB – VC <0
Because the length of the neutral base is smaller than the diffusion length for electrons in
the base, a number of electrons injected from the emitter into the base can diffuse to the
collector-base junction depletion region, at x = WB
Once there, they are accelerated by the electric field of the depletion region and transported
into the collector

WB

05 Bipolar Junction Transistors (BJTs) basics
• In modern BJTs 99% or more of the electrons injected by the emitter into the base
reach the collector.
• The magnitude of current flowing in the collector does not depend on magnitude of
the collector voltage; the collector-base junction simply needs to be reverse biased.
• This effect, in which the current in a junction is controlled by the bias applied to
another junction, is called "transistor effect".

WB

05 Bipolar Junction Transistors (BJTs) basics

Symbolic representation, applied bias, and currents
in an NPN bipolar transistor.

05 Bipolar Junction Transistors (BJTs) basics

• A BJT transistor with a for...
05 Bipolar Junction Transistors (BJTs) basics
The first bipolar transistor was realized in 1947 by Brattain, Bardeen and
Shockley. The three of them received the Nobel prize in 1956 for their
invention. The bipolar transistor is composed of two PN junctions and hence is
also called the "Bipolar Junction Transistor" (BJT).
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