B ipolar J unction T ransistors
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Transcript of B ipolar J unction T ransistors
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Bipolar Junction TransistorsECE 2204
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Three Terminal Device•Terminals
▫Emitter The dominant carriers are emitted from the region
(equivalent to the Source in a MOSFET)▫Base
These now minority carriers travel through the base region Some recombine in the base, forcing a base current to flow
▫Collector The remaining carriers from the emitter are collected
from this region (equivalent to the Drain)
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Types of BJTs• n-p-n
▫ Emitter is n+ type Electrons flow from the emitter towards the collector
▫ Base is p type Some of the electrons from the emitter recombine with the
holes in the base▫ Collector is n- type
• p-n-p▫ Emitter is p+ type
Holes flow from the emitter towards the collector▫ Base is n type
Some of the holes from the emitter recombine with the electrons in the base
▫ Collector is p- type
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Cross Section of npn Transistor
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Cross-Section of pnp BJT
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Circuit Symbols and Current Conventions
npn pnp
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CBE III
The one equation that will always be used with BJTs*
* With the exception of reverse active. Then, the equation becomes
BEC III
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Circuit Configurations
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I-V Characteristic: npn Transistor
Measured in a Common Emitter ConfigurationModified from https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif
IC = b IB when VCE > VCEsat
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Nonideal I-V Characteristic
ICEO – leakage current between the collector and emitter when IB = 0, usually equal to the reverse saturation of the base-collection diode
Effects from a change in the effective distance between emitter and collectorVA – Early Voltageb is not a constantBVCEO – breakdown voltage of the transistor
Modified from: http://cnx.org/content/m29636/latest/
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Current-Voltage Characteristics of a Common-Base Circuit
In Forward Active Region: IC = aF IE, where aF < 1
Modified from Microelectronic Circuit Analysis and Design by D. Neamen
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Simplified I-V Characteristics
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Modes of Operation•Forward-Active
▫B-E junction is forward biased▫B-C junction is reverse biased
•Saturation▫B-E and B-C junctions are forward biased
•Cut-Off▫B-E and B-C junctions are reverse biased
•Inverse-Active (or Reverse-Active)▫B-E junction is reverse biased▫B-C junction is forward biased
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npn BJT in Forward-Active
BE junction is forward biasedBC junction is reverse biased
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Currents and Carriers in npn BJT
iEn = iE – iEp
iCn = iC – iCp where iCp ~ Is of the base-collector junction
iEn > iCn because some electrons recombine with holes in the base
iB replenishes the holes in the base
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Current Relationships in Forward Active Region
F
FF
EFC
BFE
BFC
BCE
aaiai
iiii
iii
1
)1(
b
bb
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DC Equivalent Circuit for npn in forward active
npn pnp
1nkT
qV
SE
BE
eII
1nkT
qV
SE
EB
eII
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Simplified DC Equivalent Circuitnpn pnp
VBE = 0.7V VCE ≥ 50mV VEB = 0.7V VEC ≥ 50mV IB ≥ 0mA IB ≥ 0mA
IC = bF IB AND IE = (bF +1) IB
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IC ~ ISC IC ≤ bF IB
VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV
Saturationnpn pnp
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Cut-OffIC = IB = IE = 0
VBE ≤ 0.6V VEB ≤ 0.6V