Bipolar Junction TransistorsECE 2204

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)

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

Cross Section of npn Transistor

Cross-Section of pnp BJT

Circuit Symbols and Current Conventions

npn pnp

CBE III

The one equation that will always be used with BJTs*

* With the exception of reverse active. Then, the equation becomes

BEC III

Circuit Configurations

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

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/

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

Simplified I-V Characteristics

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

npn BJT in Forward-Active

BE junction is forward biasedBC junction is reverse biased

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

Current Relationships in Forward Active Region

F

FF

EFC

BFE

BFC

BCE

aaiai

iiii

iii

1

)1(

b

bb

DC Equivalent Circuit for npn in forward active

npn pnp

1nkT

qV

SE

BE

eII

1nkT

qV

SE

EB

eII

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

IC ~ ISC IC ≤ bF IB

VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV

Saturationnpn pnp

Cut-OffIC = IB = IE = 0

VBE ≤ 0.6V VEB ≤ 0.6V