Bipolar Junction Transistors
ECE 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
The one equation that will always be
used with BJTs*
I E  I B  IC
* With the exception of reverse active. Then, the equation becomes
IC  I E  I B
Circuit Configurations
I-V Characteristic: npn Transistor
IC = b IB when VCE > VCEsat
Measured in a Common Emitter Configuration
Modified from
https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif
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 collector
VA – Early Voltage
b is not a constant
BVCEO – 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 biased
BC 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
iE  iC  iB
iC  b F iB
iE  (1  b F )iB
iC  a F iE
aF
bF 
1  aF
DC Equivalent Circuit for
npn in forward active
npn
BE
 qV

I E  I S  e nkT  1


pnp
EB
 qV

I E  I S  e nkT  1


Simplified DC Equivalent Circuit
npn
pnp
IC = bF IB AND IE = (bF +1) IB
VBE = 0.7V VCE ≥ 50mV
IB ≥ 0mA
VEB = 0.7V VEC ≥ 50mV
IB ≥ 0mA
Saturation
npn
pnp
IC ~ ISC
VBE = 0.75V VCE = 50mV
IC ≤ bF IB
VEB = 0.75V VEC = 50mV
Cut-Off
IC = IB = IE = 0
VBE ≤ 0.6V
VEB ≤ 0.6V