Bipolar Junction Transistors

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2 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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B
ipolar
J
unction
T
ransistors

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*

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

Circuit Configurations

I
-
V Characteristic: npn Transistor

Measured in a Common Emitter Configuration

Modified from
https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif

I
C

=
b

I
B

when V
CE

> V
CEsat

Nonideal I
-
V Characteristic

I
CEO



leakage current between the collector and emitter when I
B

= 0, usually
equal to the reverse saturation of the base
-
collection diode


Effects from a change in the effective distance between emitter and collector


V
A



Early Voltage


b

is not a constant


BV
CEO



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: I
C

=
a
F

I
E
, where
a
F

< 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

i
En

= i
E



i
Ep

i
Cn

= i
C



i
Cp
where i
Cp

~ Is
of the base
-
collector junction


i
En

> i
Cn

because some electrons recombine with holes in the base


i
B

replenishes the holes in the base

Current Relationships

in Forward Active Region

DC Equivalent Circuit for

npn

in forward active

npn

pnp

Simplified DC Equivalent Circuit

npn

pnp

V
BE

= 0.7V V
CE

≥ 50mV




V
EB

= 0.7V V
EC


50mV


I
B



0mA








I
B



0mA


I
C

=
b
F

I
B

AND I
E

= (
b
F

+1) I
B




I
C

~ I
SC


I
C


b
F

I
B













V
BE

= 0.75V V
CE

= 50mV


V
EB

= 0.75V V
EC

= 50mV


Saturation

npn

pnp



Cut
-
Off




I
C

= I
B

= I
E

= 0














V
BE

≤ 0.6V




V
EB


0.6V