BJT Basics
ROCHESTER INSTITUTE OF TECHNOLOGY
MICROELECTRONIC ENGINEERING
Bipolar Junction Transistor - Basics
Dr. Lynn Fuller
Webpage: http://people.rit.edu/lffeee/
Microelectronic Engineering
Rochester Institute of Technology
82 Lomb Memorial Drive
Rochester, NY 14623-5604
Tel (585) 475-2035
Email: Lynn.Fuller@rit.edu
MicroE webpage: http://www.microe.rit.edu
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
1-29-15 BJT_Basics.ppt
Page 1
BJT Basics
OUTLINE
Definitions
Schematic Symbols
Theory
Integrated BJT Structure
Modes of Operation
IC-VCE Family of Curves
Modifications
References
Homework Questions
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 2
BJT Basics
DEFINITIONS
Bipolar Junction Transistor - (BJT) Both holes and electrons participate in the
conduction of current, hence the name bipolar.
Minority carrier - In a p-type semiconductor electrons are the minority carrier type,
in an n-type semiconductor holes are the minority carrier type.
Emitter - Emits minority carriers into the base region of a BJT. For example, in an
NPN BJT the n-type emitter, emits electrons into the p-type base. The emitter
usually has the highest doping levels of the three regions of a BJT.
Base - Thin region which is used to control the flow of minority carriers from the
emitter to the collector
Collector -Collects the minority carriers that make it through the base from the
emitter. The collector usually has the lightest doping concentrations of the three
regions.
DC Beta ( bdc ) - The ratio of the collector current to the base current. bdc = IC / IB
AC Beta ( bac ) - The ratio of the change in the collector current to the change in
the base current. bac = D IC / D IB
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 3
BJT Basics
BJT - BIPOLAR JUNCTION TRANSISTOR
Label
2N3904
Flat
1
2
Emitter
Base
Collector
3
Rochester Institute of Technology
Microelectronic Engineering
John Bardeen, William Shockley,
Walter Brattain, 1948
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 4
BJT Basics
SCHEMATIC SYMBOLS
npn
Base
Collector
n
p
n
pnp
Collector
p
n
p
Base
Emitter
Emitter
Collector
Collector
Base
Base
Emitter
Rochester Institute of Technology
Microelectronic Engineering
Emitter
The arrow on the emitter is in the direction that
current will flow in the Base Emitter pn junction
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 5
BJT Basics
IDEALIZED STRUCTURE
p-type
Emitter
SC
SC
N
P
n-type
Collector
N
n-type
Base
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 6
BJT Basics
ELECTRON CONCENTRATIONS IN AN NPN BJT
Base
Emitter
Collector
E
n~Nde
n~Ndc
n~very small but not zero
~ni2/Nab
BE
Space Charge Layer
x
BC
With the B-E junction forward biased, and B-C junction
reverse biased. There is a concentration gradient in the base
that forces electrons to flow toward the collector.
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 7
BJT Basics
COMMENTS
1. The concentration of electrons in n-type silicon is ~ doping
concentration in that region.
2. In p-type silicon the number of electrons is almost zero
3. A forward biased pn junction means more carriers of both types
can cross the potential barrier. So a forward biased base-emitter
junction (in an npn BJT) means more electrons on the base side than
in equilibrium (no bias).
4. A reverse biased pn junction means less carriers of both types can
cross the potential barrier. So a reverse biased base-collector junction
(in an npn BJT) means less electrons on the base side than in
equilibrium (no bias). Even closer to zero electrons in p-type base at
the edge of the B-C space charge layer.
5. The base is so narrow that few electrons are lost as they diffuse
across the base width. Diffusion is driven by a concentration
gradient. So electrons move towards the collector and current flows
in the opposite
direction.
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 8
BJT Basics
INTEGRATED BJT STRUCTURE
Collector (n)
~ 1018 cm-3
Base (p)
Emitter (n+)
~ 1018 cm-3
p-type ~ 1016 cm-3
electrons
current
Lightly doped (~1015 cm-3) n-type silicon wafer
Since the emitter is more heavily doped compared to
the base than the collector, the emitter-base junction
has a lower breakdown voltage than the base-collector
junction.
n+ means heavily doped n-type
n- means lightly doped n-type
Rochester Institute of Technology
p+ means heavily doped p-type
Microelectronic Engineering
p- means lightly doped p-type
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 9
BJT Basics
BJT TERMINAL CURRENTS
From device physics
IE = ISE eVBE/VT
where ISE = AqDni2/NAW and VT = KT/q
IC = a IE where a represents the fraction of
carriers from the emitter that make it to the collector
IC = b IB where b represents the ratio of collector
current to base current
We can show that b = a/(1-a) or a = b / (b+1)
WithRochester
the B-E
junction forward biased, and B-C junction
Institute of Technology
reverse
biased.
Microelectronic
Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 10
BJT Basics
LARGE SIGNAL MODEL IN FORWARD ACTIVE MODE
Modes
Base/Emitter
Cutoff
Reverse
Active
Forward
Inverse
Reverse
Saturation Forward
Base/Collector
Reverse
Reverse
Forward
Forward
Collector
IC
With the B-E junction forward
biased, and B-C junction
reverse biased.
a IE = Is eVBE/VT
IB
Base
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
IE
Emitter
Page 11
BJT Basics
EBERS-MOLL MODEL OF NPN BJT
This type of model works in all
four regions of operation
Collector
IC
IC = aF iDe – iDc
IE = -iDe + aR iDc
aF iDe
iDc
The diode currents are:
iDc = ISC (e Vbc/VT -1)
iDe = ISE (e Vbe/VT -1)
IB
Base
aR iDc
iDe
Transistors are modeled by determining
appropriate values of: aF, aR, ISC and ISE
IE
Emitter
Institute of Technology
Note:Rochester
b is often
given instead of a but a = b/(1+b)
Microelectronic
Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 12
BJT Basics
EARLY VOLTAGE
Increasing VCE increases the reverse bias on the BC junction increasing
the width of the BC space charge layer resulting in a decrease in the base
width and increase in concentration gradient and an increase in collector
current. To account for this the equation relating the collector current to
the VBE can be modified slightly as shown: VA is the Early voltage after
Dr. Jim Early of Fairchild Semiconductor. IC
IC = IS 1+ VCE
VA
eVBE/VT
-VA
VCE
This isRochester
one of
the many modifications to make the BJT models more accurate.
Institute of Technology
Engineering
Other Microelectronic
modifications
include resistors to account for series resistance in the
collector, base and emitter.
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 13
BJT Basics
CHARACTERISTICS OF TWO TERMINAL DEVICES
I
I
+
V
-
V
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 14
BJT Basics
BE JUNCTION, BC JUNCTION, CE
I
I
-8
0.7
V
I
-8
0.7
V
-8
0.7
Collector
I
Base +
V-
V+
Base
I
+
V
-
Emitter
I
Collector
Emitter
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
V
Page 15
BJT Basics
CHARACTERISTICS OF THREE TERMINAL DEVICES
Iin
Iout
+
+
Vin
Vout
Common
Iin
Iout
Vin
Vout
Each trace is for a different
value for Iout or Vout
Rochester Institute of Technology
Microelectronic Engineering
Each trace is for a different
value for Iin or Vin
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 16
BJT Basics
BJT IC-VCE FAMILY OF CURVES
IC
Steps of base current - IB
10 mA
9 mA
IB = 30 mA
8 mA
10 mA increments
7 mA
b dc
6 mA
5 mA
DIC = 2.5 mA b ac
4 mA
3 mA
IB = 20 mA
DIB = 10 mA
IB = 10 mA
2 mA
1 mA
VCE
Beta (b ac ) =
Beta (b dc ) =
DIC
2.5x10-3
= 250
=
DIB
10x10-6
IC
5.0x10-3
= 250
=
IB
20x10-6
The two Beta values are not
always the same!
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 17
BJT Basics
NPN COMMON EMITTER IC-VCE CHARACTERISTICS
Forward Active Mode
Base-Emitter junction is forward biased
Base-Collector junction is reverse biased
npn
IC
- +5 V
B-C junction is reverse biased
Vbc negative
base - p
IB
base
current
steps
Va > 0 is a forward biased junction
Va < 0 is a reverse biased junction
Va is defined as the voltage from p to n
0.7 V
Sweep from 0 to 5 Volts
Collector - n
+
+
Vbe positive
-
B-E junction is forward biased
Emitter - n
0V
Inverse Active Mode
Base-Emitter junction is reverse biased
Base-Collector junction is forward biased
Rochester Institute of Technology
Microelectronic Engineering
January
29, 2015
Dr. collector
Lynn Fuller, Professor
Switch the connections to©the
emitter
and
leads
Page 18
BJT Basics
TRIPLE DIFFUSED BJT STRUCTURE
Collector
Emitter
Base
n+
p-base
n+
p-wafer
n-well collector
Doping
Conc
n+ emitter
1-D Doping Profile
p-base
§ Simple BJT structure
§ Large collector series resistance
§ Large dimensions
§ Isolation issues
n-well collector
depth
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
WB
Base Width WB ~ 0.5mm
Page 19
BJT Basics
SHALLOW TRENCH ISOLATION
Collector
SiO2
n+
plug
p-wafer
Emitter
Base
n+
p-base
SiO2
SiO2
n-type epitaxial silicon
n+ buried layer
§ Process Enhancements
§ Oxide-plug isolation
§ Patterned buried sub-collector
§ Epitaxial silicon
§ Performance Improvements
§ Low collector series resistance
§ Improved collector/emitter isolation
§ smaller geometries
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 20
BJT Basics
REFERENCES
1. Sedra and Smith, 5.1-5.4
2. Device Electronics for Integrated Circuits, 2nd Edition, Kamins
and Muller, John Wiley and Sons, 1986.
3. The Bipolar Junction Transistor, 2nd Edition, Gerald Neudeck,
Addison-Wesley, 1989.
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 21
BJT Basics
HOMEWORK - BJT’S
1. Why won’t two back to back diodes behave like a BJT?
2. Sketch a figure like that on page 7 showing the hole
concentration for a pnp transistor with B-E junction forward biased
and B-C junction reverse biased. Show direction of current flow.
3. The Ic versus Vce family of
Vce
curves for a 2N3906 BJT is shown.
What is the current gain, Beta, b
Ib
4. Look up the 2N3906 and see
what the typical b is.
Emitter
5. Look up some information
about John Bardeen. Write a few
sentences.
Rochester Institute of Technology
Microelectronic Engineering
© January 29, 2015 Dr. Lynn Fuller, Professor
Page 22