ECE 3040 - Microelectronic Circuits
Lecture 10
Bipolar Junction Transistors
Introduction and Qualitative Analysis
Instructor: Dr. Shyh-Chiang Shen
Study: Pierret 10.1-10.4
Lecture Outline
•
•
•
•
Brief history of transistors
Terminology and device symbol definition
Formation of Bipolar Junction Transistor
Qualitative description of BJT operation
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
The Invention of Transistors
• Date: Dec. 16, 1947
• Who: John Bardeen and Walter
Brattain @ Bell Telephone
Laboratory, Murray Hill, NJ
• What does this thing do?
Dr. John Bardeen
(The inventor)
Dr. Walter Brattain
(The inventor)
– Current (power) amplification
• small current in, big current out
(you’ve got the current gain!!)
• Impacts:
– Start of microelectronic age in
human history
– Collapse of modern world with the
absence of transistors
Center for Compound Semiconductors
Dr. William Shockley
(The Boss)
Dr. S.-C. Shen, ECE3040B
How Does the First Transistor Work?
•
•
•
Original plan was…
– To make a point-contact rectifier using
surface states on semiconductors
Current amplification was observed
– under forward bias between “emitter”
(electrode) and the “base” (the
semiconductor), if the “collector”
electrode is placed close enough to
the emitter, the amount of the collector
current is controlled by the current
presented in the base.
– In other words, the input current (Ib)
controls the output current (Ic) in such
a way that power gain results!!
The first transistor paper:
– “Transistor, the semiconductor
triode,” J. Bardeen and W. Brattain,
Phys. Rev. 74, 230-231, 1948
Center for Compound Semiconductors
The first Transistor
“Electron and Holes in Semiconductors – with
applications to transistor electronics,” by William
Shockley, D. Van Nostrand Company, Inc., 1950
Dr. S.-C. Shen, ECE3040B
The Three Men Who Changed the World
•
http://nobelprize.org/physics/laureates/1956/
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Can Electrical Engineers Win the Nobel Prize?
The First Integrated
Circuit
(Dr. J. Kilby, Texas
Instruments)
•
Center for Compound Semiconductors
http://nobelprize.org/physics/laureates/2000/
Dr. S.-C. Shen, ECE3040B
Proliferation of Transistors –
BJTs and FETs
• The origin of the name of “Bipolar Junction Transistor (BJT)”
– The later development of transistor evolved from the original concepts
and utilized 3 sections of semiconductors with different material types
and doping concentrations to form 2 PN junctions that are adjacent to
each other.
– Both electrons and holes are involved in the carrier transportation
(Bipolar)
– Devices function like “TRANsfer reSISTORs” because the equivalent
resistance looking into collector is different from the equivalent
resistance looking into the base
• One can also make “Unipolar Transistors” that only one type
of carriers dominates the carrier transportation..
– Junction Field Effect Transistors (JFET)
– MEtal-Semiconductor Field Effect Transistors (MESFET)
– Metal Oxide Semiconductor Field Effect Transistors (MOSFET)
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Terminology
N
P
N
P
P
N
N
P
N
npn BJT
N
N
P
N
Emitter
Base
Collector
P
N
P
Emitter
Base
Collector
P
P
pnp BJT
– the emitter and collector have the same type, but not necessarily
identical doping concentration
– Transistors are called “BJT” if the E,B,C are made of the same material
– Transistors are called Heterojunction Bipolar Transistor (HBT) if (at least)
Emitter material is different from the base material (e.g. n-InGaP(E) + pGaAs(B) + n-GaAs (C) ! InGaP/GaAs HBT)
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Definition of Current Directions for PNP and NPN BJT
E
C
P
N
E
P
C
N
P
N
Emitter Base Collector
E
+
IE
+
VEB
IC
-
•
•
Emitter Base Collector
The circuit symbol shows
B
B
nominal Emit. curr. direction
C
E
C
VEC - VCE +
IB
B
- IE
VBE
IC
+ IB
B
General voltage notation definition: Vxy=Vx-Vy.. x,y = E,B,C
Current direction definition:
– IE is directing from P to N
– Direction of IC is defined in the same direction as that of IE
– IB is defined as the hole current that flows in the P-type base (npn BJT)and
the electron current that flows out of the N-type base (pnp BJT)
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Bandgap Alignment of a pnp BJT
W
Un-depleted Base Width
n
p+
p
WB
Base Width
WEB
Depletion width
for B-E junction
WCB
Depletion width
of B-C Junction
pnp BJT /w VBE=0, VBC=0
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Electrostatic Variables of a Un-biased pnp BJT
• Non-depleted base
width
– W = WB-xnEB-xnCB
– Bias dependent
• What about the
bandgap alignment and
electrostatic variables
for a npn BJT?
From R. F. Pierret, Semiconductor Device Fundamentals, Prentice-Hall Inc. © 1996. Permission
is granted for use of this file in conjunction with coursework employing the cited text.
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Bandgap Alignment of a npn BJT
p
n+
n
WB
Base Width
WEB
Depletion width
for B-E junction
WCB
Depletion width
of B-C Junction
npn BJT /w VBE=0, VBC=0
Sketch rest of the electrostatic variables for npn BJT…
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Silicon BJT Fabrication
•
Discrete BJT
•
e.g. 2N3019 NPN Power BJT
in TO-39 package. $~0.7 e.a.
Center for Compound Semiconductors
Integrated BJT
From R. F. Pierret, Semiconductor Device
Fundamentals, Prentice-Hall Inc. © 1996.
Permission is granted for use of this file in conjunction
with coursework employing the cited text.
Dr. S.-C. Shen, ECE3040B
Biasing Modes
E
IE
B
IB
IC
Biasing
Mode
E-B Junction
Polarity
C
C
IC
B
IB
Saturation
Forward
Active
Forward
Inverted
Reverse
Cutoff
Reverse
B-C Junction
Polarity
VBE
pnp
npn
+
+
+
+
-
Forward
Reverse
Forward
Reverse
VBC
pnp
npn
+
+
+
+
-
IE
E
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Modes of Operation in BJTs
•
Classification of operation modes •
with the applied voltages
VBE (npn)
VEB (pnp)
Classification of operation modes
within common-emitter currentvoltage characteristics..
IC
Active
Cutoff
Major interested
operation modes
Saturation
Inverted
VBC (npn)
VCB (pnp)
VCE (for npn)
VEC (for pnp)
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B
Qualitative Description: Active Mode (pnp BJT)
From R. F. Pierret, Semiconductor Device Fundamentals, Prentice-Hall Inc. © 1996.
Permission is granted for use of this file in conjunction with coursework employing the cited text.
•
e Diff.
e Drift
•
– Injected from E to B, across BE
junction, recombine /w e’s in B
– Survived h’s in B reach BC junction,
get swept to C
– W should be short so that significant
amount of h’s can reach BC junction..
Recombination
h Diff.
h Drift
BE junction under forward bias & BC
junction under reverse bias
Hole current:
•
3 Electron current components:
– Diffuse from in B to E (small if p+-n)
– Recombine with injected h’s from E
– Leakage current in BC ( very small
under rev. bias)
•
•
•
Center for Compound Semiconductors
The emitter current: IE = IEp+IEn
Collector current: IC = ICp+ICn
How about npn BJT under active
mode? BJTs under other modes?
Dr. S.-C. Shen, ECE3040B
KCL, KVL, and Circuit Configurations of BJT
C
E
IE
B
IC
B
IB
IB
IC
IE
C
•
E
• IE=IB+IC
• VEB+VBC+VCE=0
• A two-port network:
+
V1
-
I1
BLACK
BOX
I2
+
V2
-
For example: pnp BJT circuit configurations
C
B
IB
IC
IE
C
E
B
E
B
IC
IB
E
Common Emitter
Common Base
Center for Compound Semiconductors
IE
C
Common Collector
Dr. S.-C. Shen, ECE3040B
Definition of Performance Parameters
• Emitter Efficiency
– The fraction of the majority carrier current component in total emitter
current
– For pnp: γ=IEp/IE=IEp/(IEp+IEn) - - (How about npn?)
• Base Transport Factor
– The fraction of the minority carriers are injected into B (from E) that
are “survived” and enter C
– For pnp: αT=ICp/IEp – ( how about npn?)
• Common-base d.c. Current Gain
– The ratio of collector current to the emitter current under active mode
(αdc)
– αdc= γαT
• Common-emitter d.c. Current Gain
– Defined by β=IC/IB= αdc/(1- αdc)
Center for Compound Semiconductors
Dr. S.-C. Shen, ECE3040B