Transcript A little bit of physics…

Bipolar Junction
Transistors (1)
Dr. Wojciech Jadwisienczak
EE314
Introduction
Your goal is to explain the transistor.
It is assumed that EE314 students to which this
presentation is aimed, have not a clue to how these little
Buggers work and/or how to use them.
A real problem with previous explanations: for the sake of
"fidelity" authors' include confusing details until the
concept, or thread--of how they actually work & how to
use them--is lost.
The following presentation is comprised of several
different explanations. You should read chapter 13 and
this presentation several times, because any insight
gained from one will help in understanding another.
1.History of BJT
2.First BJT
3.Basic symbols and features
4.A little bit of physics…
5.Currents in BJT’
6.Basic configurations
7.Characteristics
Chapter 13: Bipolar
Junction Transistors
pp. 584-624
First - BJTs
The transistor was
probably the most
important invention of
the 20th Century, and
the story behind the
invention is one of
clashing egos and top
secret research.
Reference:
Bell Labs Museum
B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.
Interesting story…
Picture from previous slide shows the workbench of John Bardeen
and Walter Brattain at Bell Laboratories. They were supposed to be
doing fundamental research about crystal surfaces. The experimental
results hadn't been very good, though, and there's a rumor that their
boss, William Shockley, came near to canceling the project. But in
1947, working alone, they switched to using tremendously pure
materials. It dawned on them that they could build the circuit in the
picture. It was a working amplifier! John and Walter submitted a
patent for the first working point contact transistor.
Shockley was furious and took their work and invented the
junction transistor and submitted a patent for it 9 days later.
The three shared a Nobel Prize. Bardeen and Brattain continued in
research (and Bardeen later won another Nobel).
Shockley quit to start a semiconductor company in Palo Alto. It
folded, but its staff went on to invent the integrated circuit (the
"chip") and to found Intel Corporation. By 1960, all important
computers used transistors for logic, and ferrite cores for memory.
Point-Contact Transistor –
first transistor ever made
Qualitative basic operation of point-contact
transistor
Problems with first transistor…
First Bipolar Junction Transistors
W. Shockley invented the p-n junction transistor
The physically relevant region is moved to the bulk of the material
force – voltage/current
water flow – current
- amplification
Understanding of BJT
Basic models of BJT
npn transistor
Diode
Diode
pnp transistor
Diode
Diode
Qualitative basic operation of BJTs
Basic models of BJT
BJTs – Basic Configurations
Fluid Flow Analogy
Difference between FET (field effect transistor)
and BJT
Technology of BJTs
pnp BJT
npn BJT
BJTs – Practical Aspects
Heat sink
BJTs – Testing
BJTs – Testing
A little bit of physics…
A little bit of physics…
A little bit of physics…
A little bit of physics…
A little bit of physics…
More accurate physical description…
pnp BJT
-iC
iE
-VCE
-iB
1. Injected h+ current from E to B
2. e- injected across the forward-biased EB junction (current from
B to E)
3. e- supplied by the B contact for recombination with h+
(recombination current)
4. h+ reaching the reverse-biased C junction
5,6.Thermally generated e- & h+ making up the reverse saturation
current of the C junction
Now, you can try…
npn BJT
BJTs – Basic configurations
npn BJTs – Operation Modes
Forward & reverse polarized
pn junctions
Different operation modes:
npn BJTs – Operation Modes
•When there is no IB current almost
no IC flows
•When IB current flows, IC can flow
•The device is then a current
controlled current device
Operational modes
can be defined
based on
VBE and VBC
BJT-Basic operation
pnp BJT
npn BJT
(n+), (p+) – heavy doped regions; Doping in E>B>C
BJTs – Current & Voltage Relationships
Operation mode: vBE is forward & vBC is reverse
The Shockley equation
  v BE
i E  I ES exp 
  VT
Einstein relation
 
  1
 
D
kT

m
q
IES–saturation I (10-12-10-16A); VT=kT/q -thermal V (26meV)
D – diffusion coefficient [cm2/s]
The Kirchhoff’s laws
m – carrier mobility [cm2/Vs]
iE  iC  iB
VBE  VBC  VCE  0
It is true regardless of the bias
conditions of the junction
Useful
parameter
iC

iB
iE
the common-emitter current gain
for ideal BJT  is infinite
BJTs – Current & Voltage Relationships
Useful
parameter
iC

iE
the common-base current gain
for typical BJT  is ~0.99
The Shockley equation
once more
  vBE  
  1
iC  I ES exp 
  VT  
If we define the scale current
I S  I ES
A little bit of math… search for iB
iB  1   iE
Finally…
iC

 
iB 1  
 vBE 

iC  I S 
 VT 
  vBE  
  1
iB  1   I ES exp 
  VT  
iC  iB
BJTs – Characteristics
Schematic
Common-Emitter
iC  iB
Output
Input
VBC<0 or equivalently VCE>VBE
If VCE<VBE the B-C junction is
forward bias and IC decreases
Remember VBE has to be greater
than 0.6-07 V
Example 13.1
BJTs – Load line analysis
Common-Emitter Amplifier
Input loop
smaller
vin(t)
VBB  vin (t )  RBiB (t )  vBE (t )
if iB=0
vBE  VBB  vin
if vBE=0
iE  (VBB  vin ) / RB
BJTs – Load line analysis
Common-Emitter Amplifier
Output loop
VCC  RC iC  vCE
Example 13.2
Circuit with BJTs
Our approach: Operating point - dc operating point
Analysis of the signals - the signals to be amplified
Circuit is divided into: model for large-signal dc analysis of BJT circuit
bias circuits for BJT amplifier
small-signal models used to analyze circuits for
signals being amplified
Remember !
Large-Signal dc Analysis: Active-Region Model
Important: a current-controlled current source models the
dependence of the collector current on the base current
VCB
reverse bias
VBE
forward bias
?
?
The constrains for IB and VCE must be satisfy to keep BJT in the
active-mode
Large-Signal dc Analysis: Saturation-Region Model
VCB
forward bias
VBE
forward bias
?
?
Large-Signal dc Analysis: Cutoff-Region Model
VCB
reverse bias
VBE
reverse bias
?
?
If small forward-bias voltage of up to about 0.5 V are applied, the
currents are often negligible and we use the cutoff-region model.
Large-Signal dc Analysis: characteristics of an npn BJT
Large-Signal dc Analysis
Procedure: (1) select the operation mode of the BJT
(2) use selected model for the device to solve the circuit
and determine IC, IB, VBE, and VCE
(3) check to see if the solution satisfies the constrains for
the region, if so the analysis is done
(4) if not, assume operation in a different region and
repeat until a valid solution is found
This procedure is very important in the analysis and design
of the bias circuit for BJT amplifier.
The objective of the bias circuit is to place the operating point in
the active region.
Bias point – it is important to select IC, IB, VBE, and VCE
independent of the  and operation temperature.
Example 13.4, 13.5, 13.6
Large-Signal dc Analysis: Bias Circuit
From Example 13.6
VBB acts as a short
circuit for ac signals
Remember: that the Q point should be independent of the 
stability issue)
VBB & VCC provide this stability, however this impractical solution
Other approach is necessary to solve this problem-resistor network
Large-Signal dc Analysis: Four-Resistor Bias Circuit
Solution of the bias problem:
1
VB  RB I B  VBE  RE I E
I E    1I B
3
VBE  0.7V
VB  VBE
IB 
RB    1RE
Thevenin
equivalent
4
2
Equivalent
circuit for
active-region
model
Input
Output
RB  R1 R2 VB  VCC R2 / R1  R2 
VCE  VCC  RC I C  RE I E
BJTs – Practical Aspects
npn
V
I
R
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