Budapest University of Technology and Economics
Department of Electron Devices
Microelectronics, BSc course
Bipolar transistors 3
http://www.eet.bme.hu/~poppe/miel/en/08-bipolar3.ppt
http://www.eet.bme.hu
Budapest University of Technology and Economics
Department of Electron Devices
Characteristics of the
ideal BJT
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
2
Budapest University of Technology and Economics
Department of Electron Devices
Common base setup
input
output
Also called grounded base setup
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
3
Budapest University of Technology and Economics
Department of Electron Devices
Common base setup
input
Input characteristic:
output
Output characteristic:
normal
active
saturation
IE
normal
active
closed
saturation
closed
inverse
active
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
4
Budapest University of Technology and Economics
Department of Electron Devices
The process of amplification
common base setup
rd 
UT
IE

26 mV
 2 ,6 
10 mA
u be
in  i  rd  4 mA  2 , 6   10 , 4 mV
Au 
24-10-2011
u ki
u be

2000
 200
u out
ki  i  R t  4 mA  500   2000 mV
10
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
5
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
Also called as grounded emitter setup
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
6
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
I C   A N I E  I CB 0
I C   A N ( I B  I C )  I CB 0
IC  
AN
1  AN
IB 
I CB 0
1  AN
I C   B N I B  I CE 0
I E  I B  IC
BN 
AN
1  AN
I CE 0 
I CB 0
1  AN
B : common emitter, large signal current gain
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
7
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
is
constant
charge
increases
No current flows.
24-10-2011
Part of the base current is spent
on accumulating the base
chrage. UBE voltage increases,
the emitter current starts to flow.
The other part of the base
current is spent on recombination
with some part of the emitter
current.
charge is
constant
The charge in the base is not
increased any longer. Any
increase of the base current
recombines with a given part of
the emitter current, thus, the
emitter current will also increase.
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
8
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
Input characteristic:
Output characteristic:
saturation
saturation
inverse
active
normal
active
normal active
closed
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
9
Budapest University of Technology and Economics
Department of Electron Devices
IE 
I ES exp( U BE / U T )  1  A I I CS exp( U BC / U T )  1
I C   A N I ES exp( U BE / U T )  1 
saturation
UCE
Threshold of saturation:
saturation
UBC = 0
inverse
active
UBE = UCEnormal
active
U CES
24-10-2011
0
 U T ln
X
I CS exp( U BC / U T )  1
1
normal active
closed
AI
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
10
Budapest University of Technology and Economics
Department of Electron Devices
Characteristics of real
BJTs: secondary
effects
► Parasitic
CB diode
► Series resistances
► Early effect
► Operating point dependence of the gain
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
11
Budapest University of Technology and Economics
Department of Electron Devices
Effect of the parasitic CB diode
U CES
inner transistor
0
 U T ln
1
AI
parasitic
junction
No emitter region opposite to it, thus, in inverse oparation the
electrons injected from the collector into the base will be lost: inverse
active current gain is worsened.
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
12
Budapest University of Technology and Economics
Department of Electron Devices
Effect of series resistances
Base contact
Where is it
exactly?
The "inner base" – good approximation: RBB'
E
C
B'
RBB'
B
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
13
Budapest University of Technology and Economics
Department of Electron Devices
Effect of series resistances
Collector contact
n+ emitter
collector
p base
chip carrier (collector lead)
ICRCC' adds to UCE 
characteristics can be only on the
right hand side of the 1/RCC' line
24-10-2011
reduction of RCC' in case of
discrete transistors: epitaxial
structure (like in case of diodes)
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
14
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
Backlash: The output
voltage influences the input
characteristic
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
15
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
16
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
The Early voltage
rout
ki 
24-10-2011
dU
CE
dI C

UE
IC
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
17
Budapest University of Technology and Economics
Department of Electron Devices
Early effect at common base setup
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
18
Budapest University of Technology and Economics
Department of Electron Devices
Early effect: the backlash
~exp(UBE/UT)
CB setup
24-10-2011
CE setup
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
19
Budapest University of Technology and Economics
Department of Electron Devices
Problem
The Early effect
What is the output resistance of the transistor in
common emitter setup is the Early voltage is 80V and
the collector current in the operating point is 5mA?
rout
ki 
24-10-2011
UE
IC
rout
ki 
80 V
 16 k 
5 mA
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
20
Budapest University of Technology and Economics
Department of Electron Devices
Op.p. dependence of current gain
Voltage dependence: due to the Early effect
dA N
dU

dA N
dw B
dw B dU
BC
e  1 
BC
 tr
D p wB N B
Dn wE N E
1  wB
 1  
2  Ln




2
w B  w BM  const  U CB
dA N
dU
BC
24-10-2011

1
2
(1   e )  2 (1   tr ) 
'
SC
/ wB
U CB
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
21
Budapest University of Technology and Economics
Department of Electron Devices
Op.p. dependence of current gain
Voltage dependence: due to the Early effect
Current dependence:
high level
24-10-2011
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011
22