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
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
2
Budapest University of Technology and Economics
Department of Electron Devices
Common base setup
input
output
Also called grounded base setup
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
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
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
4
Budapest University of Technology and Economics
Department of Electron Devices
The process of amplification
common base setup
rd 
U T 26 mV

 2,6
I E 10 mA
ube
in  i  rd  4 mA  2,6   10,4 mV
uout
uki 2000
ki  i  Rt  4 mA  500   2000mV
Au 

 200
ube
10
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
5
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
Also called as grounded emitter setup
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
6
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
I C   AN I E  I CB 0
I C   AN ( I B  I C )  I CB 0
AN
I CB 0
IC  
IB 
1  AN
1  AN
I C   BN I B  I CE 0
I E  I B  IC
AN
BN 
1  AN
I CE 0
I CB 0

1  AN
B : common emitter, large signal current gain
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
7
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
is
constant
charge
increases
No current flows.
20-10-2009
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
8
Budapest University of Technology and Economics
Department of Electron Devices
Common emitter setup
Input characteristic:
Output characteristic:
saturation
saturation
inverse
active
normal
active
closed
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
9
Budapest University of Technology and Economics
Department of Electron Devices
IE 
I ES exp(U BE / U T )  1  AI I CS exp(U BC / U T )  1
I C   AN I ES exp(U BE / U T )  1  I CS exp(U BC / U T )  1
X
saturation
UCE
Threshold of saturation:
saturation
UBC = 0
inverse
active
UBE = UCEnormal
active
U CES 0
20-10-2009
1
 U T ln
AI
closed
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
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
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
11
Budapest University of Technology and Economics
Department of Electron Devices
Effect of the parasitic CB diode
U CES 0
inner transistor
1
 U T ln
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.
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
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
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
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
20-10-2009
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
14
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
Backlash: The output
voltage influences the input
characteristic
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
15
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
16
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
The Early voltage
dU CE U E
rout

ki 
dI C
IC
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
17
Budapest University of Technology and Economics
Department of Electron Devices
Early effect at common base setup
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
18
Budapest University of Technology and Economics
Department of Electron Devices
Early effect: the backlash
~exp(UBE/UT)
CB setup
20-10-2009
CE setup
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
19
Budapest University of Technology and Economics
Department of Electron Devices
The Early effect
Problem
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?
UE
rout
ki 
IC
20-10-2009
80 V
rout
 16 k
ki 
5 mA
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
20
Budapest University of Technology and Economics
Department of Electron Devices
Op.p. dependence of current gain
Voltage dependence: due to the Early effect
dAN
dAN dwB

dU BC dwB dU BC
D p wB N B
e  1 
Dn wE N E
1  wB 
tr  1   
2  Ln 
2
wB  wBM  const  U CB
'
SC
dAN
/ wB
1
 (1  e )  2(1  tr ) 
dU BC 2
U CB
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
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:
p high level
20-10-2009
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008
22