8 Bipolar Transistors

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8 Bipolar Transistors
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Small-signal NPN

• Variations of standard Bipolar -- When
  single-level Metal M1 only -- stretching
  terminals to allow one or more leads to route
  between terminals

Stretched-terminal Transistor
Stretched-Collector
Stretched-Base
3

• Upgrading NPN gt tries to increase EBJ area for
  higher Beta
• problem - pinched Base leads to high RB
  - Emitter crowding !
• OK for high Beta, moderate speed app

4

• Layout to reduce RB (for higher speed)
• but also reduces BETA (due to small
  Area/Periphery ratio)

C
B E B E B E B

• superior freq. response,
• inferior BETA

• double-Base reduces RB to about 0.25 RB

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Substrate PNP -- in standard Bipolar

• lacks full isolation

P
N
P
6

• NPN
• Doping level
• Emitter diffusion 1020 cm-3
• 10 Ohm
• Base diffusion 1017 cm-3
• typical b 150 (_at_ 0.8mA/mm2)

• Substrate PNP
• Doping level
• Emitter (Base diffusion of NPN) 1017 cm-3
• 100 Ohm
• Base (Nepi doping) much lower
• typical b 30 (_at_ 30mA/mm2)
• typical Ic 1 - 2 mA
• not practical for high current PNP !

7
Higher-current substrate PNP
E 25 mm wide Not to be wider than 25-50 mm
because of pinched sheet 10kW/sq. under Emitter.
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Lateral PNP in standard Bipolar

• not much can do to boost Switching speed
• Beta can be improved by layout.
• Narrower Base Width gt higher Beta, lower VA
• Wider Base Width gt lower Beta, larger VA
• b VA const. Approximately.

• b depends on
• g emitter injection efficiency
• NB base doping
• tB base recombination
• WB base width
• Collection efficiency
• gt under Layout designers control.

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Lateral PNP in standard Bipolar
Drawn width
Actual width
Pbase
Pbase
Pbase
E
C
C
Effective width
B
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Constructing Lateral PNP
Example layout -- basic
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Example Split-Collector Lateral PNP
C
C
1/4-1/4-1/4-1/4
1/2-1/2
1/6-1/6-1/6-1/4-1/4
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Current Mirror using Split-Collector Lateral PNP
Simplified Schematic circuit
Q1A
1/2
E
Q1B
1/2
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• Collector-Ring Circular vs. Square
• Circular
• Base width well defined and shorter
• Good Area/Periphery ratio
• Difficult to layout
• Square
• Base width longer due to longer diagonal path
• Poor Area/Periphery ratio
• Easier Layout

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Example hot-dog transistor, arrayed-emitter
transistor

• current is proportional to the of emitters
• hexagonal packing for small area

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ALTERNATIVE SMALL-SIGNAL BIPOLAR TRANSISTORS

• (1) Extensions to Standard Bipolar
• Super-b NPN gt low input current diff pair
  for opamp
• deep-P PNP

• Deep Emitter diff gt small WB lt0.1mm
• Beta gt 5000 !
• Punch-thru at 1-3 Volts VA 1-3Volts

Super-b NPN
Standard Bipolar
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• deep-P PNP

Standard Bipolar lateral PNP
Deep-P lateral PNP

• Improved emitter injection
• deep diff gt larger fraction of minority
  injection from the
  sidewalls ! gt 2-3times higher current density
• high current Beta rolloff at
  200-500mA wrt 100-200mA in base laterals.

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(2) Analog BiCMOS
- Standard Bipolar N-type (111) epi - BiCMOS
P-type (100) epi gt Nwell, graded diffusion
higher resistivity for N-collector gt if no
deep-N sinker, then soft transition
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(2) Analog BiCMOS
- CDI NPN ? - (collector diffused isolation)
(standard bipolar NPN)
gt Vop is limited to 15-20V due to shallow NWell
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(2) Analog BiCMOS
- Extended-Base NPN for higher VCEO 40-60V
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(4) Advanced Bipolars
- Special, reduced EBJ area NPN Washed-emitter
NPN
Washed-Emitter NPN
Conventional diffused NPN
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(4) Advanced Bipolars
- Polysilicon Emitter gt much thinner, precisely
controlled emitter diffusions than
Washed-emitter
gt Betas upto six times greater