8 Bipolar Transistors

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8 Bipolar Transistors
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• Most LOGIC circuits in CMOS
• Bipolar important for most Analog circuits
• Voltage current amplifiers
• Voltage current references
• Oscillators
• Timers
• Amplitude limiters
• Nonlinear signal processors
• Power switches
• Transient protectors

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Here, we shall cover topics that are important
considerations for Layout Design, but not
usually covered in elementary texts.
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8.1 Operation
NPN Transistor Model
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8.1 Operation
NPN Transistor Model
EB junction forward-biased np diode CB junction
reverse-biased np, current-controlled current
source
Ic Is exp(VBE/VT) VBE VT ln(Ic/Is)
Ic bF IB
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VBE VT ln(Ic/Is)
Is Emitter saturation current VT thermal
voltage 26 mV at 300K dVBE/dT -2mV / C ? 1
deg in DT can induce 8 mismatch in VBE !
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(1)Beta Rolloff High Ic ? b drops because
injected minority majority in Base Low Ic ? b
drops because leakage injected minority current
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BETA ROLLOFF

• Typically, NPN
• b drops at Ic gt 5mA/mil2 8 mA/mm2
• Due to high-level injection
• b drops at Ic lt 10nA/mil2 15pA/mm2
• Due to recombination in depletion region,
  recombination at oxide interface, shallow
  emitter effect,

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BETA ROLLOFF

• Lateral-PNP b of lateral PNP is much smaller
  than NPN. Why ?
• High-IC rolloff occurs earlier because high-level
  injection condition occurs earlier due to small
  base N
• Low-IC rolloff occurs earlier because current
  flows near the surface (thus higher
  recombination at ox-Si interface)
• Lateral PNP usually the High-IC rolloff and
  Low-IC rolloff meet already, and the peak beta
  is already in the high-level injection
  condition ! Tough to design.

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• (2)Avalanche Breakdown
• How high a voltage can a BJT operate ?
• VEBO EBJ breakdown with C open
• VCBO CBJ breakdown with E open
• VCEO Breakdown with B open

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• NPN VEBO 7V hot carriers generated during
  avalanche produces recomb centers at the
  oxide interface ? degrades NPN rapidly
• The recomb centers degrade low-current beta

• NPN VCBO 20V 120 V because B C are lightly
  doped.
• As CBJ is at subsurface, nor oxide interface
  problem here. Thus, not affects beta.

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• Lateral-PNP
• Both VEBO and VCBO are due to breakdown of
  Base-Epi junction. ? so, no effect on beta.

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• NPN VCEO about 0.6 VCBO
• Called beta multiplication because impact
  ionization begins to occur at V well below
  breakdown voltage

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• A 36V-rated BJT
• VCES Base shorted to Emitter, beta
  multiplication.
• When Ic begins to flow, then VCES snaps back to
  VCEO (snapback).
• For example, as soon as VCES gt 60V, it
  immediately drops (snaps back) to 43V. ? a
  sustained VCEO or VCEO(sus) is 38V. So the
  transistor is rated 36V, giving a little bit
  of safety margin.

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• Current limit above it, electromigration
  failure
• Voltage limit avalanche breakdown
• Power limit excessive temperature within the
  package
• Secondary breakdown (1) due to hot spot ( which
  leads to thermal runaway) high current density
  feeds back each other ? breakdown substantially
  below VCEO (2) emitter-current focusing (ECF)
  phenomenon causes extremely high current density
  in small portion of EBJ. ECF during Turn-Off
  Base wire extracts charge from neutralBase
  adjacent to Emitter periphery ? then proceeds
  inward to center of transistor and conducting
  portion collapsesto an ever-shrinking area.

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(4) Saturation in NPN

• Effect of Saturation in I.C. BJT vs. Discrete
  BJT
• Discrete BJT only longer Turnoff time
• I.C. BJT longer Turnoff time host of other
  problems

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• IF NPN in I.C. is Saturated gt What happens ??
• What are the host of other problems ???

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• When BCJ goes Forward (in saturation), Carriers
  are drawn into the Substrate via the
  parasitic PNP.

• Current in substrate ? substrate debiasing

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Another Problem of BJT Saturation ? Current
hogging
Definition
When one of the matched Transistors in a Mirror
circuit enters into Saturation, its Base current
increases (hogs) at the expense of the Base
currents of other transistors.
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• Saturation ? Current hogging

• Ex) Q3 saturates gt
• Q3 takes currents (IB) away from Q1 and Q2 to
  feed the IE of Qp.
• VBE of Q2, Q1 decreases, so does VBE of Q3.
• Thus, IC decreases for Q1 and Q2. gt
• Circuit malfunction.

Parasitic PNP
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• Ex) Q3 saturates gt From the Device structure
• Q3 takes currents (IB) away from Q1 and Q2 to
  feed the IE of Qp.
• VBE of Q2, Q1 decreases, so does VBE of Q3.
• Thus, IC decreases for Q1 and Q2. gt
• Circuit malfunction.

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Two ways to fight Current Hogging (or Saturation)

• Schottky Clamp
• Base-side Ballasting

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Schottky Clamp
gt Applet
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• Base-side Ballasting

Use Base resistors to reduce IB via negative
feedback.

• Q3 enters Saturation ? I3B increases ? VBE3 is
  decreased by the increased voltage drop I3BR3 ?
  Q3 leaves saturation.

• R must be matched to Q size If Q2 has 3 times
  the emitter area of Q1, then R2 R1 / 3

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8.1.5 Saturation of Lateral PNP
P
P
P
P
P
N-epi
Buried N
P-sub

• Parasitic BJT
• QP1 hole flow into substrate
• QP2 hole flow into sidewall (isolation) Tanks

IF QL saturates, THEN QP2 turns ON !
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8.1.5 Saturation of Lateral PNP
PNP
P
P
P
P
P
N-epi
Buried N
P-sub
IC for QL
Collector Efficiency IC / IE - IB
0.1 lt--gt 1.0
IE - IB feeds the collectors of QL QP1 QP2
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(6) Parasitics of Integrated-Circuit BJTs

• The PN junctions such as
• substrate PN junction
• Sidewall (isolation) PN Junction
• Do not forget these !

I.C. Designers (BiCMOS or standard Bipolar) use
four-terminal BJT symbols circuit Schematics to
remind oneself of these junctions !
NPN
PNP
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Parasitics in more detail
Vertical NPN
Complete circuit
Device Cross-section
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Parasitics in more detail
Lateral PNP
Device cross-section
Complete Circuit
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8.2 Small-signal BJT

• power BJT vs.
• small signal BJT Ic lt 10mA, power lt 100mW

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(1) Standard-bipolar NPN
N Emitter P-base Base N-well Collector N NBL
deep-N sinker.
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(1) Standard-bipolar NPN
1020 cm-3
N
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(1) Standard-bipolar NPN

• light doping for - large VCBO - good Emit.
  Inj. Effic.

P
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(1) Standard-bipolar NPN

• Too light doping, problems in - low VT gt surf.
  channel - high-Ic Beta Rolloff

P
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(1) Standard-bipolar NPN

• Compromised for Rs 100-200 Ohm/sq.

P
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(1) Standard-bipolar NPN
N

• N small, for - large VCEO - large VA
• Drift region

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(1) Standard-bipolar NPN
N
N

• NBL deep N sinker for - low Res. Path for
  Ic - Rc 1kW w/o deep N sinker ! - Rc 100W
  w. deep N sinker.

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How to Construct Small-signal NPN
C - B - E layout
C - E - B layout
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How to Construct Small-signal NPN
Emitter Junction

• Is proportional to Junction Area
• Area / Periphery Ratio gt Vertical Injection /
  Lateral Injection

Lateral larger R larger surf. Recomb.
Example
Standard Bipolar Op Amp 1.0 mil x 1.0 mil gt
peak b 290 1.5 mil x 3.5 mil gt peak b 520 !
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How to Construct Small-signal NPN
Emitter Junction

• Is proportional to Junction Area
• Area / Periphery Ratio gt Vertical Injection /
  Lateral Injection

Lateral larger R larger surf. Recomb.
Example
Standard Bipolar Op Amp 1.0 mil x 1.0 mil gt
peak b 290 1.5 mil x 3.5 mil gt peak b 520 !
A/P 1 / 4 0.25
A/P 5.25/10 0.525