9 Applications of BJT

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9 Applications of BJT
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9.1 Power BJT

• Because of high-Ic rolloff (gt1.5uA/um2), emitter
  area increases. In order to conserve space,
  work at lower beta than small-signal BJTs. b
  min. acceptable 10 (power NPN can handle 8-15
  uA/um2).
• Lateral PNP can not achieve more than 250uA/
  min.emitter
• Small-signal BJT can handle 10 mA, 100 mW
• Power BJTs for gt100mA, gt500mW need special
  layout, up to as high as 10A 100W
• Most IC power NPN Ilt2A Plt10W
• Power PNP Ilt500mA.

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Failure of Power NPN

• Emitter debiasing
• thermal runaway
• secondary breakdown

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(A) Emitter debiasing some Tr may not even
conduct
Ex) suppose Ie 50mA, R 1.2mW each ? total
Volt drop 50mA 1.2mW 100mA 1.2mW 150mA
1.2mW 3.6mV ? DIe/Ie e3.6mV/26mV -1 1.15 -1
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(A) Emitter debiasing some Tr may not even
conduct
Ex) suppose Ie 50mA, R 1.2mW each ? total
Volt drop 50mA1.2mW 100mA1.2mW
150mA1.2mW 3.6mV ? DIe/Ie e3.6mV/26mV -1
1.15 -1 15
15 morecurrent than Q1 !
Ve1
Ve1-3.6mV
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• (B) Use Emitter Ballasting R
• to reduce impact of E debiasing
• Insert R at Emitter node
• typically 50-75mV drop at max full rated current
• Ex) 50mA Em. ? 1W R

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• (B) Use Emitter Ballasting R
• to reduce impact of E debiasing
• Insert R at Emitter node
• typically 50-75mV drop at full rated current
• Ex) 50mA Em. ? 1W R
• Ex) (1) 3.6mV debiasing ? (2) current change in
  Ballasting Rs ? (3) change in Emitter terminal
  voltage

DVe -1.8mV
50mA1.8mA
1W
50mA-1.8mA
DVe 1.8mV
D -1.8mV
D 1.8mV
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Voltage drop should not exceed 5mV
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Emitter contact
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? Because exp(5mV/25mV)-1 1.22-1 22
difference between IE1/IE2
DVBE L Rs IE / 2W
Ex) 50mA along the length of 300um (L), 30um (W),
and 12mW/sq (Al, Rs) ? 3001250/230 3mV
acceptable
Emitter Current Flow Direction
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Thermal Runaway
dVBE/dT - 2 mV/C --- initially starts
with E debiasing
Hot spots conduct more current !
Secondary Breakdown If JE gt Jcrit VCE gt VCEO2
? leads to overheating metallization failure
Example BJT driving inductive load ? vulnerable
to secondary breakdown
? Keep IE lt 8-15uA/um2
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Layout of Power NPN

• Linear-mode NPN
• operates in Active mode for long time
• large VCE, large Ic ? large Power dissipation ?
  must have a large Area to dissipate heat
• General rule limit Power lt 150 uW/um2, IE lt 10
  uA/um2

• Switched-mode NPN
• lower VCE VCE,sat (about 0.2V) operates
  between Cutoff lt-gt Saturation
• Power diss. during brief Switch interval only
  Average power diss. is small
• can withstand higher Power dissipation but IE
  focusing a problem during turnoff
• Limit to IE lt 20 uA/um2.

• Pulsed-mode NPN
• NPN operates in pulsed mode when driving
  Capacitive-loads
• conducts large Ic only for a few 100ns rests
  for a few ms.
• unharmed by IE focusing because external C will
  quench conduction,
• immune to thermal runaway, high-Ic beta rolloff
  and high Rc limits Ic
• pulse duration not to exceed 1 ms, intervals
  greater than 250 ns.
• IE,ave not to exceed 20 uA/um2.

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Layout of Power NPN
Linear-mode PE lt 150 uW/um2, IE lt 8 uA/um2
Switched-mode less average Power during
operation. IE lt 16uA/um2 Pulsed-mode Large
current during a few 100 nS, immune to thermal
runaway ex) Capacitive load like MOS gate
Interdigitated-Emitter NPN oldest style,
higher speed
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• Two emitter Ballasting Rs
• extremely vulnerable to inter-finger debiasing
  must be DVinterfinger lt 5mV
• Large of short fingers better than small of
  long fingers
• compromised finger widths 8-25 um
• Emitter contact as large as possible to reduce RE

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- Previous geometry Base metallization
comb-style
- Below is Serpentine-style Base metallization
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Wide-Emitter Narrow-Contact NPN

• Narrow Emitter Fingers
• Good for high freq. due to low RB
• good for controlling Emitter crowding
• bad for intrafinger debiasing (concentrates
  conduction at one end of finger)
• ? individually ballasted sections (no one finger
  can conduct too much)
• ? use wide emitter, narrow contact for similar
  effect (distributed network of
  ballasting resistors)

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Christmas-tree Power NPN

• Widely used in Linear-mode app due to resistance
  to thermal runaway
• Rarely used for switching app
• Emitter central spine triangular prongs
• most conduction in triangular prongs
• Emitter ballasting naturally built in
• At low Ic all parts high Ic only triangular
  prongs
• best for app that dissipates high power

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Cruciform-emitter NPN

• Emitter width 75-125um for added ballasting
• Em.contact locations ? distributed 3D ballasting
• wide E/narrow contact Xmas tree
• efficient use of space better than xmas tree
• two drawbacks
• small Em.contacts ? electromigration
• compact size can cause extreme local heating
• Best suited for Switching app.

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Power Transistor in Analog BiCMOS

• can operate at IE gt 150 uA/um2
• overlap of E over contact 8-12 um
• DeepN 8 um wide

• All forms of Power NPN above plus

Wide-emitter, narrow-contact in BiCMOS

• Currents (IE, IC, IB) flow out thru Metal-2 v
  thru Viafrom Metal-1

• minimizes Rc
• blocks substrate injection during sat

To Metal-2
Diffusion masks
Metal-1 masks
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Saturation (CBJ) detecting limiting
Two examples of PNP to limit saturation
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Switching NPN Transistor -- example

• minimizes Rc
• blocks substrate injection during sat