Transcript The Christmas

Robert Colville Team
Team Members
 Noah Boydston
 Kyle Brown
 Robert Colville
 Linsy Cook
 GeeHyun Park
 Wissam Khazem
Applications of Bipolar
Transistors

Bipolar Transistors verses MOS
 Advantages
 High Transconductance



Directly Proportional to Emitter current
Independent of emitter area
Improves base-emitter voltage matching
Superior device matching
Resulting bipolar applications

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High output applications – amplifiers, output drivers
Precision output applications – voltage regulators,
voltage and current references
Bipolar Transistors

Disadvantages



Saturation is a phenomenon unique to bipolar
transistors
Can destruct under heavy loads if improperly
constructed
Vulnerable to thermal gradients if carelessly matched
Power Bipolar Transistors


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Must increase emitter area to accommodate large
currents -- exceeding device’s critical current
density results in beta degradation
Power transistors operate at lower betas to
conserve space. Typical beta minimum is 10.
NPN transistors can handle higher currents so
they are used more often than PNP transistors
Transistors handling currents over 100mA require
special layouts to avoid thermal runaway and
secondary breakdown
Power Bipolar Transistors


Integrated bipolar transistors have practical
maximum current/power limit of 2 A and 10
W
Beyond this range, discrete components are
more practical than integrated components
Failure Mechanisms


Three main problems
 Emitter debiasing
 Thermal runaway
 Secondary breakdown
Caused by:
 Large currents
 High power dissipation (heating)
Emitter Debiasing
Q1
Q2
50 mA
+ 0.6mV R1


Q3
50 mA
+ 1.2mV R2
Q4
50 mA
50 mA
+ 1.8mV R3
A non-uniform current distribution due to voltage
drops in the extrinsic base, emitter, and their leads
Overloaded sections of transistor are more
susceptible to thermal runaway and secondary
breakdown
VBE / Vt
e
Emitter Ballasting


Resistors inserted into
each emitter lead
Sized to provide voltage
drop of 50 to 75mV
Forces the emitter current
to distribute more evenly
Disadvantage: Voltage drop across resistors adds
to saturation voltage, lowers transconductance,
and increases power dissipation
Q1
Q2
RB1
R1


Q3
RB2
R2
Q4
RB3
R3
RB4
Intrafinger debiasing


Debiasing in a single, long emitter finger
Voltage drops as current flows along resistor
VBE


LRs I E

2W
Voltage drop along length (L) should not exceed 5
millivolts
Rs – sheet resistance, Ie – emitter finger current
Intrafinger debiasing

Best Methods for reducing intrafinger debiasing
 Fingers may be shortened and widened


Use a number of shorter fingers the same width as the
original
Also possible to use a ballasting technique to provide some
compensation
Thermal Runaway



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Localized current flow as transistor heats unevenly
VBE drops by 2mV / °C
Conduction region can quickly collapse to form
small hot spot
 Can cause catastrophic failure
 Overstress leads to other failures
Prevention
 Ballasting resistors to lessen debiasing
 Distributed emitter ballasting
Secondary Breakdown



Caused by an emitter current density greater than
a critical threshold Jcrit
Past this point the VCEO drops to a new value the
secondary breakdown voltage VCEO2
When avalanche starts
 Base drive circuit can not turn of the transistor
 Transistor overheats and fails
Secondary Breakdown


Transistor with
inductive loads are
extremely vulnerable
Secondary breakdown
occurs when the
collector voltage
passes VCEO2 and the
emitter current density
passes Jcrit
VCC
L1
D1
Q1
Layout of Power NPN
Transistors


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Several types of layout designs
Transistors used in linear-mode applications
Heat dissipation
Transistors used in switch-mode applications
Emitter focusing
Interdigitated-Emitter
Transistor


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Oldest style, but still in use
due to speed capabilities
Multiple emitter fingers
each with ballasting resistor
With a minimum sheet
resistance of 5 ohms/sq
Ballasting resistance of
2.5 ohms/sq per finger
Interdigitated-Emitter
Transistor

Extremely vulnerable to intrafinger debiasing

VBE  5mV
LRSIE
VBE 
2W


RS = sheet resistance of metal
IE = total current flowing out of finger
Interdigitated Emitter
Transistor

LRSIE
2W
Design Considerations
 Large number of shortened fingers
 Increased width
Slower switching, emitter focusing
 Fastest designs use minimum-width fingers
Difficult to place enough metal on narrow
fingers to prevent them from debiasing
VBE 
Interdigitated Emitter
Transistor

Design Compromises
 Emitter width of 8 to 25 um
 Contacts as large as possible
Reduce emitter resistance
Interdigitated Emitter
Transistor


Placement of base contacts
between fingers
 Reduces base resistance
 Faster switching
Placement of base contacts at ends
of array
 Ensures end fingers turn off at
same speed
 If omitted, could lead to
 Emitter current focusing
 Secondary breakdown
Interdigitated Emitter
Transistor

Width of base contacts
 Minimum width conserves space
 Beta roll-off due to high currents may call for
wider base metalization
 Designs with base–lead debiasing >2-4 mV
should be redesigned to reduce metallization
resistance
Interdigitated Emitter
Transistor


Comb style << metallization
resistance
However, may single level
metal designs don’t work
well with this style
Interdigitated Emitter
Transistor


Deep N+ only along one
side
Design will suffice for
linear-mode operation
but not for switchingmode
Interdigitated Emitter
Transistor
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For switching-mode at high currents
RC= RVN+ + RLNBL
To reduce RVN+ increase deep N+ area
To reduce RLNBL contact NBL along a longer
periphery or decrease distance between active
regions and sinkers
Other options include: placing sinkers along both
sides and unbroken ring of N+
Interdigitated Emitter
Transistor

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Can operate at higher speeds due to low base
resistance
Narrow emitter fingers
Reduce base resistance
Control emitter crowding
However, narrow emitters prone to intrafinger
debiasing
Use of ballasting resistors can help normalize
current flow in each finger, but cannot prevent
intrafinger debiasing
The Wide-Emitter NarrowContact Transistor
Focus is on use of narrow contacts that act as
individually ballasted sections

The Wide-Emitter NarrowContact Transistor

Idea Behind Design
 Each finger is divided into individually ballasted
sections
 No one portion of the emitter can conduct more
current than any other
 Not feasible to segment fingers
Narrow emitter contact on a wide finger
provides similar benefit
The Wide-Emitter NarrowContact Transistor


Wide-emitter finger with narrow contact =
Distributed network of ballasting resistors
Network is made up of

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Emitter resistance
Pinched base resistance
Emitter resistence
Largest at periphery
Smallest at center

Base resistance
Smallest at periphery
Largest at center
The Wide-Emitter NarrowContact Transistor

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The network of emitter and base resistors complement
each other

At low currents RB is small

Uniform
current flow
As currents increase
Debiasing causes the currents to move out to the
periphery
RE has increased due to increased currents
Emitter voltage drops counter act conduction
gradients due to debiasing
Therefore, RE and RB ensure uniform current flow
The Wide-Emitter NarrowContact Transistor
Specifics
 Sufficient emitter overlap of contact
 Typical emitter overlaps of 12 to 25 um
 Larger overlaps
slow frequency response
 Smaller overlaps
don’t provide enough
ballasting to prevent failure mechanisms
The Wide-Emitter NarrowContact Transistor
Other aspects of design
 Multiple base regions with
deep n+ fingers between
them
 Design minimizes Rc 
Increases area and
complicates lead routing
 Single level metal
serpentined base leads
causes significant
debiasing
The Wide-Emitter NarrowContact Transistor
Overview
 Very robust
 Distributed ballasting helps prevent failure
mechanisms allowing for increased current
densities
 Under harsh conditions ballasting resistors
inserted into leads will help
 Design does not switch as fast previous design
The Christmas-Tree Device



Nicknamed the
Christmas-tree device
Used in linear
applications
exceptional
resistance to thermal
runaway
Rarely used for switching
applications
Prone to
emitter current focusing
during turn off
The Christmas-Tree Device

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Emitter consists of a
central spine surrounded
by branches of triangular
prongs
Majority of conduction
occurs in triangular
prongs
Prongs connect to the
central spine via narrow
strips that act as
ballasting resistors
The Christmas-Tree Device

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Current increases
emitter crowding
conduction moves to periphery
current flows through
ballasting resistor
This distributed ballasting increases resistance to thermal
runaway
Due to wide emitter layout is vulnerable to emitter
currently focusing
At turn off
condition moves from periphery toward
spine
concentration of increased current leads to
secondary breakdown
The Christmas-Tree Device
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This device is best in applications that dissipate
large amounts of power without abrupt turn off
transitions
Wide-emitter narrow-contact structure exhibits
superior immunity to secondary breakdown
because of decreased width
Device chosen for:
 series-pass devices of linear voltage regulators
 Output stages of audio power amplifiers
Cruciform-Emitter Transistor
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Device is an improvement of the
wide-emitter narrow-contact
structure
Focus was to incorporate
additional emitter ballasting
while avoiding secondary
breakdown
Emitter consists of cross-shaped
sections aligned side by side
forming a continuous emitter
finger
Base contacts occupy notches
between crosses
Cruciform-Emitter Transistor
Improvements / Changes

Width of cruciform emitter increased
to (75 to 124 um) to obtain additional
ballasting

Narrow emitter contact replaced by a
number of small square contacts in
the center of each cross structure

Contact design produces 3dimensional ballasting effect that is
more efficient than the 2-dimensional
ballasting generated by the wideemitter narrow-contact layout
Cruciform-Emitter Transistor
Overview
 Cruciform design combines best features of
Christmas-tree and WENC
 Contact to emitter ratio of WENC design
 Resistance to thermal runaway due to increased
ballasting exploited by Christmas-tree design
 Out performs the Christmas-tree device with
respect to secondary breakdown
 Extremely efficient use of space
Cruciform-Emitter Transistor
Overview
 Two drawbacks associated with this design
 First, small emitter contacts
high localized
current densities in metallization
vulnerable to electromigration
 This effect can reduced by replacing small
contacts with array of minimum contacts to
increase sidewall perimeter
Cruciform-Emitter Transistor

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Second, compact design can cause extreme
localized heating at high power levels
Less area – efficient transistor are preferred
to compact designs
This structure is therefore best suited for
switching applications because these
applications are constrained more by current
handling capabilities than by power
dissipation
Power Transistor Layouts in Analog
BiCMOS

Double-level
metallization allows the
base contacts to
completely encircle each
emitter finger, whereas
in a single-level-metal
design can only reach
two or three sides of
each finger.
Metallization System


Emitter current flows
from the narrow
emitter contacts to
via and up to the
metal-2 layer.
The resistance in the
metal-1 plates
actually serves as
emitter ballasting.
Metal-1 and Metal-2 layouts
Wide-emitter Narrow-contact
Transistor

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This structure has been used to fabricate pulsepower transistors capable of operation at the
emitter current densities of more than
100mA/mil2.
The use of a solid metal-2 plate to terminate the
emitters helps minimize debiasing.
Individual emitter ballasting resistors are
unnecessary except for the most demanding
applications.
The inherent emitter ballasting distributed within
the emitter fingers prevents hot spots.
Comparison of Power
Transistor layouts

All the power transistor layouts presented in this
section have their advantages and disadvantages:
Intedigitated
Emitter
Wide-emmitter
Narrow-Contact
Christmastree
Devise
Cruciform
Transistor
Ease of Emitter
Sensing
Excellent
Fair
Poor
Poor
Thermal
Runaway
Good
Good
Excellent
Excellent
Secondary
Breakdown
Fair
Excellent
Poor
Good
Frequency
Response
Excellent
Good
Fair
Fair
Compactness
of Layout
Poor
Good
Good
Excellent
Saturation Detection and
Limiting

Substrate injection
 Effects:
 Waste supply current
 Substrate debiasing
 Device latchup
 Preventing measures:
 Inspecting minority carriers
 Preventing the transistor from saturation
Substrate Injection

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Emitter of lateral NPN injects minority
carriers into tank
Small signal transformers inject minimal
amounts of current
Large lateral PNP produces currents of
10-100 mA
Debasing occurs triggering latchup
Prevent Minority Carriers

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Unbroken ring of deep-N+
around outside edge of tank
Ring merges with NBL and
completely encloses the base
Minority carriers recombine
before reaching isolation
Compatible with BiCMOS and
less effective in CDI processes
Prevent Substance Injecting

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Incorporate a ring of base
diffusion called the
secondary collector
Unsaturated primary
collector allows little
current flow to secondary
collector
Secondary collector gathers
excess carriers
Ring collector
The Secondary Collector 1/2


Secondary collector connected to ground
 Returns carriers to ground line
 At saturation emitter current flows to ground
 Functions as an unprotected lateral transistor
without substrate debiasing
Secondary collector connected to base lead
 At saturation apparent beta rapidly declines
 Functions as a deep-N+ guardring
 Increase efficiency with guardring combination
The Secondary Collector 2/2


Analog BiCMOS processes
 Base diffusion instead of P-type Source Drain
 Less effective than standard bipolar processes
Functions as a Saturation Detector
 Current flowing in secondary collector
 No need to encircle entire transistor
 Dynamic antisaturation circuit
 Deep-N+ ring suppress transient substrate
injection
Saturating NPN Transistor


Inject current into the substance
Protection against minority carriers injection




Greater than few milliamps of base drive
Deep-N+ ring surrounding periphery of the collector
 Contains minority carriers inside the tank
 Reduces collector resistance
Base diffusion used in saturation detection
Power switching transistors incorporate small base
diffusion
NPN Transistor

An NPN switching
transistor
incorporating both a
complete deep –N+
ring and an outer
collector that
functions as a
saturation detector.
Diffusion and Metallization
Proposed Symbols

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A) Power NPN
B) Power NPN with a deep –N+ in collector
C) Lateral PNP with deep –N+ in base
D) Lateral PNP with a secondary collector
E) NPN transistor with saturation detection
References


The Art of Analog Layout, by Roy Alan
Hastings, Prentice Hall, Upper Saddle
River, NJ, ©2000.
Analysis and Design of Analog Integrated
Circuits, 4th ed., by Gray, Hurst, Lewis and
Meyer, Wiley, New York, ©2001.