Section 8.1
Heithem Souissi
Dina Miqdadi
Mohammad Butt
Ahmad Elmardini
Wyatt Sullivan
Fares Alnajjar
Beta rolloff &
Avalanche breakdown
HEITHEM SOUISSI
DINA MIQDADI
Beta
Rolloff
β Rolloff

Beta= Current gain for a BJT

Beta is commonly referred to as a constant,
although it varies considerably depending
on the collector current.
β Rolloff (con’t)

High current beta rolloff is caused by highlevel injection.

Low current beta rolloff results from
recombination, and shallow emitter effect.
β Rolloff (con’t)

The lateral PNP beta curve is different from that of an
NPN for several reasons
1.
Due to its light doping,PNP exhibits lower emitter
injection efficiency that reduces its beta peak.
2.
The flow of carriers near the surface of the PNP
increases surface recombination, causing low-current
beta rolloff.
3.
The lightly doped base of the PNP causes high level
injections to begin at relatively low current levels,
therefore high current beta rolloff.
Avalanche
Breakdown
What is Breakdown?

Deleterious effect that occurs in the presence of high
electric field.

Causes high resistance elements to allow flow of high
current.

Typically an irreversible effect permanently damaging the
element.
Avalanche/Zener Breakdown

'Zener diode' and 'avalanche diode' are terms often used
interchangeably.

Both refer to breakdown of a diode under reverse bias.
Avalanche/Zener Breakdown
(con’t)

Avalanche occurs with lightly doped PN
junctions. (Multiplication effect).

Zener occurs in highly doped junctions
(quantum tunneling effect).
Avalanche/Zener Breakdown
(con’t)

Reverse bias = Very little current flow
= Open circuit

As Reverse voltage a point is reached where current
dramatically, therefore dynamic resistance .
Avalanche Breakdown
Avalanche breakdown causes high flow of current under
reverse bias condition!
The question is: How does this happen?
Avalanche Breakdown (con’t)
Reverse bias
Thick depletion region causes high
electric field and tremendous acceleration
Very few electrons make it through
depletion region with high velocity
These electrons collide with atoms in the
depletion region and free more electrons
( Process called Multiplication).
Results in higher and higher current flow
Avalanche Breakdown (con’t)

An empirical relationship used to
describe avalanche breakdown:
M=Multiplication factor= 1/(1-|V/Vbr|^n)
Avalanche Breakdown (con’t)
By analogy, the process is named
because a single carrier can spawn
literally thousands of additional carriers
through collisions, just as a single
snowball can cause an avalanche.
Avalanche Breakdown (con’t)
Thermal Runaway, Secondary
Breakdown and Saturation in NPN
Transistors
Mohammad Butt
Ahmad Elmardini
Topics in this section (8.1.3-4)







Thermal runaway
Hot spots
Secondary breakdown
Forward-bias safe operating area(FBSOA)
Saturation in NPN transistors
Current hogging
Prevention schemes for current hogging

Thermal Runaway: Increase in temperature
leads to higher current and power dissipation,
which in turn increases the temperature
further until the device is destroyed.

Hot Spot: The region of the transistor that
conducts current and steadily shrinks as it
grows hotter is called a hot spot.
Temp.
increases
β
increases
ICB0
increases
VBE
decreases

Unstable Hot Spot: If the temperature
in the hot spot reaches 350o C to 450o
C, the transistor shorts out.

Stable Hot Spot: If the increased
current density causes beta to roll off far
enough at a high but not destructive
temperature, it stabilizes the hot spot.

A transistor with stable hot spot selfdestructs during turn-off due to
avalanche of the collector-base junction
at a voltage below the VCEO

Secondary Breakdown: High values of
VCE and IC cause burnout of a transistor
junction area. It is not necessary for the
average junction temp. to exceed the
maximum rating.

Secondary breakdown can also occur in
transistors which have not experienced
thermal runaway.

Thermal runaway and secondary
breakdown can be avoided by
restricting the operating conditions of
the transistor.
Forward-bias Safe Operating
Area(FBSOA)
Which transistor loses less FBSOA due to
second breakdown?

Transistor 1: A very robust transistor
without a heat sink.

Transistor 2: A properly heat sunk but
poorly designed transistor.
•Answer: Transistor 1
Saturation in NPN Transistors

Occurs when VBE > 0 and VBC >0

Useful in power transistors to reduce VCE(sat)
and to minimize power dissipation

Unintentional saturation is the biggest devicerelated design flaw

Affects discrete and integrated transistors in
different ways
Integrated Bipolar Transistors
Current Hogging

Caused by saturation due to flow of base
current through base-collector junction rather
than base emitter junction

Reduces the base-emitter voltage

Base current of saturating transistor
increases at the expense of other transistors
Current Mirror Transistor
Prevention of Current Hogging

Base-side Ballasting: Insertion of matched
resistors into base leads of each transistor.
Resistors must ratio inversely to the emitter
areas of their respective transistors.

Schottky clamps: Clamping diode is
connected across base-collector junction to
prevent saturation.
Base-side Ballasting
Schottky-clamped Transistor
Lateral PNP
Transistors
Wyatt Sullivan
Fares Alnajjar
Lateral Parasitic Transistance

Emitter -> P-substrate
• Major cause of collector efficiency
• NBL required to compensate

Collector -> BOI
• Creates saturation currents
• Determines Saturation voltages
NBL Explained

N-type Buried Layer (NBL)

Acts as a minority carrier repellant
• Holes are “repelled” back into the N-epi
• Drift currents cause a slight potential
Sidewall Transistance

High collector voltages create PNP
• Collector -> base -> sidewall (P-Type)
• Narrow base in high voltages
• Emitter continues to inject current
• Excess collector current flows to substrate
PNP Lateral Transistor
Models for Parasitics of BJTs

Major Parasitics
• Sidewall saturation
• Avalanche breakdown
• Leakage
• Capacitance between junctions
Models for Transistors
Transistor Models Explained

Diodes characterize
• Capacitance
• Avalanche breakdown

Resistors show internal resistances
• Rb is normally quite large
• Re is negligible
Design Considerations

Low Rb
– Higher frequency operation

Low Rc
– Higher saturation voltages
– Power applications for high current
Bibliography
“The Art of analog layout” by Alan
Hastings.
 “Principles of semiconductor devices”
by Bart Van.
