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 high-level injection.

Low current beta rolloff results from recombination, and shallow emitter effect.

β Rolloff (con’t)1. The lateral PNP beta curve is different from that of an NPN

for several reasons

2. Due to its light doping,PNP exhibits lower emitter injection efficiency that reduces its beta peak.

3. The flow of carriers near the surface of the PNP increases surface recombination, causing low-current beta rolloff.

4. 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 occurs with lightly doped PN junctions. (Multiplication effect).

Zener occurs in highly doped junctions (quantum tunneling effect).

Avalanche/Zener Breakdown(con’t)

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 underreverse 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

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)

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

increasesVBE

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 self-destructs 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 device-related 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.