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Thursday, August 1, 2013

PPT On TRANSISTOR AGING

Presentation On TRANSISTOR AGING
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TRANSISTOR AGING Presentation Transcript:
1.TRANSISTOR AGING

2.INTRODUCTION
Transistors do age ….like         Humans!!
Time by time degradation of performance of a transistor is called Transistor Aging.
The degradation of  transistors occurs over time  and  can  result in  circuit failure!!!
Several different phenomena can degrade the transistors.

3.WHAT WE ALREADY KNOW
A transistor is a three terminal semiconductor device used to  amplify and switch electronic signals.
It is the fundamental building block of modern electronic devices.
ADVANTAGES:
 Smaller, less expensive, requires
 less electricity  etc.
APPLICATIONS:
 Hearing aids,Watches,Cameras, TV etc.

4.AGING MECHANISMS
OXIDE BREAKDOWN.
HOT CARRIER INJECTION.
NEGATIVE BIAS TEMPERATURE INSTABILITY(NBTI).

5.OXIDE BREAKDOWN:
Refers to the destruction of an oxide layer.
       Defects within the gate oxide are usually
       called traps.
They are called traps because the degraded  Oxide can trap charges. Traps are usually neutral. Once these traps form a conduction path from the gate to the channel, breakdown occurs.

6.NEGATIVE BIAS TEMPERATURE INSTABILITY(NBTI):
Occur due to the application of negative bias to gate at elevated temperature.
Traps charge within the dielectric.
Whenever you apply gate voltage, charge buildup in the dielectric.

7.HOT CARRIER INJECTION:
A carrier is injected from the conducting   channel in the silicon substrate to the SiO2 layer.
 Since the charge carriers becomes trapped in the SiO2 layer of a MOS transistor, the voltage needed to turn on the transistor increases .
As a result transistor switches more and more slowly.

8.OXIDE BREAKDOWN
Gate-oxide breakdown begins when traps form in the gate-oxide.
more and more traps are created in the gate-oxide.
traps start to overlap creating a conduction path-breakdown occurs-SOFT BREAKDOWN(SBD).
Once there is conduction, new traps are created by thermal damage, which in turn allows for increased conductance.

9.This cycle of conduction leading to increased heat leads to thermal runaway.
The Silicon within the breakdown spot starts to melt, and Oxygen is released
Silicon filament is formed in the breakdown spot.

10.CROSS SECTION OF GATE OXIDE AFTER HARD BREAKDOWN THROUGH EMISSION- MICROSCOPY.

11.TRAP GENERATION
There are different theories on how defects are generated in the gate-oxide.
The two main models are :
The Thermo chemical model .
The Anode Hole Injection (AHI)model.

12.AHI MODEL
The AHI model is based on the process of electron injection into the oxide and holes are generated at the anode which then gets trapped into the oxide.
At high electric fields, the electrons arriving at the gate have a high kinetic energy
 (> 8MV/cm).

13.When these hot electrons reach the gate electrode they transfer their entire energy to a deep-valence band electron, and
this electron is promoted to the lowest available electron energy state, which is the conduction band edge .
Once the electron reaches the conduction band, it creates a hot hole, which tunnels into the oxide.

14.The holes which enter the oxide, allow for increased current density.
Due to increased current density, more electrons  tunnel in to the gate and more holes  are created.
These holes creates traps within the oxide.

15.How to limit  degradation?
Prevention cannot be done, as for most of the electronic devices, they will loose their properties over time.
To limit transistor aging effect in a circuit, various  method can be used.
Lets discuss some of them.

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