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Chapter 2.2.9,2.2.10 and 2.2.11 gives the diffusion process, recombination process and carrier concentration profile under steady state condition for optically perturbed Si-Bar.

2.2.9.Diffusion Process and Diffusion Current.

In kinetic theory of gases we have studied the concept of diffusion. Gas tends to move from higher concentration to lower concentration until gas is uniformly distributed through the room.

Just as potential gradient causes drift current in a similar manner concentration gradient causes a diffusion current. Linear concentration gradient causes a constant diffusion current as shown in Figure 2.2.40.

Conducting electrons and holes always move from high concentration to low concentration hence the rate of carrier movements per unit area or carrier flux density is proportional to the negative of the concentration gradient as given in the equation below:


Here D n and D p are diffusion coefficient of electron and holes respectively.

The unit of Diffusion Coefficient = cm 2 /s and Einstein Relation gives the inter-relation between Diffusion Coefficient and mobility namely:


Table tabulates the values of diffusion coefficient and mobility foe conducting electrons and holes in intrinsic Si at 300K.

Table Comparative values of diffusion coefficients and mobilities of conducting electrons and holes in intrinsic Si at 300K.

Material μ n (cm 2 /V-s) μ p (cm 2 /V-s) D n (cm 2 /s) D p (cm 2 /s)
Ge 3900 1900 100 50
Si 1350 480 35 12.5
GaAs 8500 400 220 10

When we assign the electronic charge to the flux density we obtain diffusion current density due to conducting electron or due to holes.


2.2.10.Life Time of Minority Carriers.

Life Time of minority carriers is the average survival time of minority carriers before it recombines.

In an ideal semiconductor, there are no impurities, no defects and hence no energy states in the band-gap. Therefore once EHP is thermally generated it has very little likelihood to recombine. kinds of recombination.

There are two kinds of recombination: in-direct recombination also known as Shockley-Hall-Reed(SHR) recombination and direct recombination known as Auger recombination.

In ideal semi-conductors, the equilibrium between thermal generation and Auger Recombination of EHP decides the equilibrium value and any perturbation of carrier concentration takes a very long time to come to the thermal equilibrium value. In such circumstances the life-time is very large in minutes and seconds.

In real semi-conductors there are energy states introduced in the energy band-gap. Impurities introduce intermediate energy states in the band gap. Imperfections in the crystal structure or crystal defects also introduce intermediate energy states. These intermediate energy states act as the stepping stone for indirect recombination or for SHR recombination as shown in Figure 2.2.41. But the effectiveness of these intermediate energy states as a recombination center depends on the relative position within the band-gap. The middle position within the band-gap makes the intermediate energy states a very effective stepping stone for in-drect recombination. For instance Au. It introduces intermediate energy states which lie in the middle of the Si band-gap hence it is a very effective stepping stone for recombination.Therefore Au is a life-time killer.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Solid state physics and devices-the harbinger of third wave of civilization. OpenStax CNX. Sep 15, 2014 Download for free at http://legacy.cnx.org/content/col11170/1.89
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