drift current in semiconductor

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Drift Current in a biased PN Junction is independent of biasing ,in unbiased PN junction minority carriers can cross the barrier and it produces drift current and is balanced by equal diffusion current.Drift current depends on temperature also


current passing through a square centimeter area perpendicular to the direction of flow
Drift current Density .
Drift current density.{Electrons}
Jn = q n μn E (A / cm2)

Where
n -Number of Electrons per cubic centimeter

μn – Mobility of electrons in cm2 / Vs

μp – Mobility of holes in cm2 / Vs

E – Applied Electric filed Intensity in V /cm

q – Charge of an electron = 1.6 × 10−19 coulomb.
 

@ blooz: as u had said that drift current is independent of biasing, but electric field affects the drifting of charged carriers, and electric field must depend on applied voltage or potential across the junction (E=V/L). so drifting must also depend on applied biasing. is it right or not??? pls give ur comments
 

Hi Nitin,

Higher the Electric Field ,means carrier will be swept faster,but the current depends on no of carriers and not their velocity ,
whether it is hole or electron their number determines the current ,higher field implies the carriers will be swept faster .
 


Electrical current is a measure of the amount of electrical charge transferred per unit time. It represents the flow of electrons through a conductive material. The SI unit of electrical current is the ampere, defined as 1 coulomb/second. So higher the velocity you will have more charge flowing per unit time. Check the drift current in equation it is directly proportional to applied electric field.
Drift current - Wikipedia, the free encyclopedia
When applied electric field is strong enough, the carrier velocity in the semiconductor reaches a maximum value, saturation velocity. When this happens, the semiconductor is said to be in a state of velocity saturation. As the applied electric field increases from that point, the carrier velocity no longer increases because the carriers lose energy through increased levels of interaction with the lattice, by emitting phonons and even photons as soon as the carrier energy is large enough to do so.
Velocity saturation - Wikipedia, the free encyclopedia
 

So higher the velocity you will have more charge flowing per unit time
Click to expand...
But the no of carriers of is important ,field determines the velocity

drift current is due to generation and recombination of carriers


Here are some good reference to clarify the concept
1.Introduction to semiconductor devices: for computing and telecommunications Applications
By Kevin F. Brennan page 50

2.Solid State Electronics Devices ,Streetman
{Indian Edition
page 194}

3.Silicon photonics: the state of the art
By Graham T. Reed
page 197
 

blooz said:
But the no of carriers of is important ,field determines the velocity

drift current is due to generation and recombination of carriers
Click to expand...

I think you are confused with the diffusion current in BJT. Answer a simple question.. You have infinite numbers of carriers in the semiconductor but applied electric field is zero. what will be the drift current?
 

Just go through the above text books ,it's crystal clear

Drift current is independent of applied bias

---------- Post added at 15:11 ---------- Previous post was at 15:02 ----------

Reference data for engineers: radio, electronics, computer, and communications
By Mac E. Van Valkenburg, Wendy Middleton

---------- Post added at 15:17 ---------- Previous post was at 15:11 ----------

A Similar Discussion
h**p://www.physicsforums.com/showthread.php?t=168627

Bottom line is minority carriers are generated much slower than they are swept due to the field, and you're limited by the generation rate.
 

As I read your references, I got the following idea;
" Drift current is generated by minority carriers in PN junction, and this is equal to diffusion current but flows in opposite direction in equlibrium.( when unbiased). Since biasing and hence Electric field only generates Majority carriers, not the minority, thats why Drift currnet is independent of biasing."
But as minority charge carriers can be thermally generated, drift current is temperature dependent.

Am I correct?
 


Yes Varunkant2k, that's the point .

" Drift current is generated by minority carriers in PN junction, and this is equal to diffusion current but flows in opposite direction in equlibrium.( when unbiased). Since biasing and hence Electric field only generates Majority carriers, not the minority, thats why Drift currnet is independent of biasing."
But as minority charge carriers can be thermally generated, drift current is temperature dependent.
 
frnds the text which taken from books specifies that drift current is independent of biasing only because biasing doesn't produce minority carriers (or drift current carriers) but if minority carriers get generated by temprature then minority carriers (or drift current carriers) will get drifted by applied biasing and hence electric field in biased condition.
Remember that drift current will only exist if there is electric field across the semiconductor specimen, and during unbiased condition electric field is produced by potential barrier formed due to diffusion of majority carriers in pn junction.
 

blooz said:
Drift current is independent of applied bias
Click to expand...

Drift current only in the case of PN junction is independent of applied bias. Because drift current in PN junction is due the minority carriers. Minority carriers are generated by temperature so drift current is temperature dependent. When the diode is forward biased drift current is present, but because diffusion current grows exponentially, it dominates. The total current flowing through the depletion region under forward biasing is made up of mostly majority carrier diffusion. But this is not true in case of the MOSFET. Drift current in MOSFET is due to majority carriers. In case of MOSFET drift current increase with applied electric field till the Velocity saturation.
 


Yes ,the First post clearly states that we are considering a PN junction ,and MOSFET Case is Different as the underlying Device physics is different ,
 

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