question about mobility

[yadhu]

why mobility of electrons is more in GaAs compared to Si......

 

[not-a-moderator]

Because of Physics.
Semiconductor mobility depends on the impurity concentrations (including donor and acceptor concentrations), defect concentration, temperature, and electron and hole concentrations. It also depends on the electric field, particularly at high fields when velocity saturation occurs.
Electron mobility is the measure of electron scattering in a semiconductor, also the proportionality factor between the electron drift velocity and electric field as well as between carrier concentration and the conductivity of the semiconductor.
It is measured in unit cm2/V s.
Electron mobility is different for different semiconductors. The electron mobility at 300 K for three key semiconductors: Si -1500 cm2/V s, GaAs -7500 cm2/V s, 6H-SiC -400 cm2/V s.

 

[jeffrey samuel]

GaAs has more electron mobility why this can be found in almost all the electronic books

 

[yadhu]

can you give the reason behind it.....

 

[jeffrey samuel]

The size of Gallium and Arsenic is the main reason the outter electrons are more free in them and so need lesser force to be free

this free electrons have higher mobility as the imparted energy is not lost in any place in the total process of electron liberation

this causes the electrons to move faster

 

[yadhu]

As atomic no, is 33 and that of Ga is 31 if combination of these two gives more than five times the mobility of silicon then why Ge(atomic no,32) has less mobility.......

 

[jeffrey samuel]

As size is larger pal and Ge has lesser mobility when used in its own form with dopants and not as a compound in semi conductors

 

[yadhu]

i didn't understood what you explained above pal means what?

 

[Marvel_tronix]

Mobility, quantitatively, is given as
µ = q * <τ> / (m*)
where q=charge
<τ> = mean free time between collisions
m* = conductivity effective mass
Now, standard values for mn*(conductivity effective mass of electron) & mp*(conductivity effective mass of holes) are

	Si	Ge	GaAs
mn*/mo	0.26	0.12	0.068
mp*/mo	0.39	0.30	0.50

Mathematically u can get an idea why mobility has such different values for different materials.
Also, remember m* is not the actual mass of a particle. It is the "EFFECTIVE" mass which lumps up all the (relevant) effects on a particle;for e.g. internal crystalline fields, quantum mechanical effects.

 

[fendouyingcai]

hello marvel_tronix ，why the mobility of mass of holes listed here is lager than the mobility of mass of electron ? Because we learn from books ,the mobility of mass of electron is lager .

 

[Marvel_tronix]

hello marvel_tronix ，why the mobility of mass of holes listed here is lager than the mobility of mass of electron ? Because we learn from books ,the mobility of mass of electron is lager .

What do u mean by "mobility of mass"?

 

[fendouyingcai]

What do u mean by "mobility of mass"?

sorry ，my expression may be confusing.I mean the mobilty of electronic is lager than the mobility of holes. :smile:

 

[Marvel_tronix]

Yeah mobility of electron is larger than that of hole.
The given values are of conductivity effective masses to which the mobility is inversely proportional.
Since, for electron m* value is lower than that for hole, its simply comes out that µ is larger for electrons than holes.
See carefully the listed values and their rows-columns' titles.

 

[fendouyingcai]

Yeah mobility of electron is larger than that of hole.
The given values are of conductivity effective masses to which the mobility is inversely proportional.
Since, for electron m* value is lower than that for hole, its simply comes out that µ is larger for electrons than holes.
See carefully the listed values and their rows-columns' titles.

wow,thanks .