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holes are the absence of electrons in the lattice, right? then how come we say holes have a +ve charge? shouldnt it be zero?

also, why is the mobility of electrons more than holes?

thanks

- Thread starter jablonsky27
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- #1

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holes are the absence of electrons in the lattice, right? then how come we say holes have a +ve charge? shouldnt it be zero?

also, why is the mobility of electrons more than holes?

thanks

- #2

Defennder

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As for the other question, I'm guessing it's something to do with the magnitude of their effective masses.Consider this in 1D:

We all know that J=nev where n is the concentration of electrons, v is the drift velocity.

Suppose the valence band is completely full of N electrons, then it goes without saying there can be no current flow:

[tex]J_{x} = \sum_i^N -ev_{i} = 0[/tex]

But on the other hand suppose there is one missing electron, one vacancy in the valence band where an electron should be:

[tex]J_{x}=\sum_i^{N-1}-ev_{i} = \sum_i^N -ev_{i} -(-ev_{j}) = ev_{j}[/tex]

Notice that the final expression on the right can be thought of as current due to the drift of a single positive charge, since there is no minus sign. That is why we are justified in thinking that we can treat absence of negative charges as positively charged holes.

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what would the effective mass of a hole be??

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Where would you usually find a hole? And where would you usually find an electron?

Where == which band?

Yes, the effective masses of holes and electrons are different. The mobility is a function of the effective mass, which is a theoretical tool brought in to simplify the description of a charged particle in a crystal. Classically, the concept of an effective mass is analogous to that of a 'psuedo force' in Newtonian mechanics.

In the simplified, http://en.wikipedia.org/wiki/Drude_model" [Broken], for a fixed mean free time.

Where == which band?

Yes, the effective masses of holes and electrons are different. The mobility is a function of the effective mass, which is a theoretical tool brought in to simplify the description of a charged particle in a crystal. Classically, the concept of an effective mass is analogous to that of a 'psuedo force' in Newtonian mechanics.

In the simplified, http://en.wikipedia.org/wiki/Drude_model" [Broken], for a fixed mean free time.

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