# Semiconductors

Discussion in 'Physics' started by VVS, Mar 28, 2008.

1. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
Hi I am studying EEE and we have a subject called devices. we are studying how semiconductors behave and looking at the mobility of electrons and holes. I found often that in tables the mobility of holes is often much less because the effective mass is greater.
Could somebody please explain to me why it is that holes and electrons have different effective masses when the existance of holes arises from symmetry arguments???

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Electrons have mass. Holes are the absence of electrons. Since there should be an electron where the hole is the net charge increase is positive, but only by the electrons absence. Not totally sure of this answer, but it's been a long time since school for me.

3. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
i understand why the magnitude of the charges of the electron and the hole have to be equal, but shouldnt the mass be same as well??? if u look in tables u will find different effective masses for electrons and holes although holes are just the absence of electrons.

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Think about how electrons and holes move around in the presence of an applied electric-field.

In n-type semicondutor material where the charge carrier is the electron, the application of the electric-field causes the migration of electrons through the material.

In p-type semicondutor material where the charge carrier is the hole, the application of the electric-field still causes the migration of electrons, however they now move to fill the hole - in this sense the hole moves opposite to the movement of the electrons which respond to the application of the electric-field.

Therein, the idea of symmetry doesn't wholly apply here.

In terms of mass think of the following picture from the e-book: http://www.allaboutcircuits.com/vol_3/chpt_2/5.html#03404.png

Picture (b) is n-type, and Picture (c) is p-type.

For a given unit volume (the volumes of these two pictures are the same), and in the absence of other variables (i.e. changing thermal effects) to keep the concept clear:

In n-type semiconductors you have a one mobile electron that will respond to the electric-field and move over the material - the others are bound to the semiconductor material. The mobility of the electron is largely unresticted beacuse the mass, as in how much, mobile charge is low.

However, in the p-type material there are a whole host of mobile electrons that can move to occupy the vacated hole when an electric-field is applied. The mobility of these electrons are largely restricted by one another because of the mass (how much) mobile charge there is in that unit volume.

This is where the idea of effective mass comes in. I have never really heard it described in the way you state in your OP, but that is my take on it.

Dave

5. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
thanx dave makes sense to me now i think.
the effective mass is a property which arises from the multitude of electric fields acting on the carriers, and since holes have the oposite charge of electrons they will feel have a different effective mass.
It now makes sense to me, thanx!!!!

6. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
The other way round - an electric-field (singular) acts on a multitude of mobile charge. Because there is more effective mobile charge in the p-type (more holes) its mobility is restricted.

Dave