Is an n-type semiconductor negatively charged or neutral?

[cowgiljl]

Does an n-type semiconductor exhibit a negitive charge, or is it electrically neutral? Why?

I know that a n-type semiconductor is negitive charge

thanks joe

 

[chroot]

No, an n-type semiconductor is not negatively charged.

Here's the whole story: if you make a crystal of pure silicon, it will not conduct electricity very well. Each atom of silicon has four valence electrons, and each atom bonds with each of its four neighbors in the crystal lattice. All of the electrons are involved in chemical bonds, and thus cannot move. Pure silicon (called "intrinsic semiconductor") is essentially an insulator.

To make n-type semiconductor, you impregnate the pure silicon crystal with atoms of another element like phosphorus. The phoshorus atoms fit into the crystal lattice just like silicon, but with one important difference: phosphorus atoms have five valence electrons. One of the phosphorus atoms' electrons is free to move about throughout the crystal. The resulting material, "doped" with phosphorus, can carry electricity via these mobile electrons.

To make a p-type semiconductor, you dope the pure silicon with an element like boron, which only has three valence electrons. The resulting material is "missing" electrons. It has "holes" where electrons are absent. P-type semiconductor can carry electricity via the movement of these holes (or, by the movement of electrons through those holes, if you want to be pedantic). Interestingly, the conduction of electricity by a p-type semiconductor is quite well described as due to the movement of positive particles. In essence, the holes behave just like mobile positive particles.

Let us know if you have any more questions.

- Warren

 

[cowgiljl]

thanks i did find something like that in one of my old circuit anay. books. It is always a good thing to keep them around when going back to school

thanks joe

 

[appy]

but why is it neutral even tho it has excess of negative charge

 

[antonantal]

but why is it neutral even tho it has excess of negative charge

It doesn't have excess of any type of charge. Like chroot said, the silicon is doped with atoms of phosphorus or boron. And atoms are electrically neutral.

For example, in the n-type silicon, although the phosphorus atom has one mobile electron, it has equal numbers of electrons and protons. So if the mobile electron moves from the atom, the atom becomes positively charged (it becomes a positive ion).
In the p-type silicon, although the boron atom has one mobile vacancy (hole), it has equal numbers of electrons and protons. So if the mobile hole moves from the boron atom (meaning that an electron from a neighbour silicon atom moved in the vacancy), the atom becomes negatively charged (it becomes a negative ion).
So, in both types of doped silicon, the number of mobile charges of one polarity is equal to the number of fixed ions of opposite polarity, and thus the doped silicon is neutral.

 

[appy]

got it! thanks a lot

 

[i.mehrzad]

Look at it this way. An n type semiconductor is made up of atoms that are each elictrically neutral. The coincidence of the extra electron is because Si can't accommodate it.