Question

how does adding a donor atom (n type extrinsic semi con) or adding an acceptor atom (p-type extrinsic semicon) lowers the energy gap btwn conduction and valence band?

Answer 1

What your talking about is typically called doping. One would take an intrinsic semiconductor, a pure semiconductor without any significant dopant species present, and add different atoms, this is the doping process, to induce the electrical properties desired. Now in an n-type material, the dominant carrier is the electron (n stands for negative charge). In a p-type material, the dominant carrier are holes, which are the absence of electrons leaving a so-called "positive charge" (p stands for positive).

Now a donor atom is one that more valence electrons than the atoms they replace in the intrinsic semiconductor lattice. Examples of these atoms are Phosphorus and Arsenic. Acceptor atoms have less valence electrons than the atoms they replace in the intrinsic semiconductor lattice. Examples of these atoms are Boron and Aluminum.

In n-type doping, the new atom that has replaced the old one in the lattice has more electrons, specifically the 5th electron which is weakly bound, this loose electron requires very little energy to leave the energy state it's in and enter the conduction band, becoming a carrier. Now if i haven't made it obvious i will state it explicitly now; it would require much more energy if this doped material wasn't added since you would have to rip away electrons from a complete shell, or a somewhat complete shell. Remember when there is only one electron in the valance band it is shielded by the inner atoms making it much easier to rip away from the nucleus.

The same thing in reverse is applied to the p-type material. Since the doped atom has one less electron, this atom is looking to complete its shell by ripping away any electrons near it making holes where the electron originate from. These holes now become dominant carrier and lower the energy level since they are easier to create.

Hope this helps sorry it was lengthy you picked a tough topic ha ha.

Answer 2

hey it helps but look at this
that line of ele