Problem 6.13

If Ge is added to GaAs, the Ge is about equally distributed between Ga and As sites. Which sites would Ge prefer if Se is added also? Would GaAs doped with Se be an n-type or a p-type semiconductor?
 
 

Answer:

Ge is located between Ga and As on the Periodic Table, so is slightly smaller than Ga and slightly larger than As and is able to fit into either lattice sight. Se is smaller than either Ga or As but closest in size to As so would fit into the As lattice sites. Although Se would also fit into the larger Ga lattice sites, the Se is too small and would "rattle" around a bit and have less overlap with its neighbors. Thus, Se would prefer to be in the As sites.

Since Se has more valence electrons than either Ga or As, this would provide extra electrons to the conduction and create an n-type semiconductor.