27
Review of Basic Device Physics
a freely moving hole that contributes to electrical conduction. These impurities (e.g., B) are called acceptors, since they accept electrons from the VB, and
the doped silicon is called p-type that contains excess holes.
Thus, we can see from Figure 2.3, the donor and acceptor atoms occupy
substitutional lattice sites and the extra electrons or holes are very loosely
bound, that is, can easily move to the CB or VB, respectively. In terms of
energy band diagrams, donors add allowed electron states in the bandgap
close to the CB edge as shown in Figure 2.4a whereas acceptors add allowed
states just above the VB edge as shown in Figure 2.4b. Figure 2.4 also shows
the positions of the Fermi level due to donors (Figure  2.4c) and acceptors
(Figure 2.4d). Donor levels contain positive charge when ionized (emptied).
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+5
As
(a)
+4
Si
Conduction
electron
−q
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+3
B
+4
Si
+q
Hole
(b)
FIGURE 2.3
Extrinsic semiconductors forming covalent bonds: (a) an arsenic donor atom in silicon providing one electron for conduction in the CB and (b) a boron acceptor atom in silicon creating a
hole for conduction in the VB.