Question E6A04:
What is the name given to an impurity atom that adds holes to a semiconductor crystal structure?  

Choices:  
A: Insulator impurity  
B: N-type impurity  
C: Acceptor impurity  
D: Donor impurity    

Correct Answer is  C: Acceptor impurity  

To add holes to a semiconductor, you use an acceptor impurity. These are trivalent atoms (e.g., boron in silicon) that “accept” an electron from the crystal, leaving behind a hole (a positive charge carrier). This creates p-type material where holes are the majority carriers.

Why C (Acceptor impurity) is correct:
- An acceptor atom has one fewer valence electron than the host (e.g., B has 3 vs. Si’s 4), so it accepts an electron from the lattice.
- This leaves a hole in the valence band, increasing hole concentration and making the semiconductor p-type.

Why the other choices are incorrect:
- A: Insulator impurity — Not a standard doping term; an insulator would not intentionally create free carriers (holes or electrons).
- B: N-type impurity — N-type means donors dominate; majority carriers are electrons, not holes.
- D: Donor impurity — Donors supply extra electrons (e.g., phosphorus in silicon) and create n-type material; they do not add holes.

Useful rules and relations:
- Acceptor → p-type → holes are majority carriers; typical dopants: Group III (B, Al, Ga).
- Donor → n-type → electrons are majority carriers; typical dopants: Group V (P, As, Sb).
- In p-type with acceptor concentration $N_A$ (assuming full ionization and $N_A \gg n_i$):
  $$p \approx N_A$$
- Mass action law (always holds at equilibrium):
  $$n \, p = n_i^2$$
- Energy view: acceptor levels lie just above the valence band; capturing an electron from the valence band creates a hole.