Question E6A02:
Which of the following semiconductor materials contains excess free electrons?  

Choices:  
A: N-type  
B: P-type  
C: Bipolar  
D: Insulated gate    

Correct Answer is  A: N-type  

Correct Answer: A — N-type

1) Why A is correct
- N-type semiconductor is doped with donor impurities (e.g., phosphorus, arsenic) that contribute extra electrons to the conduction band.
- Electrons are the majority carriers; holes are the minority carriers.
- Memory aid: N = Negative charge carriers (electrons).

2) Why the other choices are incorrect
- B: P-type
  - Doped with acceptor impurities (e.g., boron) that create holes.
  - Holes (positive carriers) are in excess, not electrons.
- C: Bipolar
  - Refers to device type (BJT) that uses both electrons and holes; it is not a material type. A BJT contains both N and P regions.
- D: Insulated gate
  - Refers to a device structure (e.g., MOSFET). The material channel can be N- or P-type; “insulated gate” is not a semiconductor material classification.

3) Useful formulas and rules of thumb
- Majority/minority carrier relation:
  $$n p = n_i^2$$
  where $n$ = electron concentration, $p$ = hole concentration, and $n_i$ = intrinsic carrier concentration.
- In N-type material with donor density $N_D$ (and $N_D \gg n_i$):
  $$n \approx N_D,\quad p \approx \frac{n_i^2}{N_D}$$
- Conductivity:
  $$\sigma = q\,(n\mu_n + p\mu_p)$$
  In N-type, $n \gg p$, so electrons dominate conduction.
- Rule of thumb: Donors → N-type → extra electrons. Acceptors → P-type → extra holes.