Q4. Sort these dopants for silicon into those that make n-type and those that make p-type material: B, P, Al, As, In, Sb.
n-type (pentavalent donors): P, As, Sb. p-type (trivalent acceptors): B, Al, In (NCERT pages 329–330).
Q5. Why must a dopant atom be nearly the same size as the semiconductor atom it replaces?
So that it can replace host atoms without distorting the original pure semiconductor lattice; a necessary condition is that the sizes of the dopant and the semiconductor atoms should be nearly the same (NCERT page 329).
Q6. Assertion: an n-type sample has fewer holes than a pure sample of the same semiconductor at the same temperature. Reason: the many extra electrons recombine with holes faster. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (NCERT page 330).
Q7. A doped Si sample has nh = 4.5 × 1022 m−3. With ni = 1.5 × 1016 m−3, find ne, and say whether it is n-type or p-type.
ne = (2.25 × 1032)/(4.5 × 1022) = 5 × 109 m−3. Holes are the majority carriers, so it is p-type (NCERT Eqs. (14.4)–(14.5), pages 331–332).
Q8. A p-type crystal has many more holes than electrons. Why is it not positively charged?
Each acceptor that supplies a hole becomes a fixed negative ion, so the charge of the extra holes is equal and opposite to that of the ionised cores (NCERT page 331).
Q9. A Si sample (ni = 1.5 × 1016 m−3) has 5 × 1022 As and 5 × 1020 In atoms m−3. Find ne, nh and the type.
Each In atom takes up one donor electron: ne ≈ 4.95 × 1022 m−3, nh = (2.25 × 1032)/(4.95 × 1022) ≈ 4.5 × 109 m−3; n-type (beyond the NCERT text; NCERT Eq. (14.5), page 332).