1.
3. Which of the following doped semiconductors will be p-type and which will be n- type? (a)
arsenic in germanium, (b) germanium in silicon, (c) indium in germanium, (d) silicon on
antimony sites in indium antimonide (InSb) and (e) Mg on gallium sites in gallium nitride (GaN)
4. Explain why silicon is used for solar cell but not for LEDs?
Solution
3. Sol:-
n-type semiconductors :- This types of semiconductors are produced when an element of 15 th
group is introduced in extermely pure silicon or germanium (i.e group 14 th
elements ) . The 15 th
group elements have five valence electrons . after forming four covalent bonds with group 14
elements , one electrons remains free . this free electron serves to conduct electricity . Since the
conductivity is due to negative charged electrons , these are called n-type semiconductors . "n"
stands for negative .
p-type semiconductors :- This types of semiconductors are produced when an element of 13 th
group is introduced in extermely pure silicon or germanium (i.e group 14 th
elements ) . The 13 th
group elements have three valence electrons . after forming three covalent bonds with group 14
elements , one electron deficient bond or hole is produced . these holes are positively charged
and can move across the crystal , thus current is carried across the crystal . since the conductivity
is due to positively charged holes , these are called p-type semiconductors . "p" stands for
positive .
Elements of group 13 - B , Al , Ga , In , Tl
Elements of group 14 - Si , Ge ,
Elements of group 15 - P , As , Sb , Bi
(a) Arsenic (As) in Germanium (Ge) = n-type semiconductor
(b) Germenium (Ge) in Silicon (Si) = Ge doped silicon is a material which is used for high
frequency applications .
(c) Indium (In) in Germenium (Ge) = p-type semiconductor

2.
(d) Silicon (Si) on Antimony (Sb) in indium (In) antimonide (InSb) = p-type semiconductor
(e) Mg on Gallium (Ga) sites in gallium nitride (GaN) = p-type semiconductor
Explanation :- In GaN , n-type doping is most common and can be achieved by adding the
elements such as Si or Ge . however p- type doping in GaN is only observed with the addition of
Mg .