Add a pentavalent atom (5 valence electrons) like Arsenic, Phosphorus or Antimony to Si/Ge and you get an n-type semiconductor (extra electrons). Add a trivalent atom (3 valence electrons) like Boron, Indium or Aluminium and you get a p-type semiconductor (extra holes). Memory hook: "5 gives Negative electrons, 3 makes Positive holes" — pentavalent = n-type, trivalent = p-type.
At a glance
Valence electrons
Pentavalent: 5 (As, P, Sb)
Trivalent: 3 (B, In, Al)
Periodic group
Group 15
Group 13
Role
Donor (gives electron)
Acceptor (creates hole)
Type formed
n-type
p-type
Majority carrier
Electrons
Holes
Dopant selection for silicon or germanium: pentavalent Group 15 atoms (As, P, Sb) donate electrons and give n-type, while trivalent Group 13 atoms (B, In, Al) create holes and give p-type.
Your doubts, answered
Is Boron used for n-type or p-type?
Boron makes p-type. Boron has 3 valence electrons (trivalent, Group 13). When it replaces a Si/Ge atom it cannot fill the fourth bond, so it leaves a hole. Holes become the majority carriers, giving p-type. This is exactly what NEET 2023 asked: Germanium doped with Boron is p-type.
Which dopants make an n-type semiconductor?
Pentavalent atoms with 5 valence electrons: Arsenic (As), Phosphorus (P) and Antimony (Sb) are the common ones (all Group 15). Four electrons bond with Si/Ge and the fifth is nearly free, so it becomes a mobile electron. These atoms donate electrons, so they are called donor impurities.
Is Arsenic a donor or an acceptor?
Arsenic is a donor. It is pentavalent (5 valence electrons), so it gives one extra electron to the crystal and makes n-type. Donor = gives electron = pentavalent = n-type. Acceptor = takes electron / creates hole = trivalent = p-type.
Are Group 13 elements trivalent or pentavalent?
Group 13 (B, Al, Ga, In) are trivalent — they have 3 valence electrons and make p-type. Group 15 (P, As, Sb) are pentavalent — 5 valence electrons and make n-type. Silicon and Germanium sit in Group 14 (tetravalent, 4 electrons), so dopants are picked from the neighbouring Group 13 or Group 15.
Does Indium give p-type or n-type?
Indium (In) gives p-type. It is trivalent (Group 13), same family as Boron and Aluminium, so it creates holes. NCERT lists the trivalent acceptors as Indium, Boron and Aluminium.
⚠️ The NEET trap ✗ Boron is pentavalent so Germanium doped with Boron gives n-type. ✓ Boron is trivalent (3 valence electrons, Group 13), so it creates holes and gives p-type. Only Arsenic, Phosphorus and Antimony (pentavalent) give n-type. 🧠 NTA loves swapping donor and acceptor lists — read the valency, not the name.
Real NEET questions
2023
A p-type extrinsic semiconductor is obtained when Germanium is doped with:
A · Arsenic
B · Boron ✓
C · Antimony
D · Phosphorous
Solution: Step 1: A p-type semiconductor needs a trivalent (acceptor) dopant that creates holes. Step 2: Check valency of each option. Arsenic, Antimony and Phosphorous are pentavalent (5 valence electrons, Group 15) — they are donors and give n-type. Step 3: Boron has 3 valence electrons (trivalent, Group 13), so it leaves a hole and makes p-type. Answer: Boron (B).
2019
For a p-type semiconductor, which of the following statements is true?
A · Electrons are the majority carriers and trivalent atoms are the dopants.
B · Holes are the majority carriers and trivalent atoms are the dopants. ✓
C · Holes are the majority carriers and pentavalent atoms are the dopants.
D · Electrons are the majority carriers and pentavalent atoms are the dopants.
Solution: Step 1: p-type is made by doping with trivalent (acceptor) atoms such as B, In, Al. Step 2: Each trivalent atom is short of one electron for the fourth bond, so it creates a hole. Step 3: Therefore holes are the majority carriers and the dopants are trivalent. This matches option B.
Pentavalent (5 valence electrons) = donor = n-type. Trivalent (3 valence electrons) = acceptor = p-type. Since Si/Ge are tetravalent (Group 14), you pick dopants from the neighbouring Group 15 or Group 13.
NCERT notes the dopant must fit into the crystal lattice without distorting it, so its atomic size should be close to that of Si or Ge. That is why nearby Group 13 and Group 15 elements are used, not far-away ones.
Does doping change the total charge of the crystal?
No. The crystal stays electrically neutral. A donor gives a free electron but keeps a fixed positive ion core; an acceptor creates a mobile hole but keeps a fixed negative ion core. Only the number of mobile carriers changes.
How much impurity is added during doping?
Only a few parts per million (ppm). Even this tiny amount hugely increases conductivity because it adds many more carriers than the small intrinsic count at room temperature.
