An n-type semiconductor is pure silicon or germanium (4 valence electrons) doped with a pentavalent atom (5 valence electrons) like Phosphorus (P), Arsenic (As) or Antimony (Sb). Four electrons of the dopant bond with the four neighbouring Si/Ge atoms; the fifth electron is very loosely held and becomes free to conduct. So the dopant "donates" an electron and is called a donor. In n-type, electrons are the majority carriers and holes are the minority carriers (ne >> nh). Memory hook: "n = extra Negative electrons, dopant has oNe extra electron (penta = 5)."
A pentavalent donor (e.g. As) bonds with four Si neighbours; its fifth electron is loosely held and becomes a free conduction electron, making electrons the majority carriers (ne >> nh) while the crystal stays neutral.
Your doubts, answered
Is an n-type semiconductor negatively charged overall?
No. It is electrically neutral. The dopant atom brought its own 5 electrons and 5 protons, so total charge stays zero. The extra fifth electron is free to move, but the donor atom it left behind is now a fixed positive ion (+1). The mobile negative electron and the fixed positive ion cancel out. 'n-type' only means the majority mobile carriers are Negative electrons, not that the crystal has extra charge.
Which impurity makes an n-type semiconductor?
A pentavalent (valency 5) impurity: Phosphorus (P), Arsenic (As), or Antimony (Sb). These are group-15 elements with 5 valence electrons. Doped into tetravalent Si or Ge, one electron is left over per dopant atom to conduct. Trap: Boron, Aluminium and Indium are trivalent (valency 3) and make p-type, not n-type.
Why is the fifth electron free to move in n-type?
Four of the dopant's five electrons form covalent bonds with the four neighbouring Si/Ge atoms. The fifth electron sees these four bonded electrons as part of the atom's core, so it is only weakly bound. The energy to free it is very small: about 0.05 eV for silicon and 0.01 eV for germanium. This is far less than the band gap (1.1 eV for Si, 0.72 eV for Ge), so at room temperature this electron is already free.
Are there holes in an n-type semiconductor?
Yes, but very few. Holes are still generated by thermal breaking of Si-Si bonds (intrinsic generation). They are the minority carriers. Their number drops because of recombination, and it follows the mass action law ne x nh = ni^2. Since ne is large (from the donors), nh becomes very small.
Is the pentavalent dopant a donor or an acceptor?
A donor. It donates one extra free electron for conduction. Remember: peNtavalent -> doNor -> n-type. Trivalent dopants are acceptors (they accept an electron, creating a hole) and give p-type.
In n-type, how do I compare ne and nh?
Use ne >> nh (electrons far outnumber holes). More precisely, ne (approx) equals ND, the donor concentration, because thermally generated carriers (ni approx 10^16 per m^3) are tiny compared to doping levels. Then nh = ni^2 / ne.
⚠️ The NEET trap ✗ Students think an n-type semiconductor is negatively charged because it has 'extra electrons', and that a pentavalent dopant is an acceptor. ✓ An n-type semiconductor is electrically neutral (fixed positive donor ions balance the free electrons). The pentavalent dopant is a DONOR, giving one free electron. Electrons are majority carriers; holes still exist as minority carriers. 🧠 n-type = neutral crystal, Negative majority carriers, pentavalent DONOR. Never say 'negatively charged'.
Real NEET questions
NEET 2021
The electron concentration in an n-type semiconductor is the same as the hole concentration in a p-type semiconductor. An external electric field is applied across each of them. Compare the currents in them.
A · Current in n-type > current in p-type ✓
B · No current will flow in p-type, current will only flow in n-type
C · Current in n-type = current in p-type
D · Current in p-type > current in n-type
Solution: Step 1: Drift current I = n e A vd, and drift velocity vd = mu x E, so I is proportional to (carrier number) x (mobility) x (field). Step 2: Given ne (n-type) = nh (p-type) and the same field E and same geometry, the only difference is mobility. Step 3: Electron mobility mu_e is greater than hole mobility mu_h (electrons move more easily than holes). Step 4: Therefore current in n-type (carried by electrons) > current in p-type (carried by holes). Answer: A.
It is pure silicon or germanium doped with a pentavalent (5-valence-electron) atom like P, As or Sb. The extra electron from each dopant is free to move, so electrons become the majority charge carriers.
Which elements are used to make n-type semiconductors?
Pentavalent group-15 elements: Phosphorus (P), Arsenic (As) and Antimony (Sb).
What are the majority and minority carriers in n-type?
Electrons are the majority carriers and holes are the minority carriers. So ne >> nh.
Is n-type semiconductor positively or negatively charged?
Neither. It is electrically neutral overall, because the fixed positive donor ions exactly balance the free negative electrons.
What is the energy needed to free the donor electron?
Very little: about 0.05 eV for silicon and 0.01 eV for germanium. This is far below the band gap, so the donor electron is free even at room temperature.
How do I find the hole concentration in an n-type sample?
Use the mass action law ne x nh = ni^2. Take ne (approx) equal to the donor concentration ND, then nh = ni^2 / ne.
