Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
Pure silicon is an intrinsic semiconductor, not a conductor. Each Si atom has 4 valence electrons that form covalent bonds with 4 neighbours, so at low temperature no electrons are free to carry current. Its energy gap is only 1.1 eV, so heat can free a few electrons - this small, temperature-dependent conductivity is what makes it a semiconductor, sitting between metals and insulators.
At T = 0 K there is no thermal energy to break the covalent bonds. Every valence electron is locked in a bond, the conduction band is empty, and no charge carriers exist. With no free electrons or holes, no current flows, so pure Si or Ge behaves exactly like an insulator at 0 K. Raising the temperature above 0 K frees some electrons and conduction begins.
Yes. In a pure semiconductor every free electron is created by one broken bond, and that same broken bond leaves behind one hole. So electrons and holes are always made in pairs. This gives ne = nh = ni, where ni is the intrinsic carrier concentration. This equality is true only for intrinsic (pure) material - doping (extrinsic) breaks it.
Germanium has a smaller energy gap (about 0.7 eV) than silicon (about 1.1 eV). A smaller gap means less energy is needed to lift an electron from the valence band to the conduction band, so at the same temperature Ge generates more electron-hole pairs than Si. More carriers means higher conductivity (lower resistivity), so pure Ge conducts more than pure Si at room temperature.
All three are group IV elements with the same diamond-like lattice and 4 valence electrons, but their energy gaps differ hugely: C (diamond) 5.4 eV, Si 1.1 eV, Ge 0.7 eV. Carbon's 5.4 eV gap is far too large for thermal energy at room temperature to push electrons into the conduction band, so diamond is an insulator, while the smaller gaps of Si and Ge let them behave as semiconductors. (NCERT Example 14.1.)
On the basis of electrical conductivity, which one of the following materials has the smallest resistivity?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a pure semiconductor (like pure Si or Ge) with no added impurity, in which the number of free electrons equals the number of holes (ne = nh = ni), both produced only by thermal breaking of covalent bonds.
Silicon (Si) and Germanium (Ge). Both are group IV elements with 4 valence electrons and a diamond-like covalent structure. Their energy gaps are about 1.1 eV for Si and 0.7 eV for Ge.
A hole is the empty space left when an electron leaves a covalent bond. It behaves as a positive charge carrier of magnitude equal to the electron charge (+1.6 x 10^-19 C) and moves opposite to the electrons.
No. Because electrons and holes are always equal (ne = nh), neither is in majority. Majority and minority carriers exist only in extrinsic (doped) semiconductors - n-type or p-type.
It increases with temperature. More heat breaks more covalent bonds, creating more electron-hole pairs, so more carriers are available and resistivity falls. This is why semiconductors have a negative temperature coefficient of resistance.
