Electronic Principles, Semiconductor Materials (3a)

What is the atomic structure of matter?

• Matter is made of elements, each composed of atoms of the same kind.

• Atoms:

  • Nucleus (protons + neutrons, except H) → dense, positively charged core

  • Electrons orbit nucleus at large distances relative to its size

• Electron: mass = 9.11×10−319.11 \times 10^{-31}9.11×10−31 kg, charge = −1.602×10−19-1.602 \times 10^{-19}−1.602×10−19 C

• Proton: mass = 1.673×10−271.673 \times 10^{-27}1.673×10−27 kg (1836 × electron), charge = +1.602×10−19+1.602 \times 10^{-19}+1.602×10−19 C

• Neutron: no charge, mass ≈ proton’s mass

What are the atomic structures of hydrogen, helium, and carbon?

• Hydrogen: 1 proton in nucleus, 1 electron orbiting (~10−1010^{-10}10−10 m orbit diameter).

• Helium: Nucleus = 2 protons + 2 neutrons; 2 electrons in K-shell.

• Carbon: Nucleus = 6 protons + 6 neutrons (charge = +6e); 6 electrons (2 in K-shell, 4 in L-shell).

What are conduction electrons and ions in atomic structure?

• Electrons farther from the nucleus are easier to detach.

• In metals, outer electrons can become free (conduction electrons), moving randomly but drifting under external influence.

• An ion is an atom that has lost or gained electrons:

  • Lost electrons → Positive ion (more protons than electrons).

  • Gained electrons → Negative ion.

What is a covalent bond in atomic structure?

• In a covalent bond, atoms share valence electrons with adjacent atoms.

• Example: Atom A shares its 4 valence electrons with atoms B, C, D, and E.

• Each shared pair of electrons orbits around both bonded atoms, holding them together.

What is an N-type semiconductor and how is it formed?

• Formed by doping silicon/germanium with a pentavalent element (e.g., phosphorus, arsenic, antimony).

• 4 of the 5 valence electrons form covalent bonds; the 5th becomes a free electron.

• Donor atoms (pentavalent impurities) introduce these free electrons.

• The crystal remains electrically neutral, but conductivity increases due to mobile electrons.

• Called N-type (negative-type) semiconductor because electrons are the majority carriers.

How do donor atoms and an applied potential difference affect electron movement in an N-type semiconductor?

• Donors provide fixed positive ions and equal numbers of free electrons.

• More impurity → more free electrons per unit volume → higher conductivity.

• With no potential difference (p.d.), electrons move randomly (no net drift).

• With a cell connected (S positive, T negative), the electric field causes a drift of electrons toward the positive electrode (S), superimposed on their random motion.

• Electrons leaving the semiconductor at S are replaced by electrons entering at T, keeping density

What is a P-type semiconductor and how is it formed?

• Made by doping silicon with a trivalent element (e.g., indium, gallium, boron, aluminum).

• Trivalent atom provides 3 valence electrons, leaving one incomplete bond (hole).

• A hole is a vacancy that behaves like a positive charge carrier.

• Holes attract electrons from neighboring atoms, causing the hole to move through the lattice.

• Thus, in P-type semiconductors, holes are the majority carriers.

How do holes behave in a P-type semiconductor without an external electric field?

• Holes move randomly between covalent bonds, at about half the speed of free electrons.

• Random movement keeps hole density uniform, preventing charge buildup.

• Each atom with a hole has a net positive charge (e).

• The movement of holes can be regarded as the movement of positive charges within the P-type semiconductor.

What defines a P-type semiconductor and what are acceptors?

• Holes move randomly, about half the speed of electrons, keeping hole density uniform.

• Each atom with a hole is an ion with net positive charge (e).

• Hole movement is treated as positive charge movement.

• P-type semiconductors are formed when silicon/germanium is doped with trivalent impurities.

• These impurities are called acceptors because they can accept electrons from nearby atoms.

What happens at the junction of a P-type and N-type semiconductor in a diode?

• P-type: has mobile holes + fixed negative ions (neutral overall).

• N-type: has mobile electrons + fixed positive ions (neutral overall).

• Diffusion: Holes move into N-region, electrons move into P-region.

• This creates region A (negative) and region B (positive) near the junction.

• These charges form a potential barrier at the junction, preventing further carrier migration.

How does a diode behave under forward bias?

• Forward bias: Positive voltage on P-side (S), negative on N-side (T).

• Holes drift toward the junction in the P-region; electrons drift toward the junction in the N-region.

• At the junction, electrons and holes combine, allowing current to flow.

• Current results from:

  • Hole flow in P-region

  • Electron flow in N-region

  • Recombination near the junction

How does a diode behave under reverse bias, and what are minority carriers?

• Reverse bias: P-side connected to negative, N-side to positive.

• Holes move toward negative electrode, electrons toward positive, creating a depletion layer with almost no carriers → junction acts as an insulator.

• Small current exists due to thermally generated electron–hole pairs.

  • Lifetime: ~50 ms in silicon, ~100 ms in germanium

  • Rate increases with temperature, reducing intrinsic resistance.

• Minority carriers: Thermally generated carriers that are few compared to majority carriers.

  • P-type: holes = majority, electrons = minority

  • N-type: electrons = majority, holes = minority

What is the reverse current in a silicon diode under reverse bias?

• Reverse bias: Current remains nearly constant from ~0.1 V up to the breakdown voltage.

• This constant current is called the saturation current (IsI_sIs​).

• In practice, reverse current slightly increases with voltage due to surface leakage.