Introduction to Semiconductors Quiz

Introduction to Semiconductors

• Electronics is the science and technology of charge motion in gases, vacuum, or semiconductors.
• There are three types of materials: conductors, insulators, and semiconductors.
• Conductors have low resistance and allow current flow.
• Insulators have high resistance and suppress current flow.
• Semiconductors can either allow or suppress current flow, depending on conditions.

Materials

• Conductivity (and resistivity) of semiconductors falls between conductors and insulators.
• Resistivity is measured in Ω-m.

The Atom (Bohr Model)

• Atoms have a nucleus (protons and neutrons), and orbiting electrons.
• Electrons orbit the nucleus in specific energy levels (shells).
• Electrons near the nucleus have lower energy than those farther out.

Electrons and Shells

• Electrons orbit at discrete distances from the nucleus, corresponding to specific energy levels called shells.
• Each shell has a maximum number of electrons it can hold.
• Valence electrons are those in the outermost shell, and are especially important for chemical bonding and material properties.

Valence Electrons

• Valence electrons farther from the nucleus have higher energy and are less tightly bound to the atom.
• The force of attraction between the nucleus and valence electrons decreases with distance.
• Electrons in the outermost shell are called valence electrons.

Ionization

• When an atom absorbs energy, its electrons can jump to higher energy levels.
• Losing a valence electron is called ionization.
• An atom that loses an electron becomes a positive ion.
• An atom that gains an electron becomes a negative ion.

Insulators

• Insulators have tightly bound electrons in their outer shells.
• Large amounts of energy are needed to free the electrons for conduction.
• Insulators do not conduct electricity under normal conditions.

Conductors

• Conductors have loosely bound valence electrons in their outer shells.
• Low energy is needed to free the electrons from their orbits.
• Electrons can move freely from atom to atom, facilitating current flow.

Current in Conductors

• Current (I) is the rate of charge flow (Q) over time (T).
• Drift velocity (v_d) is the average velocity of charge carriers in a conductor.
• Electron mobility (µ) is a measure of how easily electrons move in a material.
• Electric field (ε) is the force per unit charge.
• Electrical field can cause drift velocity and current.
• Current density (J) is the current per unit area (A).
• Ohm's law defines the relationship between current, voltage, and resistance (V = IR).
• Conductivity (σ) is the inverse of resistivity (ρ) , σ = 1/ρ;
• Current (I) = current density J x Area (A)
• Electrical field (ε) = Voltage(V)/Length (L)

Intrinsic Semiconductors

• Pure semiconductors, like silicon and germanium, are neither good conductors nor good insulators at moderate temperatures.
• At very low temperature (0K) intrinsic semiconductors act as insulators.
• At room temperature, thermal energy causes some valence electrons to jump to the conduction band, becoming free electrons.
• The absence of an electron in the valence band is called a hole.
• Electron-hole pairs are created in intrinsic semiconductors due to thermal energy.

Electron Movement in Silicon

• Free electrons move through the lattice structure.
• Holes represent the empty spaces where electrons have moved.
• Heat provides energy for the creation of electron-hole pairs.
• This electron movement allows current flow.

Intrinsic Conduction

• Applying a potential difference sets up an electric field across the material.
• Electrons and holes are accelerated in the electric field, and current flows.
• Applying more heat creates more electron-hole pairs, increasing the conductivity of the semiconductor.
• The silicon crystal behaves like a thermistor , its resistance decreases with increasing temperature.

Extrinsic Semiconductors

• Doping is adding impurity atoms to an intrinsic semiconductor to modify its electrical properties.
• N-type semiconductors have extra electrons (donor atoms), making them electron-rich.
• P-type semiconductors have missing electrons (acceptor atoms), making them hole-rich.
• The number of electrons in n-type or holes in p-type can be controlled during doping.

The Mass-Action Law

• Under thermal equilibrium, the product of the electron and hole concentrations is a constant (independent of dopant concentration).
• np = n_i²
• n_i is the intrinsic carrier concentration, determined by temperature.

Charge Densities in a Semiconductor

• For n-type material, the electron concentration is approximately equal to the donor atom concentration.
• For p-type material, the hole concentration is approximately equal to the acceptor atom concentration.

The Thermistor

• A thermistor is a heat-sensitive resistor.
• Its resistance decreases as temperature increases .
• Used in temperature measurement and control applications.
• A thermistor exhibits negative temperature coefficient when the temperature increase causes the resistance to decrease and vice versa.

The Light Dependent Resistor (LDR)

• An LDR resistance changes based on light (light energy instead of heat energy).
• Its resistance decreases when exposed to light, and increases in darkness.
• Used in automatic lighting systems and light meters.