MODULE 1: Introduction to Semiconductors

A set of practice flashcards covering key concepts from Module 1: Introduction to Semiconductors.

What invention revolutionized electronics as a solid-state device?

The transistor.

What class of materials has conductivity that can be tuned by temperature, optical excitation, or impurities?

Semiconductors.

Name some common solid-state device roles that semiconductors can play.

Switches, amplifiers, logic devices, memory, photodiodes, LEDs/laser diodes.

How are materials classified based on electrical conductivity at room temperature?

As conductors, insulators, or semiconductors.

How are semiconductors classified by composition?

Elemental and compound semiconductors.

Give an example of an elemental semiconductor and a compound semiconductor.

Elemental: silicon (Si). Compound: gallium arsenide (GaAs).

Why is silicon the most widely used semiconductor material?

It is abundant in Earth's crust, has a larger bandgap than Ge allowing higher-temperature operation, and can be obtained in high-purity form.

What are the main energy bands in solids and what is the bandgap?

Valence band and conduction band separated by the bandgap Eg.

What is an intrinsic semiconductor?

A pure crystal with no intentional dopants; at thermal equilibrium, no free electrons in CB and no holes in VB.

What is an extrinsic semiconductor?

A semiconductor doped with impurities to increase conductivity (n-type or p-type).

What dopants create n-type semiconductors and what are the majority carriers?

Pentavalent dopants (e.g., P, As, Sb) create n-type; electrons are the majority carriers.

What dopants create p-type semiconductors and what are the majority carriers?

Trivalent dopants (e.g., B, Al) create p-type; holes are the majority carriers.

What is the effective mass in a semiconductor and what symbols denote electron and hole effective masses?

The mass that describes a carrier’s response in a crystal; mn for electrons and mp for holes; it can be positive, negative, or infinite.

What is the Fermi-Dirac distribution function?

f(E) = 1 / [1 + exp((E − EF) / kT)].

Where is the Fermi level located in intrinsic, n-type, and p-type semiconductors?

Intrinsic: in the middle of the bandgap; n-type: closer to the conduction band; p-type: closer to the valence band.

What is the Mass Action Law in semiconductors?

n p = ni^2; for intrinsic, n = p = ni.

What are Nc and Nv and how are they related to the conduction and valence bands?

Nc and Nv are the effective densities of states in the conduction and valence bands; Nc ≈ 2(2π me* kT / h^2)^{3/2}, Nv ≈ 2(2π mh* kT / h^2)^{3/2}.

What is space charge neutrality in a doped semiconductor?

Total positive charges equal total negative charges; at room temperature donors and acceptors are assumed ionized.

What are quasi-Fermi levels and what do Fn and Fp represent?

In non-equilibrium with excess carriers, Fn is the electron quasi-Fermi level and Fp is the hole quasi-Fermi level; their separation indicates deviation from equilibrium.

Differentiate direct and indirect bandgap semiconductors in terms of luminescence.

Direct: valence band maximum and conduction band minimum occur at the same k, allowing efficient photon emission; indirect: occur at different k values and require phonons, making light emission inefficient (e.g., Si, Ge).

What are the three main recombination mechanisms for excess carriers?

Direct band-to-band recombination; indirect recombination via trap centers; Auger recombination.

What is carrier lifetime?

The characteristic time for excess carriers to recombine; under low-level injection, n(t) = n0 e^{-t/τ}, where τ is the lifetime.

What is photoconductivity?

Increase in conductivity due to optical generation of excess carriers when light excites electrons to the conduction band.

What are the three types of luminescence and their excitation mechanisms?

Photoluminescence (excited by photons), cathodoluminescence (excited by high-energy electrons), and electroluminescence (excited by current injection).

What happens during optical generation in terms of carrier concentrations?

Photons with energy greater than Eg are absorbed, exciting electrons to the conduction band; total electron concentration becomes n = n0 + Sn and holes p = p0 + Sp.

What is the difference between low-level and high-level injection?

Low-level: Sn, Sp << no (or Po); high-level: Sn, Sp are comparable to no (or Po), and the majority carrier concentration is altered.