Optical Sources in Semiconductors

Optical Sources

• Optical sources are used in semiconductor devices.
• Valence electrons occupy a band of energy levels in a semiconductor, called the valence band.
• The valence band is the lowest band of allowed energy levels.
• The conduction band is the next higher band of allowed energy levels for electrons.
• Excitation of an electron from the valence band to the conduction band creates free electrons and holes.
• In a pure crystal at low temperatures, the conduction band is completely empty of electrons, and the valence band is completely full.
• The two bands are separated by an energy gap (bandgap), where no energy levels exist.
• As the temperature increases, some electrons are thermally excited across the bandgap, for silicon (Si) this excitation energy is greater than 1.1 eV, which is the bandgap energy.
• The concentration of electrons (n) and holes (p) in an intrinsic semiconductor is known as the intrinsic carrier concentration (ni).
• For a perfect material with no imperfections or impurities, ni is given by the formula: n = p = ni = Kexp(-Eg/2kBT) where K is a constant, Eg is the bandgap energy, kB is the Boltzmann constant, and T is the temperature.
• Semiconductor materials can be classified as either direct-bandgap or indirect-bandgap depending on the shape of the bandgap as a function of momentum (k).
• In direct bandgap materials, electrons and holes have the same momentum during recombination, which can be accompanied by the emission of a photon.
• In indirect bandgap materials, electron and hole recombination involves a third particle (e.g., phonon).
• LEDs (Light-Emitting Diodes) are used in optical communication.
• LEDs have advantages over lasers in optical communication applications due to lower complexity, lower cost for fabrication, and higher yield.
• LED structures have light-guiding layers, substrate, metalization, and heat sinks.
• The bandgap energy and output wavelength of an AlxGa1−xAs material vary as a function of aluminum mole fraction (x).
• The spectral emission pattern of a Ga1−xAlxAs LED with x = 0.08 has a width of 36 nm at its half-power point.
• Relationships exist between the crystal lattice spacing, energy gap and wavelength of diode emission at room temperature for semiconductor materials.

N-type Material

• The ionization of donor impurities increases the electron concentration distribution in the conduction band.
• Donor level in the conduction band.

P-type Material

• The ionization of acceptor impurities increases the hole concentration distribution in the valence band.
• Acceptor level in the valence band.

Extrinsic Semiconductors

• In extrinsic semiconductors one type of charge carrier increases, causing a decrease in the other, while the product remains constant at a given temperature.
• The product of the two types of carriers remains constant at a given temperature. This gives rise to the mass-action law pn = ni^2.
• Majority carriers are either electrons in n-type or holes in p-type material.
• Minority carriers are either holes in n-type or electrons in p-type material.
• Semiconductor devices rely on the injection and extraction of minority carriers.

pn Junction

• Electron diffusion across a pn junction creates a barrier potential (electric field) in the depletion region.

Reverse Bias

• Reverse bias widens the depletion region but allows minority carriers to move freely with the applied field.

Forward Bias

• Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.

Direct and Indirect Bandgaps

• Electron transitions in semiconductors can take place with the absorption or emission of a photon, but energy and momentum must be conserved for these to occur.
• A photon has considerable energy but very little momentum.
• Direct bandgap materials have the electrons and holes recombining with the same momentum value.
• Indirect bandgap materials require a third particle (e.g., phonon) to conserve momentum.

Typical Spectral Patterns

• Spectral patterns of edge-emitting and surface-emitting LEDs broaden with increasing wavelength.
• Typical characteristics of surface- and edge-emitting LEDs are shown in a table.

Quantum Efficiency and LED Power

• Excess carrier density decays exponentially with time.
• The rate at which carriers are generated, externally supplied rate thermally generated rates
• Total rate at which carriers are generated=J/qd
• The equilibrium condition equating to yield

Internal Quantum Efficiency

• Internal quantum efficiency (ηint) is the ratio of radiative recombination rate to the total recombination rate.
• For exponential decay of excess carriers, the radiative recombination time is tr = n/Rr and nonradiative time is tnr = n/Rnr.
• Internal quantum efficiency ηint = 1 / ( 1 + τnr / τr )

Optical Power Generated Internally to LED

• The total number of recombinations per second = I / q
• Internal optical power Pint = ηint * I * h * c / λ * q

Example of InGaAsP LED

• A double-heterojunction InGaAsP LED has radiative and nonradiative recombination times for examples.
• The drive current and the internal quantum efficiency and internal power level are to be found.
• Examples for external quantum efficiency and refractive index are included.