Optical Sources in Semiconductors

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Questions and Answers

What does the internal quantum efficiency ηint represent?

The ratio of radiative recombination rate to the total recombination rate (correct)

The ratio of the external power emitted to the internal photon generation

The ratio of the total current density to the thickness of the recombination region

The ratio of the internal power generated to the external power emitted

Which factor does not contribute to the externally generated optical power of an LED?

The total rate of carriers generated

The internal quantum efficiency

The external quantum efficiency

The thickness of the recombination region (correct)

How is the external quantum efficiency ηext defined?

The ratio of the photons emitted from the LED to the number of internally generated photons (correct)

The ratio of the internal radiation emitted to the total power generated

The ratio of radiative to nonradiative recombination rates

The ratio of the photons emitted from the LED to the total number of internal electrons

Which statement about the generation of carriers is accurate?

The supplied rates and generated rates must be equal for equilibrium Signup and view all the answers

What role does the critical angle Φc play in an LED's operation?

It signifies the angular limit for photon emission from the device Signup and view all the answers

What property of the material in regions 1 and 5 creates an optical barrier around the waveguide region?

Low index of refraction Signup and view all the answers

What is the emission pattern of the output beam for an edge-emitting LED?

Lambertian in the plane of the pn junction Signup and view all the answers

What relationship does the bandgap energy E have with the peak emission wavelength λ?

Inversely proportional to wavelength Signup and view all the answers

In the context of LED spectral emission, what happens to the spectral patterns with increasing wavelength?

They broaden Signup and view all the answers

What is the significance of the parameter Γ in the relation for excess carrier density?

It denotes the time constant of the carrier lifetime Signup and view all the answers

Which notation is typically used for simplicity when referring to GaAlAs and InGaAsP materials?

Common abbreviations Signup and view all the answers

How does the excess carrier density decay over time?

Exponentially with time Signup and view all the answers

In a typical edge-emitting LED design, what is the angle of the output beam that is highly directional perpendicular to the pn junction?

30° Signup and view all the answers

What is the primary effect of injecting a reverse bias into a pn junction?

It widens the depletion region and allows minority carriers to move freely. Signup and view all the answers

What characterizes a direct-bandgap semiconductor?

The conduction-band minimum and the valence-band maximum occur at the same momentum value. Signup and view all the answers

For indirect-bandgap materials, what assists in electron recombination for photon emission?

A third particle, typically a phonon. Signup and view all the answers

Which of the following is NOT an advantage of using LEDs in optical communication?

LEDs can operate at high thermal and optical stabilization. Signup and view all the answers

What occurs during electron diffusion across a pn junction?

A barrier potential is created in the depletion region. Signup and view all the answers

What is required for electron transitions to occur in semiconductors?

Both energy and momentum must be conserved. Signup and view all the answers

What is a typical feature of Light-Emitting Diodes (LEDs) for optical applications?

They must exhibit high radiance output and high quantum efficiency. Signup and view all the answers

What effect does lowering the barrier potential with a forward bias have in a pn junction?

It permits majority carriers to diffuse across the junction. Signup and view all the answers

What is the valence band in a semiconductor?

The lowest band of allowed states for valence electrons Signup and view all the answers

At low temperatures, how are the conduction and valence bands in a pure crystal organized?

The conduction band is empty and the valence band is full Signup and view all the answers

What does the term 'intrinsic carrier concentration' refer to?

The concentration of thermally excited electrons and holes in a pure semiconductor Signup and view all the answers

What effect does increasing donor impurities have on n-type materials?

It reduces the hole concentration in the valence band Signup and view all the answers

What is the significance of the bandgap in a semiconductor?

It separates the valence band from the conduction band Signup and view all the answers

What phenomenon occurs in extrinsic semiconductors concerning carrier types?

Increasing one type of carrier decreases the other type Signup and view all the answers

What are majority carriers in semiconductors?

Electrons in n-type materials or holes in p-type materials Signup and view all the answers

What does the mass-action law describe in terms of charge carriers in semiconductors?

The product of electron and hole concentrations at thermal equilibrium Signup and view all the answers

Study Notes

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.

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Description

Explore the fundamentals of optical sources in semiconductor devices. This quiz covers the concepts of valence and conduction bands, energy gaps, and intrinsic carrier concentration. Test your understanding of how temperature affects electron excitation and the behavior of semiconductors.