Optical Sources PDF

The Optical sources Energy Bands ▪In a semiconductor the valence electrons occupy a band of energy levels called the valence band. ▪This is the lowest band of allowed states. The next higher band of allowed energy levels for the electrons is called the conduction band. ▪Energy level diagrams sh...

The Optical sources Energy Bands ▪In a semiconductor the valence electrons occupy a band of energy levels called the valence band. ▪This is the lowest band of allowed states. The next higher band of allowed energy levels for the electrons is called the conduction band. ▪Energy level diagrams showing the excitation of an electron from the valence band to the conduction band. The resultant free electrons and holes move under the influence of an external electric field E Energy Bands ▪In a pure crystal at low temperatures, the conduction band is completely empty of electrons and the valence band is completely full. ▪These two bands are separated by an energy gap, or bandgap, in which no energy levels exist. ▪As the temperature is raised, some electrons are thermally excited across the bandgap. For Si this excitation energy must be greater than 1.1 eV, which is the bandgap energy. ▪equal electron and hole concentrations in an intrinsic semiconductor created by the thermal excitation of electrons across the bandgap. The concentration of electrons and holes is known as the intrinsic carrier concentration ni, and for a perfect material with no imperfections or impurities it is given by where N-type Material The ionization of donor impurities increases the electron concentration distribution in the conduction band Donor level in an n-type material P-type material the ionization of acceptor impurities increases the hole concentration distribution in the valence band. Acceptor level in a p-type material Extrinsic semiconductor For an extrinsic semiconductor, the increase of one type of carrier reduces the number of the other type. In this case, the product of the two types of carriers remains constant at a given temperature. This gives rise to the mass-action law which is valid for both intrinsic and extrinsic materials under thermal equilibrium. Since the electrical conductivity is proportional to the carrier concentration, two types of charge carriers are defined for this material: 1. Majority carriers refer either to electrons in n-type material or to holes in p-type material. 2. Minority carriers refer either to holes in n-type material or to electrons in p-type material. The operation of semiconductor devices is essentially based 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 A 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 ▪In order for electron transitions to take place to or from the conduction band with the absorption or emission of a photon, respectively, both energy and momentum must be conserved. ▪Although a photon can have considerable energy, its momentum hv/c is very small. Direct and Indirect Bandgaps ▪Semiconductors are classified as either direct-bandgap or indirect-bandgap materials depending on the shape of the bandgap as a function of the momentum k. ▪Let us consider recombination of an electron and a hole, accompanied by the emission of a photon. ▪The simplest and most probable recombination process will be that where the electron and hole have the same momentum value. This is a direct-bandgap material. ▪For indirect-bandgap materials, the conduction-band minimum and the valence-band maximum energy levels occur at different values of momentum. Here, band-to-band recombination must involve a third particle to conserve momentum because the photon momentum is very small. Phonons (i.e., crystal lattice vibrations) serve this purpose. Electron recombination and the associated photon emission for a direct-bandgap material electron recombination for indirect-bandgap materials requires a phonon of energy E and momentum k ph ph Light-Emitting Diodes (LEDs) Advantage of LED in optical communication: 1. bit rates less than approximately 100–200 Mb/s 2. used with multimode fiber-coupled optical power in the tens of microwatts 3. These LEDs require less complex drive circuitry than laser diodes. 4. No thermal or optical stabilization circuits are needed 5. They can be fabricated less expensively with higher yields. 6. To be useful in fiber transmission applications, an LED must have a high radiance output, a fast emission response time, and a high quantum efficiency. LED Structures (a) Cross-sectional drawing (not to scale) of a typical GaAlAs double- heterostructure light emitter. In this structure, x > y to provide for both carrier confinement and optical guiding; (b) energy band diagram showing the active region, and the electron and hole barriers that confine the charge carriers to the active layer; (c) variations in the refractive index; the lower index of refraction of the material in regions 1 and 5 creates an optical barrier around the waveguide region.  Light-Emitting Diodes (LEDs) Schematic (not to scale) of a high-radiance surface-emitting LED. The active region is limited to a circular section having an area compatible with the fiber-core end face. Light-Emitting Diodes (LEDs) Schematic (not to scale) of an edge-emitting double-heterojunction LED. The output beam is lambertian in the plane of the pn junction (q|| = 120°) and highly directional perpendicular to the pn junction (q^ ª 30°). Light Source Materials Bandgap energy and output wavelength as a function of aluminum mole fraction x for AlxGa1–x As at room temperature. Light Source Materials Spectral emission pattern of a representative Ga1–x Alx As LED with x = 0.08. The width of the spectral pattern at its half-power point is 36 nm. Light Source Materials Relationships between the crystal lattice spacing, energy gap, and diode emission wavelength at room temperature. The shaded area is for the quaternary alloy InGaAsP. The asterisk (*) is for In0.8Ga0.2As0.35P0.65 (Eg ª 1.1 eV) lattice-matched to InP. Light Source Materials ▪For simplicity, the notations GaAlAs and InGaAsP are generally used unless there is an explicit need to know the values of x and y. ▪Other notations such as AlGaAs, (Al, Ga)As, (GaAl)As, GaInPAs, Ga1–xAlxAs and InxGa1–xAsyP1–y are also found in the literature. ▪Using the fundamental quantum mechanical relationship between energy E and frequency v, the peak emission wavelength λ in micrometers, bandgap energy Eg in electron volts Example Example Typical spectral patterns for edge- emitting and surface-emitting LEDs at 1310 nm. The patterns broaden with increasing wavelength and are wider for surface emitters  Quantum Efficiency and LED Power The excess carrier density decays exponentially with time according to the relation n0 is the initial injected excess electron density Γ is the time constant of the carrier lifetime. The total rate at which externally thermally carriers are generated = supplied rate + generated rates The equilibrium condition is found by equating to zero, yielding J is the current density in A/cm2, q is the electron charge, d is the thickness of the recombination region internal quantum efficiency If the radiative recombination rate is Rr and the nonradiative recombination rate is Rnr, then the internal quantum efficiency ηint is the ratio of the radiative recombination rate to the total recombination rate: optical power generated internally to LED optical power generated internally to the LED is Pint external quantum efficiency Not all internally generated photons will exit the device. To find the emitted power, one needs to consider the external quantum efficiency ηext. This is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. Only light falling within a cone defi ned by the critical angle Φc will be emitted from an optical source. external quantum efficiency The End