CHAPTER 9: PHOTONIC DEVICES Part 2

Semiconductor lasers Semiconductor lasers are diodes that emit coherent light by stimulated emission. They consist of a p-n junction inside a slab of semiconductor that is typically less than a millimeter in any dimension. Excitation is provided by current flow through the device, and the cleaved ends of the diode provide the feedback mirrors.

Semiconductor laser structures Homojunction 40,000 A/cm2 Single heterojunction 10,000 Double heterojunction 1,300 Double heterojunction, large optical cavity 600

Top: Homojunction. Middle: Single heterojunction Top: Homojunction. Middle: Single heterojunction. Bottom: Double heterojunction

SEMICONDUCTOR LASER Semiconductor Laser Materials: Have direct Eg Laser emission : range from 0.3 to 30m 3 most important III-V compound: GaxIn1-xAsyP1-y GaxIn1-xAsySb1-y AlxGa1-xAsySb1-y

Figure 9.18. Energy band gap and lattice constant for three III-V compound solid alloy system.

SEMICONDUCTOR LASER LASER OPERATION To enhance stimulated emission –need population inversion – consider p-n junc. Or a heterojunction formed between degenerate semicond. Both sides of the junc. is high – EFV below EV edge on the p-side – EFC is above the EC edge on the n-side When large bias is applied – high concen. of electrons & holes are injected into the transition region – region d contains large conc. of electrons in EC & large conc. of holes in the EV Condition necessary for population inversion: (EFC-EFV)>Eg

Figure 9.20. Comparison of some characteristics of (a) homojunction laser and (b) double-heterojunction (DH) laser. Second from the top row shows energy band diagrams under forward bias. The refractive index change for a homojunction laser is less than 1%. The refractive index change for DH laser is about 5%. The confinement of light is shown in the bottom row.

SEMICONDUCTOR LASER CARRIER & OPTICAL CONFINEMENT Double-hetero (DH) laser carriers are confined on both sides of the active region by the heterojunction barriers Optical field is also confined in the active region (see fig. 9.21) Refractive indices: ray angle 12 at the layer 1/layer 2 interface – exceed the critical angle by: Similar situation occurs for 23 at the layer 2/layer 3 interface When refractive index (in active layer) > index of its surrounding layers – propagation of the optical radiation is confined in a direction parallel to the layer interfaces

SEMICONDUCTOR LASER Figure 9.21. (a) Representation of a three-layer dielectric waveguide. (b) Ray trajectories of the guided wave.

SEMICONDUCTOR LASER c: constant : difference in the refractive index Confinement factor  (ratio of the light intensity within the active layer to the sum of light intensity – within & outside the active layer) c: constant : difference in the refractive index d: thickness of the active layer Larger & d  higher 

SEMICONDUCTOR LASER : refractive index in semicond. OPTICAL CAVITY & FEEDBACK Condition necessary to produce laser action: population inversion Photons released by stimulated emission – cause further stimulations as long as there is population inversion  phenomenon of optical gain To increase gain – multiple passes of a wave must occur – achieved using mirrors placed at either end of the cavity (reflection planes at left side & right side) Reflectivity R at each mirror: : refractive index in semicond. corresponding to the wavelength Generally a function of 

SEMICONDUCTOR LASER If an integral no. of ½  fit between the 2 planes – reinforced & coherent light will be reflected back & forth within the cavity For stimulated emission – length L of cavity must satisfy the condition m: integral number

Many values of  can satisfy the condition in fig. 9.22(a) Figure 9.22. (a) Resonant modes of a laser cavity. (b) Spontaneous emission spectrum. (c) Optical-gain wavelengths. Many values of  can satisfy the condition in fig. 9.22(a) Only those within the spontaneous emission spectrum will be produces – fig. (b) Optical losses in the path traveled by the wave mean that only strongest lines will survive – leading to a set of lasing modes – fig. (c) Separation  between allowed modes in the longitudinal direction – is the difference in the  corresponding to m & m+1

SEMICONDUCTOR LASER Fig.9.23(a) – basic p-n junction laser (homojunction laser) A pair of parallel planes are cleaved/polished perpendicular to the 110 axis – laser light will be emitted from these planes To remaining sides of the diode are roughened to eliminate lasing in other directions Structure called a Fabry-Perot cavity Fig.9.23(b) – Double heterostructure (DH) laser thin layer is sandwiched between layers of different semicond. Broad area laser – entire area along the junction plane can emit radiation Fig.9.23(c) - DH laser with a stripe geometry Oxide layer isolates all but stripe contact The lasing area is restricted to a narrow region under the contact Advantages of stripe geometry – reduced operating current, elimination of multiple emission areas along the junction & improved reliability

SEMICONDUCTOR LASER Figure 9.23. Semiconductor laser structure in the Fabry-Perot-cavity configura-tion. (a) Homojunction laser. (b) Double-heterojunction (DH) laser. (c) Stripe-geometry DH laser.

Photo detectors Semicond. device that can convert optical signal into electrical signal If light of the proper wavelength is incident on the depletion region of a diode while a reverse voltage is applied, the absorbed photons can produce additional electron-hole pairs. Photon detectors may be further subdivided into the following groups: • Photoconductive. The electrical conductivity of the material changes as a function of the intensity of the incident light. Photoconductive detectors are semiconductor materials. They have an external electrical bias voltage. • Photovoltaic. These detectors contain a p-n semiconductor junction and are often called photodiodes. A voltage is self generated as radiant energy strikes the device. The photovoltaic detector operate without external bias voltage. A good example is the solar cell used on spacecraft and satellites to convert the sun’s light into useful electrical power. • Photoemissive. These detectors use the photoelectric effect, in which incident photons free electrons from the surface of the detector material. These devices include vacuum photodiodes, bipolar phototubes, and photomultiplier tubes.

PHOTODETECTOR Operations: Carrier generation by incident light, carrier transport &/or multiplication by whatever current gain mechanism Interaction of current with the external circuit to provide output signal Applications: Infrared sensors in opto-isolators & detectors for optical-fiber communications Consist of: Photoconductor Photodiode Avalanche Photodiode (APD)

PHOTOCONDUCTOR When incident light falls on the surface of photoconductor – electron-hole pairs are generated – conductivity increased For intrinsic photoconductor - Increasing of conductivity under illumination – due to the increase in the no. of carriers For extrinsic photoconductor – photoexcitation may occur between the band edge & energy level in the Eg Photocurrent between electrodes: Primary photocurrent: = carrier transit time : quantum efficiency Popt:incident optical power hv: photon energy : carrier lifetime E: electric field Photocurrent gain:

PHOTOCONDUCTOR (Cont.) Figure 9.30. Schematic diagram of a photoconductor that consists of a slab of semiconductor and two contacts at the ends.

PHOTODIODE basically a p-n junction or a metal-semicond. contact operated under reverse bias. When optical impinges the photodiode – depletion region separate the photogenerated electron-hole pairs For high freq. operation – depletion region must be kept thin – to reduce transit time Quantum efficiency (no. of electron-hole pairs generated for each incident photon): Response speed is limited by 3 factors: Diffusion of carriers Drift time in the depletion region Capacitance of the depletion region

p-i-n photodiode A p-i-n photodiode (also called PIN photodiode) is a Photodiode with an intrinsic (i) (i.e., undoped) region in between the n- and p-doped regions. Compared with an ordinary p-n photodiode, a p-i-n photodiode has a thicker depletion region, which allows a more efficient collection of the carriers and thus a larger quantum efficiency, and also leads to a lower capacitance and thus to higher detection bandwidth.

P-i-n Photodiode (Cont.) Its depletion region thickness can be tailored to optimize the quantum efficiency & freq. response Light absorption in the semicond. Produces electron-hole pairs Pairs produced in the depletion region or within a diffusion length – separated by the electric field current flows in the external circuit as carriers drift across the depletion layer Figure 9.32. Operation of a p-i-n photodiode. (a) Cross-section view of a p-i-n photodiode. (b) Energy band diagram under reverse bias. (c) Carrier absorption characteristics.

Figure 9.33. Metal-semiconductor photodiode.

                                                                                                                                                              avalanche photodiode A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. The avalanche photodiode (APD), is also reverse-biased. The difference with the PIN diode is that the absorption of a photon of incoming light may set off an electron-hole pair avalanche breakdown, creating up to 100 more electron-hole pairs. "This feature gives the APD high sensitivity" (much greater than the PIN diode). Figure 9.34. A typical silicon avalanche photodiode: (a) device structure and (b) quantum efficiency.

A solar cell consists of two layers of semiconductor, p-type and n-type, sandwiched together to form a 'pn junction'. This pn interface induces an electric field across the junction. When 'photons are absorbed by the semiconductor, they transfer their energy to some of the semiconductor's electrons, which are then able to move about through the material. For each such negatively charged electron, a corresponding mobile positive charge, called a 'hole', is created. In an ordinary semiconductor, these electrons and holes recombine after a short time and their energy is wasted as heat. SOLAR CELL Advantages: Can convert sunlight directly to electricity with good conversion efficiency Provide nearly permanent power at low operating cost Non polluting

P-N JUNCTION SOLAR CELL Figure 9.36. Schematic representation of a silicon p-n junction solar cell.

P-N JUNCTION SOLAR CELL (Cont.) Constant-current source – parallel with the junction Source IL: results from the excitation of excess carriers by solar radiation Is: diode saturation current RL: load resistance Figure 9.37. (a) Energy band diagram of a p-n junction solar cell under solar irradiation. (b) Idealized equivalent circuit of a solar cell.

P-N JUNCTION SOLAR CELL (Cont.) Figure 9.38. (a) Current voltage characteristics of a solar cell under illumination. (b) Inversion of (a) about the voltage axis.

will be tomorrow on Thursday Test 2 will be tomorrow on Thursday 16th Oct. 2008 @ K. Perlis (DKP1) 8.30pm – 9.30pm

The attendance is Compulsory The presentation of Miniprojects (with Assignments) will be on Wednesday 22/10/2008 @ K. Perlis (DKP1) 8-10am The attendance is Compulsory