1、Semiconductor Devices for Illumination Engineering,LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The quanta of light energy released is approximately proportiona
2、l to the band gap of the semiconductor.,What is LED?,Applications of LEDs,Your fancy telephone, i-pod, palm pilot and digital camera,Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diodes life in comparison to an incandescen
3、t bulb (10 000 hours life time compared to 1000 hours for incandescence light bulb) Efficiency: Diodes emit almost no heat and run at very low amperes. Greater Light Intensity: Since each diode emits its own light Cost: Not too bad Robustness: Solid state component, not as fragile as incandescence l
4、ight bulb,Advantages of Light Emitting Diodes (LEDs),LED chip is the part that we shall deal with in this course,LED design,Luminescence is the process behind light emission,Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of en
5、ergy. Electroluminescence excitation results from the application of an electric field In a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased,Excitation,Electron (excited by the biased forward voltage) is in the conduction band,Ho
6、le is in valance band,Normally the recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band (band exterma). The emission of light is therefore; hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition),E,k,How does i
7、t work?,A typical LED needs a p-n junction,Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen,There are a lot of electrons and holes at the junction due to excitations,Electrons from n need to be injected to p to promote recombination,Recombin
8、ation produces light!,Recombination and Efficiency,EC,EV,EF,Ideal LED will have all injection electrons to take part in the recombination process In real device not all electron will recombine with holes to radiate light Sometimes recombination occurs but no light is being emitted (non-radiative) Ef
9、ficiency of the device therefore can be described Efficiency is the rate of photon emission over the rate of supply electrons,Fig. 1 Visible and near-visible electromagnetic spectrum. The visible portion is expanded at the top, and divided into major color bands. Also indicated is relative luminosit
10、y function V() as defined by the CIE for normal photopic vision.,Fig. 2 Basic recombination transitions in semiconductor. ED, EA, E, are donor-type, acceptor-type, and deep-level traps respectively.,Basic recombination transitions in semiconductor,Fig. 3 The three basic optical processes between two
11、 energy levels.,Fig. 4 (a) Composition dependence of the direct and indirect bandgap for GaAs1-xPx.,Near the band edges, the energy of the emitted photon is governed by the relationship,where mr* is the reduced effective mass,A joint density of states can be obtained as,The distribution of carriers
12、is governed by the Boltzmann distribution,The spontaneous emission rate is proportional to the product,Fig. 5 Theoretical spectrum of spontaneous emission.,Theoretical spectrum of spontaneous emission,Fig. 6 (a) GaAs diode emission spectra at 300 and 77 K. (b) Dependence of emission peak and half-po
13、wer width as a function of temperature.,Experimental spectrum of spontaneous emission,Device Structures,Fig. 7 (a) Under forward bias of a p-n junction, electrons injected from n-side recombine with holes injected from p-side. (b) Higher carrier densities and improved carrier confinement in a double
14、 heterojunction.,Materials of Choice,Fig. 8 Semiconductors of interest for LEDs, including the relative luminosity function of the human eye.,Fig. 9 (a) Quantum efficiency of GaAs,PX vs. alloy composition, with and without isoelectronic impurity nitrogen. (b) Peak emission wavelength vs. composition
15、,Quantum efficiency,Definitions of Efficiencies,Internal Quantum Efficiency,The internal quantum efficiency in is the efficiency of converting carrier current to photons, defined as,For low-level injection, the radiative recombination rate in the p-side of the junction is given by,where Rec is the r
16、ecombination coefficient and n is the excess carrier density which is much larger than the minority carrier density in equilibrium n npo,Rec, is 10-10 cm3/s for direct-bandgap materials, and 10-15 cm3/s for indirect-band gap materials.,For low-level injection n p0 the radiative lifetime r is related
17、 to the recombination coefficient by,and,External Quantum Efficiency,where op is the optical efficiency,Optical Efficiency,Fig. 10 Optical paths at the semiconductor ambient interface. A: Normal incidence has little effect. B: Angles of refraction (0 s,) corresponding to Snells law. C: Ray outside t
18、he light escape cone (s c) has total reflection.,For GaAs (ns = 3.66) and GaP (ns = 3.45), the critical angle is about 160-170.,Three major loss mechanisms reduce the quantity of emitted photons: Absorption within the LED material, Fresnel loss (reflectance), Critical-angle loss. The absorption loss
19、 for LEDs on GaAs substrates is large since the substrate is opaque to light and it absorbs about 85% of the photons emitted at the junction. For LEDs on transparent substrates such as GaP with isoelectronic centers, photons emitted downward can be reflected back with only about 25% absorption. The
20、solid angle of the light-escape cone can be calculated to be,Fig. 11 LED structures for optical-efficiency consideration: (a) planar. (b) hemispherical. c) parabolic. (d) Their normalized Lambertian emission patterns.,Fig. 12 Historic development of maximum light extraction efficiency for AlGaInP (r
21、ed circles) and InGaN (blue squares) LEDs.,Power Efficiency,The power efficiency p is simply defined as the ratio of the light power output to the electrical power input,Since the bias is approximately equal to the energy gap and light energy (qV hv), it follows that the power efficiency is similar
22、to the external quantum efficiency p exLuminous Efficiency,The brightness of light output is measured by the luminous flux (in lumens),The maximum luminous efficiency has a value of 683 lm/W.,Table 1. Radiometric and photometric quantities and units. The photometric units are lumen (lm), lux (lx = l
23、m/m2) and candela (cd = lm/sr),Fig. 13 LED structures showing the direction of emitted light from (a) surface emitter and (b) edge emitter,Two types of LED,Frequency Response,The frequency response is another important parameter to be considered in the design of LEDs for high-speed applications such
24、 as optical-fiber communication systems. It determines the maximum frequency at which the LEDs can be turned on and off, And, thus, the maximum transmission rate of data. This cutoff frequency of an LED is given by,where is the overall lifetime defined as,White LEDs,Fig. 14 Different strategies to g
25、enerate white light with LEDs. (a) Addition of R, G, and B LEDs, (b) blue LED and yellow phosphor, (c) UV LED (invisible) and R, G, and B phosphors.,Fig. 15. (a) Scheme and (b) image of color conversion LED, (c) Spectrum (solid line) of white LED with blue LED pumping a yellow phosphor together with
26、 eye-sensitivity curveV () (dashed line).,Fig. 16 Historic development of the flux (in lumen) and cost (in $/lm) for semiconductor LEDs.,Quantum Well Light-Emitting Diode,As a practical example, we here investigate single-quantum-well (SQW) InGaN/GaN blue light-emitting diodes. These devices exhibit
27、 an output power of 4.8 mW at 20 mA injection current and 3.1 V forward voltage. The low voltage results in a relatively high power efficiency of 7.7% (ratio of light power to electrical power). The external quantum efficiency is as high as 8.7% despite the large dislocation density of about 1010 cm
28、2 .,Fig. 17 Schematic of single quantu well band diagram at zero bias. Electrons and holes are confined in the well and recombine,Polarization Effects,Fig. 18 Recombination in a quantum well in the absence of a piezoelectric field, and (B) in the presence of a transverse electric field where the ove
29、rlap of wave functions decreases,Fig. 19 Blue InGaN/GaN MQW LED layer structure. The active region is composed of alternating InGaN quantum wells with GaN barriers.,Multi Quantum Well (QW) LED,Fig. 20 Energy band diagram for the InGaN MQW Blue LED.,Energy band diagram,Quantum Dot LED,Fig. 21 (a) Sch
30、ematic cross-section of QD LED. Current is fed to a single QD via an oxide aperture. (b) Plan-view SEM image of QD LED. (c) Electroluminescence (EL) spectrum (T = 10K, U = 1.65 V, I = 0.87 nA) of single In-GaAs/GaAs QD LED (diameter of oxide aperture 0.85 m, thickness 60 nm). The single line is due
31、to (neutral) exciton recombination. The inset shows dependence of EL spectrum on injection current; at higher currents a second peak due to biexciton recombination (XXX) appears.,Organic LED,Fig. 22 Typical OLED design for (a) bottom and (b) top emission,Fig.23 (a) Transparent OLED panel. (b) Flexible OLED display. (c) 3mm thin, 11 inch diagonal OLED TV.,
