This invention relates generally to lighting techniques, and in particular to techniques for high current density LED devices fabricated on bulk gallium and nitrogen containing polar, semipolar or nonpolar materials. The invention can be applied to applications such as white lighting, multi-colored lighting, lighting for flat panel displays, other optoelectronic devices, and similar products.
In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to AC power or DC power. The conventional light bulb can be found in houses, buildings, and outdoor lighting, as well as in other areas requiring light. Unfortunately, more than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb routinely fails often due to thermal expansion and contraction of the filament element.
Fluorescent lighting overcomes some of the drawbacks of the conventional light bulb. Fluorescent lighting uses an optically clear tube structure filled with a noble gas and mercury. A pair of electrodes is coupled between the halogen gas and couples to an alternating power source through a ballast. Once the gas has been excited, the resulting mercury vapor discharges to emit UV light. Usually the tube is coated with phosphors excitable by the UV emission to make white light. Many building structures use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket.
Solid state lighting techniques are also known. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Red LEDs use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue emitting LEDs. The blue colored LEDs lead to innovations such as state white lighting, and other developments. Other colored LEDs have also been proposed, although limitations still exist with solid state lighting. Further details of such limitations are described throughout the present specification and more particularly below.
A challenge for solid state lighting is the high cost of LED-based lighting. Cost often is directly proportional to the semiconductor material real estate used to produce a given amount of light. To reduce cost, more lumens must be generated per unit area of semiconductor material. Conventional InGaN LEDs, however, suffer from efficiency “droop” where internal quantum efficiency reduces as current density is increased. The current density for maximum efficiency, Jmax, is typically 1 A/cm2 to 10 A/cm2 which is a very low current density. Also, at higher power densities, current crowding and thermal gradients can result in poor performance and reliability issues. These phenomena make it difficult to reduce cost by increasing current density, as a minimum efficiency is necessary to provide energy savings for LEDs above conventional approaches like fluorescent and incandescent lighting. These and other limitations are described in further detail throughout the present specification and more particularly below.
From the above, it is seen that techniques for improving optical devices is highly desired.
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