This invention relates generally to light-emitting semi-conductor devices, or light-emitting diodes (LEDs) according to more common parlance, and particularly to those which lend themselves to use, by being arrayed, in small displays, now known as microdisplays, and in the printheads of LED printers, among other applications. The invention also specifically pertains to a method of fabricating such LED arrays.
Microdisplays and printheads incorporating an array of LEDs are now winning ever-increasing commercial acceptance. Generally, an LED array may be envisaged as a group of LEDs arranged in rows and columns on a substrate of electrically insulating or highly resistive material. The individual LEDs are electrically interconnected by conductors in a required pattern for individual excitation.
Among the materials that have been used for LED array substrates are sapphire, silicon carbide, and silicon. Both sapphire and silicon carbide permit layers of semiconducting compounds to be grown favorably thereon. Offsetting this benefit is their expensiveness, being considerably more costly than silicon. An additional drawback of sapphire and silicon carbide is that they are both more permeable to light, blue in particular, than is silicon. The light radiated by the LEDs toward the substrate may penetrate the same and hit its bottom, thereby to be diffusely reflected back through the neighboring LEDs (i.e., dots or pixels) and hence to leak out.
The silicon substrate has no such shortcomings. Besides being cheap, it is free from light leakage. Silicon is less in permeability to light than is sapphire or silicon carbide, so much so that the light from the LEDs is mostly absorbed by the substrate, with little or no leakage through the neighboring LEDs.
Japanese Unexamined Patent Publication No. 2004-195946 is hereby cited as dealing with an LED array on a high-resistance silicon substrate. The LEDs are each constituted of a first semiconductor layer of, say, n-type gallium arsenide (GaAs) and, thereover, a second semiconductor layer of, say, p-type GaAs for emission of light in response to voltage application to these constituent layers. The LEDs glow individually by having their first semiconductor layers electrically interconnected in rows or columns by first wires bonded to their surfaces, and their second semiconductor layers electrically interconnected in columns or rows by second wires bonded to their surfaces. The first semiconductor layers partly protrude from under the second semiconductor layers to provide ledges to which the first wires are bonded.
An objection to this known LED array is that the first semiconductor layers with their protruding ledges must themselves provide parts of the current paths to the LEDs. In order for the first semiconductor layers to perform this function to the full, they must be much thicker than in the case where they are coupled directly to the first wires or equivalent parts, without any such protruding parts. The thicker first semiconductor layers make the complete device correspondingly thicker. Additionally, the thicker first semiconductor layers add to the costs of the materials needed for their fabrication and to the lengths of time required for their epitaxial growth. Thus the prior art LED array was unnecessarily bulky and costly.
A further inconvenience with the prior art manifested itself when the first semiconductor layers of the LEDs were connected to one drive terminal. Such connection required the second wires to be branched at several points, necessitating highly complex LED wirings.