Shunting layer arrangement for LEDs
A shunting pattern on a surface of an LED die comprises an array of metal dots having widths that are on the order of 2Lt-5Lt (where Lt is transfer length) so as not to block a significant amount of light, yet have low contact resistance to the semiconductor current spreading layer. Contact resistance is not significantly reduced with widths greater than 2Lt. Each dot represents a current injection area. For a minimum 2Lt width and 50 square dots per mm2, the top surface area of an LED die will have about 1% of its surface covered by the dots. To cause the current to be evenly distributed over the top surface of the LED, the dots are connected with a grid of very thin metal connectors, having widths much less than 2Lt. In one embodiment, a wire bond electrode is formed near the middle of the top surface of the LED to create a more uniform current distribution.
1. A light emitting diode (LED) device comprising:
an LED die comprising a light emitting semiconducting layer epitaxially grown over a growth substrate and extending substantially entirely across the LED die, the LED die having a top surface comprising a current spreading layer covering the semiconductor layer; and
a metal electrode pattern on only a portion of the top surface for conducting current through the LED for energizing the LED, the electrode pattern comprising:
a plurality of opaque metal contacts over the top surface having widths between about 2 and 10 times a transfer length L t of the contacts, where the transfer length is defined as,
L
t
=
ρ
c
R
s
,
wherein R s is the sheet resistance in ohms per square of the current spreading layer, and ρ c is the contact resistivity of the interface of the contact and the current spreading layer expressed in ohm·m 2 ,
wherein the opaque metal contacts block light emitted by the light emitting semiconducting layer; and
metal connectors connecting ones of the contacts together, the metal connectors having widths less than 2L t .
2. The device of claim 1 wherein the plurality of metal contacts over the top surface have widths between about 2 and 5 times a transfer length L t of the contacts.
3. The device of claim 1 wherein a total area of the contacts is less than 2% of a light emitting surface of the LED die.
4. The device of claim 1 wherein a total area of the contacts is less than 5% of a light emitting surface of the LED die.
5. The device of claim 1 wherein a total area of the contacts is less than 10% of a light emitting surface of the LED die.
6. The device of claim 1 wherein the contacts are substantially circular, and the widths are diameters of the contacts.
7. The device of claim 1 wherein the contacts are polygons.
8. The device of claim 1 further comprising a wire bond electrode connected to the contacts by at least one of the metal connectors.
9. The device of claim 8 wherein the metal connectors form a grid of parallel and perpendicular connectors.
10. The device of claim 8 wherein the metal connectors radially extend from the wire bond electrode.
11. The device of claim 8 wherein at least some of the contacts increase in size as the contacts are further from the wire bond electrode.
12. The device of claim 8 wherein a density of the contacts increase as the contacts are further from the wire bond electrode.
13. The device of claim 8 further comprising a dielectric layer between the wire bond electrode and the current spreading layer to reduce a current density between the wire bond electrode and the current spreading layer.
14. The device of claim 13 wherein the wire bond electrode extends over an edge of the dielectric layer by a distance Wx around the dielectric layer, wherein 0.5L t <Wx<L t .
15. The device of claim 8 further comprising a concentric shunting ring surrounding the wire bond electrode at a certain distance to reduce current crowding under and around a periphery of the wire bond electrode, wherein there are no metal contacts between the shunting ring and the wire bond electrode.
16. The device of claim 15 wherein a width of the shunting ring is between 0.1L t and L t , and wherein a diameter of the shunting ring is at least 20% larger than a diameter of the wire bond electrode.
17. The device of claim 1 further comprising a metal shunt around a perimeter of the LED die top surface, the shunt having a first width along edges of the die and a narrower width at corners of the die for reducing current density at the corners of the die.
18. The device of claim 17 wherein the first width of the shunt along the edges of the die is larger than L t , and the narrower width of the shunt at the corners of the die is less than 0.1L t for reducing current density at the corners of the die.
19. The device of claim 1 wherein the growth substrate has been removed.