Thanks for the explanation. Maybe I am totally wrong but I guess 
that it is theoretically possible to cut 1 single LED chip from
a big wafer (the unused part will then be cut into small dies 
for smaller LEDs). In that case, a die is only slightly smaller
than the wafer. 

I am not so sure about the technology used for the bigger LEDs. 
Smaller ones use InGaAlP or similar. I guess that the
wafer will be much smaller than 300mm.

Power MOSFET is a somewhat like parallel of lots of smaller
MOSFETs and that is the reason why going to smaller geometries
can drive the Rds-on lower and lower. Still the gate charge 
capacitance goes higher and limit how good a power MOSFET 
can be. In the case of LED emitter, maybe the only limit 
is the thermal limitation.

Regards,
Xiaofan


-------Original Message-----
From: William "Chops" Westfield
Sent: Thursday, September 22, 2005 2:33 PM

On Sep 21, 2005, at 11:02 PM, Chen Xiao Fan wrote:

> Just curious, how big can this one watt LED die be?
> How big can an LED die be (I think it will not
> be 12inch/300mm).
>
> Isn't the single chip LED also cut from a bigger die?
>
I think we have a word definition issue.

A "die" is the smallest piece of circuit that exhibits the desired
circuit behavior.  I think you're thinking of a "wafer", which is a
whole bunch of individual circuit dice laid out on a thin disk of
semiconductor.  "die" == "chip", and a single chip LED has a single
die from the wafer mounted to some sort of lead frame and embedded in
plastic.  I have no idea how big a wafer full of LED dice is; remember
that it's NOT silicon, so it's probably not 300mm either.

It's vaguely interesting that you don't hear about anyone doing
"waferscale" power LED emitters (where "waferscale" implies a single
piece of wafer containing multiple individual devices.)  You'd think
the sort of parallelism that I gather works for power mosfets would
work good for LEDs, too.  Maybe not.  Or maybe that's how they DO work.

High power LEDs in general consist of unusually large LED dice on a
substrate aimed at dissipating a lot of power.  The 5W LEDs out there
seem to have 4 individual LED dice/chips...

BillW

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