Why dont they make a "Light Emitting TRIODE"

Can you list some of the advantages they might have?

You could sell them to the crowd that believes in global warming ;-) ...Jim Thompson

One immediately comes to mind: power, ground, and modulation input are the three wires required for a laser transmitter. I generally see these in SFP form, with receiver wires as well... the seventeen other wires kinda obscure the triode nature of the beast.

Because what's the use?

If nothing else, it wouldn't be a triode in this day and age, while a "diode" did morph from describing a two element tube to a two element semiconductor device, other terms have been used to describe the terminal solid state devices.

A diode is the way the device generates light. A "triode" implies some level of control, and that can be had with a separate device. It's not like in the old days where a magic eye tube was put in a glass envelope along with a triode, back then if you needed a triode to control the magic eye, leaving it separate would mean another tube, which were expensive and bulky, and would need another tube socket.

There may be LEDs with seemingly built in controlling devices, but they would then be on the level of an integrated circuit, a transistor feeding an LED, not some device that combines a bipolar transistor and LED.

Some optoisolators give the same function, I imagine. If the put a transistor in to drive the LED, then that's the same effect, you never notice if it's one device or not.

It's a different case with an bipolar transistor that is light activated, I can't remember the name suddenly. There, the base is light sensitive, and since they figured out some functions for that (they needed to detect light, and needed some amplification) then they exist. Indeed, they are the receivers in those optoisolators.Sometimes the base is even brought out, so you can adjust the bias on the base, along with it being light controlled.

Michael

Often photo Darlingtons are used in opto-couplers.

... Phil

That's what I was thinking. The only thing I can think of is to lower the l oad on whatever drives thee LED. They already pull little enough that micro processors can usually drive the anyway but there is always the wattage rac e. You could have power and ground busses where the LEDS are mounted and th e processor coud drive the base which would presumably need less current. T he base resistors could be built in like a digital transistor and it also x ould be confihued as a current regualtor eliminating the limiting resistor and making the device tolerant of more of a range of voltage.

I wouldn't be surprised if of all the companies in the world one of them tr ied the idea but nobody bought them. I can only think of that one advantage to it, and that would not be worth the extra cost IMO. Also, if the transi stor is used as a current regulator then you have dissipation. Unless you g et wild and put a switcher in there. Then you are talking a coil csuae you can't pulse the LED with overcurrent and expect it to last. The cost might end up being a dollar insrtead of the few cents a regular LED and resistor cost.

Making bipolar transistors out of III-V semiconductors (e.g. GaAs, InAs, AlGaAs, InGaAs, InP, GaN, etc) is difficult, and they don't work very well. Plus the extra constraints would make the LET much harder to build.

The main issue in LED design is that the primary photogeneration is very efficient, but the photons are generated deep inside the die, and it's really hard to get them out efficiently. If you just use a regular GaAs PN diode, all the light gets absorbed by the top semiconductor layer.

Then there's the 95% or so that you lose to total internal reflection (taking into account the Fresnel reflection of the light that does make it out), and the 50% you lose due to half the light starting out going the wrong way....

Modern LEDs are heterojunctions, with transparent semiconductor layers top and bottom to reduce absorption, and controlled roughening of the surfaces to let the light rattle around till it escapes.

Cheers

Phil Hobbs

SiGe transistors are super-duper ;-) ...Jim Thompson

Yup, the BFP640 is a fave. But the hole mobility of III-Vs is so horrible, and the minority carrier lifetime so short, that you can't get any beta no matter what you do. Those pesky direct bandgaps again. ;)

Cheers

Phil Hobbs

Rattle around, that's so sophisticated of you, Phil!

Jamie

Well, that's pretty much what happens. Listen carefully and you'll hear it. ;)

Cheers

Phil Hobbs

IIRC that was the holdup with the blue LEDs in the beginning. the rest were two cents and the blue ones were a buck.

One thing I found out is that the white ones mostly have a phosphor. That is probably what made them suitable for use in LCD TVs. If you had red, green and blue LEDs the output was too spikey for good colorimetry.

That was probably a bad move for TV engineers because dropping the TV would bereak the CCFLs in there sometimes. I guess they'll just have to make the PC boards thinner and brittle.

Why were the blue ones so costly?

Are you saying that LED tv screens use white LEDs? (as if they were using florescent bulbs?). I thought the colors themselves were created by RGB the same way as the old CRT sets? (I have not kept up with these newer equipment).

You're not suppposed to drop tv sets :)

They have a large bandgap (an energy range where the quantum mechanics of the solid material does not allow any solution to the electron wave equation). Only a single-event hop, across a large bandgap, by an electron, can create light emission of the required type.

Large bandgap => high frequency light emission (blue end of spectrum) Small bandgap => low frequency light emission (infrared)

That means most materials are unsuitable, right off the bat.

Then, it implies that the possible impurities that would create solutions in the bandgap range are many (and the semiconductor junction won't emit light if those are allowed, or even if microscopic strains and voids are allowed). The earliest usable semiconductor devices were Ge based, because the low bandgap made impurity sensitivity less troublesome than with Si. It was only after decades of Si device production that the first 'coppermine' chips used copper for wiring, because copper is an important impurity and would ruin silicon devices if it weren't carefully contained/controlled.

Another problem, that's hard to explain, is the three-dimensional nature of the 'bandgap'; some materials (silicon being one) have an "indirect bandgap", meaning that an electron transition from lowest-energy-of-high-band to highest-energy-of-low-band requires a momentum change. There's no (not much) momentum in the photon, so another particle must be involved: the complication here, is that the 'single-event hop of an electron' is a fast event, while a dance with other particles involved is slow (and for practical purposes, that means it's disallowed).

Those guys with the Nobel prize for getting it all right, explored many paths of the maze before they found a useful blue glow.