Monday, 6 May 2013

Fun with LEDs.

Warning! Techy, nerdy stuff.
When I were a lad (!), I remember the invention of the Light-Emitting Diode (LED). They were originally made out of Gallium (Ga) & Arsenic (As) instead of Germanium (Ge and now I'm really showing my age!) and Silicon (Si). Gallium & Arsenic are used to "dope" Silicon to form P & N regions respectively (Gallium has 3 electrons in its outer shell & Arsenic has 5. Germanium & Silicon have 4).

GaAs red LEDs weren't very bright. By adding Aluminium, Indium, Phosphorus, Nitrogen etc, new colours & higher-efficiency old colours were invented. Orange. Yellow. Green. Brighter green. Even brighter green. Really bright green. I thought that blue LEDs would never be invented. Wrong!

Nowadays, OLEDs are so efficient that they can be used for lighting and they are more efficient and longer-lasting than CFLs. I thought that OLEDs would never catch on. Wrong! The superb display on my Samsung phone uses AMOLED technology. But anyway...

What I found interesting about GaAs red LEDs was their I-V characteristic.
Pretty techy!
Over a wide range of currents, the voltage is ~1.75V. The steep slope means that the dynamic resistance (δV/δI) is very low. I thought to myself "Hmmm, voltage regulator!" Zener diodes are usually used as voltage regulators, but they are very noisy. A forward-biased P/N junction produces less thermal noise than a resistor with the same value as the dynamic resistance of the P/N junction. As the voltage (~1.75V) is temperature-dependent (-2mV/ºC), the relative temperature variation of a red GaAs LED is less than that of a Silicon diode (~0.7V when biased on).

I used two "strings" of red GaAs LEDs as an ultra-low-noise voltage limiter in a high-power oscillator using LDMOS MOSFETs that had just been invented by Mullard (which later became Philips). It produced 1W (+30dBm) over a frequency range of 30 to 88MHz and had a Carrier to Noise Ratio (C/N) of >190dBc/Hz @10% frequency offset. Typical RF Signal Generators of that era had a C/N of ~145dBc/Hz at that offset.

I hope that you've enjoyed this little tour around my brain!


Praguestepchild said...

This reminds me of how much I've forgotten of EE, it was mostly over my head.

Nigel Kinbrum said...

It was easy to tell if my oscillator was working, as all of the LEDs were lit! I thought that it was about time I did another post based on my old job.

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