Fun with LEDs.

Warning! Techy, nerdy stuff.

Pretty!

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 They were originally made out of Gallium (Ga) & Arsenic (As) instead of Germanium (Ge and now I'm really showing my age!)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!