They were filled with a pure inert gas such as neon because mixtures made the output temperature-dependent. Their burning voltage was under 200 V, but they needed optical priming (pre-ionizing) by a 2-Watt incandescent lamp prior to ignition by an anode voltage spike in the 5-kV range. For lower frequency noise bands glow lamps filled with neon have been used. The circuit was similar to the one for spike/needle pulses. One miniature thyratron found an additional use as a noise source, when operated as a diode (grid tied to cathode) in a transverse magnetic field. Forward-biased semiconductor diode Another possibility is using the collector current in a transistor. Reverse-biased semiconductor diode Reverse-biased diodes in breakdown can also be used as shot noise sources. Voltage regulator diodes are common, but there are two different breakdown mechanisms, and they have different noise characteristics. The mechanisms are the Zener effect and avalanche breakdown. Zener diode Reverse-biased diodes and bipolar transistor base-emitter junctions that breakdown below about 7 volts primarily exhibit the Zener effect; the breakdown is due to internal field emission. The junctions are thin, and the electric field is high. Zener breakdown is shot noise. The flicker (1/f) noise corner can be below 10 Hz. The noise generated by zener diodes is a simple shot noise. Avalanche diode For breakdown voltages greater than 7 volts, the semiconductor junction width is thicker and primary breakdown mechanism is an avalanche. The noise output is more complicated. There is excess noise (i.e., noise over and above the simple shot noise) because there is avalanche multiplication. For higher power output noise generators, amplification is needed. For broadband noise generators, that amplification can be difficult to achieve. One method uses avalanche multiplication within the same barrier that generates the noise. In an avalanche, one carrier collides with other atoms and knocks free new carriers. The result is that for each carrier that starts across a barrier, several carriers synchronously arrive. The result is a wide-bandwidth high-power source. Conventional diodes can be used in breakdown. The avalanche breakdown also has multistate noise. The noise output power randomly switches among several output levels. Multistate noise looks somewhat like flicker (1/f) noise. The effect is process dependent, but it can be minimized. Diodes may also be selected for low multistate noise. A commercial example of an avalanche diode noise generator is the Agilent 346C that covers 10 MHz to 26.5