Diodes are usually referred to as "D" for diode on PCBs. Low energy transient voltages lead to a voltage limitation similar to a zener diode. The crystal detector was the earliest type of semiconductor diode. "R" provides a bias current for both the Zener diode and the transistor. The 1N4148 is a standard silicon switching signal diode. The tunnel diode "TD" has voltage controlled negative differential resistance. Negative resistances can be used as amplifiers, such as the tunnel diode amplifier. Round had constructed a light emitting diode (LED). This is either filtered out from daylight or generated with a monochromatic light-emitting diode (LED). Schottky barriers have rectifying characteristics, suitable for use as a diode. Most laser sights use a red laser diode. It was a near pocket-sized radio featuring 4 transistors and one germanium diode. The inductor would be in parallel with the series diode and load. This is due to the higher current density in the Schottky diode. LPE afforded the technology of making heterojunction diode lasers.