Semiconductor diodes are solid-state devices that conduct electrons in a single direction and use a joined positive (P)-type and negative (N)-type semiconductor. When the N-type material is negative, electron donors release electrons toward the more positive P-type semiconductor, resulting in a forward bias conduction. A reverse bias condition occurs when the P-type material is negative and the N-type material is positive. Semiconductor diodes are very much like one-way valves used for water pumps. When the pump is turned off, water does not flow back because the one-way valve prevents it, but when the pump is running, water flows through as if the valve is not there at all.
The first semiconductor diodes were gaseous, had a directly heated cathode and a plate, and were inside a vacuum tube. When a negative charge is available at the cathode, thermal energy makes electrons fly through the vacuum and get attracted to the positively charged plate. With a positive cathode, there are no electrons flowing from the plate. This mechanism made the first power rectifiers possible, which converted alternating current (AC) to direct current (DC).
Small signal diodes have very low forward voltage drop, making them useful for signal detection and low-voltage switching. For radio frequency applications, germanium semiconductors with a metal to semiconductor junction are used for low-level detection and other low-signal-level conversions. Various types of small signal switching diodes are categorized by several factors, including switching speed and junction capacitance.
Schottky diodes are semiconductor diodes that are specially constructed using a semiconductor joined to a metal. The resulting forward voltage drop is about 0.5-volt direct current (VDC). Schottky diodes are used for clamping applications that protect circuitry from experiencing transient voltages more than 1 VDC above the positive DC supply level. This is possible by connecting the anode of a Schottky diode to the signal line being protected while connecting the cathode to the positive supply bus.
Tuning diodes make use of the diode's reverse bias capacitance. When the reverse bias voltage is increased, the capacitance usually decreases due to the effect of virtually decreasing the junction surface area under increased reverse voltage. The DC circuit may handle this adjustable capacitance of the tuning diode. This capacitance is part of an AC circuit that may alter its center frequency based partly on the adjustable capacitance of the tuning diode, resulting in a diode’s ability to tune its circuit.
Silicon diodes typically have a 0.7 VDC forward voltage drop, while germanium diodes have 0.3 VDC. The maximum reverse voltage, known as the breakdown voltage, and the maximum forward currents depend on specific diode designs. For most circuit needs, there are diodes available with the special characteristics needed. If a single diode does not meet the requirements, multiple diodes in series or parallel operation may suffice.