Diodes are passive electronic components that primarily act like gates or valves in a circuit, allowing electric current to flow in only one direction. They can also do much more, but we’re jumping ahead here. Let’s start with the Edison Effect.
The Edison Effect (discovered by Thomas Edison in 1884) is the term for what happens when electrons flow from a heated element to a cooler element in a vacuum. The electrons will flow in one direction only - from the hot element to the cool one. This is known as thermionic emission, and with the electrons only flowing one way this action is similar to the function of a check valve in your water system at home.
Electronic components that have this property are called diodes. Current in a diode only flows from one of its two electrodes to the other. These electrodes are called the anode (+), where current enters the device, and the cathode (-), where the current leaves the device. A diode conducts current when it is forward biased and it blocks current when it is reverse biased (bias refers to the flow of current).
At a minimum level of charge difference between the anode and cathode, called forward breakover, current will flow. If the charge is less than or equal then current will not flow. So if the cathode voltage is positive relative to the anode by a large enough amount, the diode will then conduct current. This threshold is called the avalanche voltage, and varies depending on the materials from which the diode is constructed.
Early diodes were made using metal electrodes in a vacuum or gas-filled glass chamber (vacuum tubes). Today’s diodes are constructed from silicon, germanium, or selenium, although silicon is the most widely used. Other materials may also be added depending on the function of the diode, adding to the usefulness of these little devices. The general public is well aware of them through the prolific use of LEDs (light-emitting diodes) in electronic devices, as well as home and industrial lighting. LEDs emit visible light when a current is applied to them.
Semiconductor diodes can also be designed to produce a direct current (DC) when light energy (visible light, infrared (IR), or ultraviolet (UV) hits them. These photovoltaic cells are used for sensors and for generating electricity in solar energy systems. Analog signals can also allow diodes to be used for modulation (influencing the data on a carrier signal), demodulation (recovering the original data from a carrier signal), and signal mixing (creating new frequencies by mixing signals).
Some of the various types on diodes in use today include the following:
- PIN - manufactured to operate as a variable resistor at radio frequency (RF) and microwave frequencies.
- Shottky - uses a junction of semiconductor and metal to produce fast switching action and low forward voltage drop. Used for rectification (conversion of AC to DC), signal conditioning and switching.
- Zener - permits current to flow either forward or reverse when a specific voltage is applied. Used as voltage regulators, surge suppressors and reference sources.
- Laser - emits coherent (laser) light when a current passes through it.
- LED - emits visible light when a current passes through it.
- Photodiode - produces current when light strikes it. Used as sensors to detect light or generate electricity when struck by light.
- Tunnel - in a tunnel (quantum effect) diode, current decreases as the voltage increases. Used as very fast switching devices in computers, high frequency oscillators, and amplifiers.
- Gunn - convert a constant voltage into high frequency oscillations. Used for generating microwave signals.
Diodes are available in a broad range of voltage and current ratings, package sizes, and mountings, depending on required specs and system function. Since electronic devices and systems all work because of electric current, at some point diodes are invariably used in the circuit to do various things, some of which include:
- Rectifying voltage (converting AC to DC)
- Controlling a signal
- Mixing signals
- Protecting circuits from surges and transients
- Regulating voltage
- Protecting against reverse currents
- Functioning as simple digital logic gates
The type of diodes used in a circuit depends on the application, and also the parameters of the final device. Diode specs should always be chosen based on the highest possible reverse voltage (DC blocking voltage) that may be encountered across the component. Other factors that should be considered include forward current rating, forward power dissipation rating, and operating junction temperature. The operating junction temperature is the internal junction temperature of the diode, not the temperature of the system. Overall system temperature and/or exposure to radiation can also affect diode performance characteristics.