A junction diode is a semiconductor crystal with two electrical terminals that conducts current in one direction. It can change AC to DC, sense temperature, protect circuits, create and perceive light, and form logic gates. When a forward bias voltage is applied, current flows easily, while a reverse bias voltage widens the depletion layer and makes current flow more difficult. Junction diodes have many uses, including rectification, surge protection, and sensing light. They can also act as controlled variable voltage capacitors and are used in various electronic devices.
A junction diode is a semiconductor crystal, usually silicon, with two connected electrical terminals. A PN junction diode is the most common type of semiconductor diode. The characteristics of the junction diode usually allow it to easily conduct current in one direction but not the other. Junction diodes can be used to change alternating current (AC) to direct current (DC), sense temperature, and protect circuits from damaging voltages. They can also create and perceive light, form logic gates, and perform many other functions. Different types of junction diodes are used in devices such as radios, TVs and CD drives, among many other electronic devices.
When a junction diode is made, its crystal is implanted with p-type positive charge carriers, called holes, on one side. The other side is implanted with n-type negative charge carriers, which are electrons. The thin region in between is known as the PN junction. Some electrons wander through the junction to combine with holes and vice versa. This creates a narrow region of neutral charge around the junction, called the depletion layer.
When a forward bias voltage is applied across the junction diode, it typically forces more electrons into the n-type region. It also forces more holes into the p-type region. As this voltage increases, the depletion layer shrinks. This makes it easier for current to flow through the junction. Once the forward bias exceeds a certain voltage, current can flow quite easily.
If the opposite, a reverse bias voltage is applied, more holes can be extracted from the p-type region and more electrons from the n-type region. Holes and electrons are pushed away from the junction, widening the depletion layer. This typically makes current flow more difficult. As the reverse bias voltage increases, the current through the junction slows to near zero. The remaining “leakage” current is often very small but can increase with diode junction temperature.
A junction diode has many uses related to its ability to conduct current in only one direction. For example, it can convert AC to DC, also known as rectification. It can also separate the audio signal from the radio frequency (RF) signal in a radio receiver. In control circuits, junction diodes can provide surge protection when a high-current device, such as a motor or relay coil, is turned on or off. Many types of integrated circuits use diodes on each pin to prevent excessive external voltages from damaging the chip.
Junction diodes can be very sensitive to light without the dark plastic they are typically encased in. They are commonly used as photodiodes to sense light and in solar cells to convert light into electricity. A light emitting diode (LED) is a junction diode that generates photons. LEDs exist in a variety of colors and can produce light from infrared to near ultraviolet. They are also often used as status indicators in electronic devices. A laser diode generates light of a single wavelength which is usually focused through a shiny cavity in its package. Laser diodes are often used in high-speed communications and consumer CD/DVD drives.
Other applications of junction diodes include logic gates, keyboard arrays, temperature sensors, and voltage regulators. A junction diode can also act as a controlled variable voltage capacitor; a radio or television tuning circuit can vary the size of the diode’s depletion layer, which in turn changes the capacitance.
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