In its forward bias, the positive side is connected to the p-type (anode) and the negative side to the n-type (cathode) semiconductor. Charge carriers (electrons and holes) flow towards the depletion region, thinning the region and allowing current to flow. The direction of current and charge flow are opposite.
In reverse biasing, the positive pole is connected to the n-type semiconductor and the negative to the p-type, an arrangement that increases the width of the depletion region. This prevents current from flowing through, so the PN junction effectively blocks the current. Its operation is similar to a check valve.
A diode is a semiconductor device based on the PN junction, so its basic property is that it allows current to flow in one direction and blocks it in the other. The diode's V-I curve is non-linear, with the current increasing exponentially from the forward voltage. The curve is highly dependent on material and type.
The full V-I characteristic also includes the reverse bias behaviour. In the plot, it is clearly shown that there is a small leakage current that increases significantly beyond the breakdown voltage, meaning current will flow.
A special diode that is designed to break down at a specific voltage, called the Zener voltage, in reverse bias. At this voltage, the diode starts to conduct but is not damaged, so it can be used in regulator (stabilizer) circuits where it is important to maintain a fixed voltage.
A diode in which a semiconductor-to-metal junction is used instead of a PN junction. Schottky diodes have the advantage of low forward voltage (0.2-0.3 V) and fast switching. They are excellent for high frequency signals, but cannot withstand as high of a reverse voltage as conventional diodes.
The Transient Voltage Suppression diode can absorb sudden voltage spikes that can occur during for example electrostatic discharge (ESD) or other transients. The TVS diode is designed to protect circuit elements from the harmful effects of high voltage. Its use (and design) is similar to that of the Zener diode.
A similar component is the varistor (VDR), which is a non-linear voltage-dependent resistor. Under normal conditions its resistance is high, in high-voltage cases it is very low, so when connected to the input of a device it can protect it from damage. A TVS diode is able to absorb lower energy, faster transients (e.g. ESD), while a varistor protects against higher energy, slower transients (e.g. mains disturbances).
The LED emits light when current flows through it. When an electron passes from an n-type semiconductor to a p-type semiconductor in the PN junction, it releases energy, and in the case of an LED, the energy is released as light. The wavelength (colour) of the light emitted depends on the materials used, which also means that the forward voltage is highly dependent on the colour.
The special feature of a varactor (also called varicap) diode is that its capacitance changes as the reverse voltage changes. They can be used in radio frequency (RF) circuits, for example for tuning or controlling the frequency of oscillators. Essentially, they are voltage-controlled capacitors.