A diode is a non-linear electronic component. When the voltage over the diode is high enough, the diode has a small resistance for the current conducted through the diode. The voltage that is enough for the diode to conduct is called forward voltage and can be for instance 0.65 V. The diode has a high resistance when the current runs in the opposite direction. Leakage current is a small current going in the opposite direction. Since the diode conducts only in one direction, the diode has polarity. The terminal closest to the positive voltage is called anode and the side closest to the negative voltage is called cathode.
The standard semiconductor diode in silicon has one side with p-doping and another with n-doping. The n-doping side has an excess quantity of electrons and the p-side lacks electrons. The boarder between the two sides is called a junction, p-n junction. When the n side is connected to a negative voltage and the p side to a positive voltage, the diode conducts. Electrons on the n side are pushed to the p side, where they fill the “holes” or the lack of electrons.
A rectifier diode is used to protect for instance capacitors from wrong polarity and to convert alternating voltage to direct voltage. Main characteristics of rectifier diodes are current rating (for instance 3 A) and leakage current (for instance 5 µA).
Fast Recovery Diodes
A Fast Recovery Diode (FRD) has a p-n junction optimized for fast recovery (trr). Fast recovery is the time it takes for the diode to start conducting when the reverse current has been removed. FRD-diodes are used to protect power MOSFET’s in inverters from reverse currents caused by inductive loads. To minimize switching losses, high frequency MOSFETs requires fast recovery diodes. The MOSFETs often come with integrated FRD-diodes.
Zener diodes has the same characteristics as other diodes in forward conduction mode. However, they start to conduct in the reverse direction with low resistance when the Zener voltage is reached. Zener diodes are mainly used for limiting voltages and they are offered with standardized values.
Schottky diodes has faster response time which means they can operate at higher frequencies. They often protect the input on digital IC’s from negative transients. The forward voltage drop is often between 0.2 and 0.4 V, depending on the current running through the diode.
Silicon carbide (SIC) has 10 times the breakdown voltage resistance of standard silicon devices. A Schottky-barrier diode in SiC withstands higher voltage, has lower losses and withstands higher currents than diodes in silicon. SIC Schottky diodes rated to 650 V are used in fast power conversion, for instance in power factor correction, battery charging or uninterruptable power supplies.
Transient Voltage Surpressor
A so-called Transient Voltage Surpressor, TVS, is similar to a Zener diode but has a larger surface in the pn junction in order to handle more power. A larger surface and better heat dissipation make it possible to handle larger pulses. TVS ICs often protects fast speed data interfaces because of their fast reaction times.