Figure 1 shows the V-I characteristics of a diode. When forward-biased, the diode begins to conduct current as the voltage across its anode (with respect to its cathode) is increased. When the voltage approaches the so-called knee voltage, about 1 V for silicon diodes, a slight increase in voltage causes the current to increase rapidly. This increase in current can be limited only by resistance connected in series with the diode.
When the diode is reverse biased, a small amount of current called the reverse leakage current flows as the voltage from anode to cathode is increased, this simply indicates that a diode has a very high resistance in the reverse direction. This large resistance characteristic is maintained with increasing reverse voltage until the reverse breakdown voltage is reached. At breakdown, a diode allows a large current flow for a small increase in voltage. Again, a current-limiting resistor must be used in series to prevent the destruction of the diode.
In power electronics, we deal with high voltages and currents. Therefore, the detailed characteristic of a diode (for example, Figure 2) is not important; we can treat a diode as an ideal element (Figure shows the ideal characteristic of a diode) Note that when the diode is forward biased, it has no voltage across it. The current through the diode then depends on the source voltage and other circuit elements. When the diode is reverse-biased, it has no current through it. The voltage across the diode then depends upon the source voltage and other circuit elements.
This characteristic of an ideal diode makes it like a switch that conducts current in only one direction. The switch can turn on and turn off by itself, depending on the polarity of the voltage. Figure 3(a) shows a forward-biased diode, and Figure 3(b) shows its switch equivalent circuit. When a diode’s anode is more positive than its cathode, it can be considered to act like a closed switch. Figure 3(c) shows a reverse-biased diode, and Figure 3(d) shows its switch equivalent. When the diode’s anode is more negative than its cathode, it can be considered to act like an open switch.
The Schottky diode is a low-voltage, high-speed device that works on a different principle from that of the PN Junction diode. It is constructed without the usual PN junction. Instead, a thin barrier metal (such as chromium, platinum, or tungsten) is interfaced with the N-type semiconductor. This construction results in a low on-state voltage (about 0.6 V) across the diode when it conducts. Furthermore, it can turn off much faster than a PN junction diode, so the switching frequency can be high. However, the reverse leakage current is much higher, and the reverse breakdown voltage is lower compared with that of a PN junction diode. Schottky diodes are therefore used as rectifiers in low-voltage applications where the efficiency of conversion is important. These diodes are also widely used in switching power supplies that operate at frequencies of 20 kHz or higher.