Power diodes, Rectifier diodes, Large signal diodes

Two types of diodes most commonly used are signal diodes and power diodes. The term signal diode generally refers to small signal diodes. The small-signal diodes have low power and current ratings. These diodes are designed for applications like switching circuits, reverse current protection, clipping, clamping, and waveshaping.

One of the most important applications of diodes is voltage rectification. The electrical power is transmitted as alternating current. Power is transmitted as a very high voltage for transmission to long distances, which eventually limits the current. That is why electrical power is transmitted as alternating current. However, the end appliances require direct current for any useful operation. So, all electrical appliances require to convert AC into DC. This may be achieved either by a built-in power unit or in the case of electronic devices (3V~15V) with the help of a power adaptor. We can say that power rectification is a very common process.

Small signal diodes are not designed to handle high power transmitted as alternating current. They have a low forward current rating as well as low peak inverse voltage. If a small signal diode is subjected to a rectification application, it will end up burning instantly. So, there are diodes that are specifically designed for rectification and power applications. These diodes are called rectifier diodes or power diodes. These are also called large-signal diodes as they operate high peaks of electrical signals.

What are power diodes?

Power diodes are semiconductor diodes designed for rectification. As a diode device, these work as one-way electrical valves. The power diodes are designed to have a large PN junction area. This enables them to have a large forward current and tolerate large reverse voltage. A power diode’s typical forward current rating can be as high as in kA, and peak inverse voltage can be in kV. The mains supply voltage is hundreds of voltages – typically 120V ~230V –all around the world. The power diodes are well designed to handle such large voltages. The electrical symbol of a power diode is the same as a generic diode. However, in a proper diagram, the symbol has anode and cathode indicated essentially as A and K, respectively.

Symbol of power diode

Rectifier diode construction

When discussing signal diodes, we had talked about mesa diode construction. The mesa diode is a more reliable structure with many predictable specifications. The power diodes are mostly designed with a mesa construction.

Mesa construction of power diode

A heavily doped N+ region forms the cathode. There is a lightly doped N-epitaxy over it to which the heavily doped P+ region is diffused. The N-epitaxy is called the drift layer. It has an important role in determining the overall junction area. As it is lightly doped, it contributes to the all Ohmic resistance of the diode. High diode resistance causes a high peak inverse voltage.

The power diodes are usually constructed from silicon. That is why they are also called Silicon Power diodes. Many special-purpose power diodes are constructed from gallium arsenide too. The materials used as dopants for the anode are Arsenic, Phosphorous, and Germanium. The materials used as a dopant for cathode are Aluminium, Boron, and Gallium.

Example of a rectifier diode

The power diodes are always marked polarity. A band indicates the cathode on one end of the diode. In some diode models, the cathode end is a bit rounded in shape. Many diodes have the diode symbol printed on the casing or have the polarity indicated by + and – signs. Power diodes are available in DO (diode outline), TO (transistor outline), D2PAK (discrete package), SOD (small outline diode), metal electrode leadless, SOT (small outline transistor) packages.

It should be noted that rectifier diodes are suitable for applications involving frequencies less than 1 MHz. For high-frequency rectification, Schottky diodes are better suited.

Power diode characteristics

As any diode has two possible biasing and two characteristic regions, the power diode too has two distinct electrical regions of operation – forward bias and reverse bias.

When a power diode is forward biased, the holes form heavily doped P+ regions are injected into the lightly doped N- region. The drift layer is lightly doped and cannot recombine all the holes generated. The holes pass through the N- region and penetrate the heavily doped N+ region attracting its electrons. Due to the attraction of positive charge carriers, the electrons from the heavily doped N+ region are also injected into the drift layer and reach the heavily doped P+ region. This is called double injection. All the recombination of holes and electrons happen in the drift layer. Due to the resistive nature of the drift layer and all holes-electrons recombination happening in it, the forward bias current of a power diode is almost linear.

In reverse bias, due to the large current capacity and resistive nature of the diode, a power diode is capable of handling large reverse voltages. Like the peak inverse voltage of a power diode is in hundreds and thousands of volts, its reverse current (inclusive of reverse saturation current) is hundreds of milli ampere. At knee voltage, the reverse current through the diode rises very sharply, resulting in a sudden current surge. That is why power diodes are designed to have a very high PIV rating compared to the voltages they have to deal with. The typical voltage-current characteristics of the power diode are shown below.

VI characteristics of a rectifier diode

Power diode specifications

Some of the important specifications of the power diode are following.

Average forward current: This is the average rectified forward current typically for 50/60 Hz sine wave signal. It is averaged on the period diode is conducting for one half of the AC signal and the other half when the diode is non-conducting.
Maximum repetitive forward current: It is the maximum current a power diode can conduct without damage. This is usually indicated for power rectification of mains line voltage.
Maximum DC reverse voltage: It is the maximum continuous voltage that a power diode can handle. Any voltage spike should be within this tolerable limit. Rectifier diodes are usually connected to a capacitor in their parallel to smooth high voltage spikes.
Repetitive peak reverse voltage is the maximum reverse voltage of an alternating signal that the power diode can handle. The repetitive peak reverse voltage is always smaller than the maximum DC reverse voltage.
Maximum working peak reverse voltage: This is the maximum reverse voltage that the diode can handle at any time. Any peak voltage of an alternating signal or amplitude of a continuous signal should not exceed this value.
Reverse leakage current: It is the reverse current that flows through the power diode in reverse bias. This current is due to the thermal effect and is contributed by minority charge carriers. The leakage current of a power diode can be hundreds of mA.
Reverse recovery time: When the diode switches from forward bias to reverse bias, the time required to drop current from forward current levels to leakage current level is called reverse recovery time. It is typically in nanoseconds. This is an important parameter when a power diode has to be used in high-speed switching applications like the SMPS.
Maximum temperature: It is the maximum temperature that the diode can tolerate. In forward bias, the diode gets heated due to the flow of current. This is the junction temperature of the diode. The ambient temperature also heats the diode. With the temperature rise, the current through the diode increases, and the diode temperature also rises with the increase in current. This can eventually damage the diode or result in unpredictable behavior. Therefore, the diode must be deployed at appropriate operating conditions only.

Rectifier diode reference

The table below lists some of the popular rectifier diode models.