Posted on 27 November 2012

Rectifier Diode Basics

 

 

 

 

 

 

 

Rectifier diodes are components with two terminals and are used to rectify alternating currents. Today, the semiconductor diodes used to rectify line voltages are produced mainly on the basis of monocrystalline silicon. Rectifier diodes have an asymmetric current-voltage characteristic (Figure 1).

Current-voltage characteristic of a rectifier diode with voltage directions, current/voltage areas and equivalent resistance line

Figure 1. Current-voltage characteristic of a rectifier diode with voltage directions, current-voltage areas, and equivalent resistance line

A distinction is made between diodes whose rectifying effect is caused by the transition of mobile charge carriers from an n-doped to a p-doped area in the semiconductor, pn-diodes, and Schottky diodes, where a metal-semiconductor junction produces the rectifying effect.

Schematic layout of a pn-diode

Figure 2. Schematic layout of a pn-diode (a) and a Schottky diode (b); The glassivation and the guard ring with oxide cover provide protection from environmental impacts and stabilize reverse currents

pn-diodes (pin-diodes)

A pn-diode consists of a heavily p-doped p+ layer with many free-moving holes, a heavily n-doped n+ layer with many free-moving electrons and a weakly n-doped n- layer (also called i layer, i meaning intrinsic) in between, whose width wp and doping determine the maximum blocking voltage. The electrons and holes present in the vicinity of the pn-junction recombine with each other, which is why they are no longer available for current transmission. Thus, an insulating thin layer with no free-moving charge carriers is created. This is called the space charge region, since a potential difference between p-doped and n-doped silicon is built up here as a result of the non-mobile charges of ionized doping atoms. This happens without external voltage being applied.

If a negative voltage is applied to the p-silicon and a positive voltage to the n-silicon, free electrons in the n-silicon are pulled toward the cathode, and the holes in the p-silicon toward the anode. Owing to this effect, the insulating space charge region is widened and the electric field intensity in the vicinity of the pn-junction increases. The diode is poled in reverse direction and there is almost no current flow. Even if a diode is poled in reverse direction, a small current will flow. This is called the leakage current. This current results from the fact that free charge carrier pairs are generated in the space charge region as a result of thermal energy or irradiation. These charge carrier pairs are separated in the field of the space charge region and drained toward the terminals.

If a positive voltage is applied to the p-silicon and a negative voltage to the n-silicon, the free electrons in the n-silicon and the holes in the p-silicon are pushed into the space charge region. The space charge region is flooded by mobile charge carriers and disappears. A current flows and more charge carriers are supplied from the outer circuit. The diode is poled in forward direction (see figure 1).

Schottky diodes

In Schottky diodes, the metal-semiconductor contact (Schottky contact) carries out the tasks of the pn-junction. The biggest difference between pn-diodes and Schottky diodes is that in pn-diodes both electrons and holes assist in current transmission (the pn-diode is a bipolar component), whereas only one type of charge carrier is responsible for current transmission in Schottky diodes (unipolar component). This has a particularly strong affect on the dynamic behaviour.

 

For more information, please read:

P-N Junction Diode

Criteria for Successful Selection of Diodes and Thyristors

Rated Current Load for Rectifier Diodes

Snubber Circuits Based on Silicon Avalanche Diodes

 

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