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The Diode Help

By — McGraw-Hill Professional
Updated on Sep 11, 2011

Introduction

The term semiconductor arises from the ability of certain materials to conduct part time. Various mixtures of elements can work as semiconductors. There are two types of semiconductors, called n type, in which most of the charge carriers are electrons, and p type, in which most of the charge carriers are electron absences called holes.  We will learn a little about semiconductor electronic components.

The Diode

When wafers of n-type and p-type material are in physical contact, the result is a p-n junction with certain properties. Figure 16-1 shows the electronic symbol for a semiconductor diode . The n-type material is represented by the short, straight line in the symbol and is the cathode . The p-type material is represented by the arrow and is the anode .

In a perfect diode, electrons flow in the direction opposite the arrow but cannot flow in the direction the arrow points. If a battery and resistor are connected in series with a diode, current flows if the cathode is negative relative to the anode (Fig. 16-2a) but does not flow if the battery is reversed (see Fig. 16-2b). This is an example of something with which you should by now be familiar: the idealized scenario! In the real world, diodes can approach but never attain this state of perfect one-way conduction.

 

Semiconductors The Diode

Fig. 16-1 . Anode and cathode symbology for a diode.

Semiconductors The Diode

Fig. 16-2 . ( a ) Forward bias of a diode results in a flow of current, ( b ) Reverse bias normally produces near-zero current.

Forward Breakover

It takes a certain minimum voltage for conduction to occur when a p-n junction is connected in the manner shown by Fig. 16-2 a . This minimum voltage is called the forward breakover voltage (or simply the forward breakover) for the diode. Depending on the type of material from which a diode is manufactured, the forward breakover voltage can range from about 0.3 V to about 1 V. If the voltage across the p-n junction is not at least as great as the forward breakover, the diode will not conduct appreciably.

Forward breakover voltages of multiple diodes add together as if the diodes were batteries. When two or more diodes are connected in series with their p-n junctions all oriented the same way, the forward breakover voltage of the combination is equal to the sum of the forward breakover voltages of each diode. When two or more diodes are connected in parallel with their p-n junctions all oriented the same way, the forward breakover voltage of the combination is the same as that of the diode whose forward breakover voltage is the smallest. The p-n junction is unique in this respect. It doesn’t conduct perfectly in the forward direction, but it doesn’t act quite like a dc resistor when it is conducting.

Bias

In a p-n junction, when the n-type material is negative with respect to the p-type material, electrons flow easily from n to p . This is forward bias ; the diode conducts well. When the polarity is switched so that the n -type material is positive with respect to the p -type material, it is in a state of reverse bias , and the diode conducts poorly.

When a diode is reverse-biased, electrons in the n -type material are pulled toward the positive charge, away from the junction. In the p -type material, holes are pulled toward the negative charge, also away from the junction. The electrons (in the n-type material) and holes (in the p -type material) become depleted in the vicinity of the junction. This impedes conduction, and the resulting depletion region behaves as a dielectric or electrical insulator.

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