The Diode Help (page 2)
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.
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.
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.
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.
Under conditions of reverse bias, a p-n junction can act as a capacitor. A special type of diode called a varactor (a contraction of the words variable reactor) is manufactured with this property specifically in mind. The junction capacitance of a varactor can be varied by changing the reverse-bias voltage because this voltage affects the width of the depletion region. The greater the reverse voltage, the wider the depletion region gets, and the smaller the capacitance becomes. A good varactor can exhibit a capacitance that fluctuates rapidly, following voltage variations up to high frequencies.
If a diode is reverse-biased and the voltage becomes high enough, the p-n junction will conduct. This is known as avalanche effect . The reverse current, which is near zero at lower voltages, rises dramatically. The avalanche voltage varies among different kinds of diodes. Figure 16-3 is a graph of the characteristic current-versus-voltage curve for a typical semiconductor diode showing the avalanche point . The avalanche voltage is considerably greater than and is of the opposite polarity from the forward breakover voltage. A Zener diode makes use of avalanche effect. Zener diodes are specially manufactured to have precise avalanche voltages. They form the basis for voltage regulation in dc power supplies.
Fig. 16-3 . Characteristic curve for a semiconductor diode.
A rectifier diode passes current in only one direction under ideal operating conditions. This makes it useful for changing ac to dc.
Generally speaking, when the cathode is negative with respect to the anode, current flows; when the cathode is positive relative to the anode, current does not flow. The constraints on this behavior are the forward breakover and avalanche voltages. During a little less than half the cycle, the diode conducts, and during a little more than half the cycle, it does not conduct. This cuts off slightly more than 50 percent of every ac cycle. Depending on which way the diode is connected in the circuit, either the positive part or the negative part of the ac cycle is blocked.
A diode can recover the audio signal from radiofrequency (rf) ac. This is called demodulation or detection . If diode detection is to be effective, the diode should have low junction capacitance. Then it works as a rectifier at rf, passing current in one direction but not in the other.
Some rf diodes are microscopic versions of the old cat whisker , in which a fine wire was placed in contact with a crystal of a mineral called galena . Components of this type are known as point-contact diodes and are designed to minimize the junction capacitance. In this way, as the frequency becomes higher and higher (up to a certain maximum), the diodes keep acting like rectifiers rather than starting to behave like capacitors. This makes point-contact diodes good for use at rf.
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