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The Diode Help (page 2)

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

Junction Capacitance

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.

Avalanche

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.

Semiconductors The Diode Detection

Fig. 16-3 . Characteristic curve for a semiconductor diode.

Rectification

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.

Detection

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.

Practice problems of these concepts can be found out: Semiconductors Practice Test

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