1. Zener diodes are designed to operate in the reverse voltage breakdown (avalanche) region. Its voltage rating Vz and the maximum power it can absorb are given by the manufacturer. Their most important application is in voltage regulation.
Figure-1 : Symbol for a Zener diode
2. Zener diodes can also be used as limiters and limiting can occur during the positive half−cycle of the input voltage, during the negative half−cycle, or during both the positive and negative half−cycles of the input voltage.
Figure-2 : Zener diode operating region
3. A Schottky diode is a junction of a lightly doped n−type semiconductor with a metal electrode. The junction of a doped semiconductor usually n−type with a special metal electrode can produce a very fast switching diode which is mainly used in high (up to 5 MHz) frequency circuits or high speed digital circuits.
Figure-3 : The components of a Schottky diode and its symbol
4. Schottky diodes find wide application as rectifiers for high frequency signals and also are usedin the design of galliun arsenide (GaAs) circuits.
5. The forward voltage drop of a conducting Schottky diode is typically 0.3 to 0.5 volt compared to the 0.6 to 0.8 found in silicon junction diodes.
6. A tunnel diode is one which is heavily doped with impurities, itwill have a region of negative resistance. The negative resistance characteristic makes the tunnel diode useful in oscillators and microwave amplifiers.
Figure-4 : Tunnel diode characteristics and symbol
7. The backward diode is a variation of the tunnel diode. It is used as a rectifier and in circuits with small amplitude waveforms.
8. A varactor, or varicap, is a diode that behaves like a variable capacitor, with the PN junction functioning like the dielectric and plates of a common capacitor. A varactor diode uses a PN junction in reverse bias and has a structure such that the capacitance of the diode varies with the reverse voltage.
Figure-5 : Symbol for varactor
9. A voltage controlled capacitance is useful in tuning applications. Typical capacitance values are small, inthe order of picofarads. Varactors have now replaced the old variable capacitor tuning circuits as in television tuners.
Figure-6 : A varactor tuner circuit
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