Zener Diode
This is a special kind of diode which permits current flow not only in forward direction, but also in reverse direction. A normal diode, when operated in reverse bias, gets damaged if the reverse current above a certain value is passed through it. This “certain value” is called as the Breakdown voltage.
The breakdown voltage of a Zener diode is very low. But this diode allows the reverse current to pass through it, once this breakdown voltage is exceeded. That breakdown voltage is called as Zener Voltage. Hence there is a controlled breakdown which does not damage the diode when a reverse current above the Zener voltage passes through a Zener diode.
A Zener diode in its reverse bias, exhibits a controlled breakdown voltage and it allows the current flow to keep the value of voltage across that Zener diode close to the Zener breakdown voltage value. This value of Zener breakdown voltage makes any Zener diode to be chosen for certain applications.
Avalanche diode is another diode which has the similar characteristics of Zener diode. The avalanche breakdown takes place across the entire PN junction, when the voltage drop is constant and is independent of current. This avalanche diode is used for photodetection.
V-I Characteristics of a Zener diode
The V-I Characteristics of a Zener diode are common for any diode when operated in forward bias. But the reverse bias operation of a Zener diode makes it very important to consider. Let us have a look at the graph.
The point where the bent is shown in the reverse bias operation, is the Zener breakdown voltage, after which the diode allows high reverse currents through it. This Zener voltage is indicated by VZ. This incredible quality of Zener diode made it the most reliable one and have got many applications too.
Applications of Zener diode
This diode has many applications such as −
- It is mostly used as a Voltage Regulator.
- Provides fixed reference voltage in transistor biasing circuits.
- For peak clipping or limiting in wave shaping circuits.
- As a Surge protector in many circuits.
- For meter protection against damage from accidental applications.
Switching Diode
This is a normal single PN junction diode which is especially designed for switching purposes. This diode can exhibit two states of high and low resistance clearly which can be used alternatively.
The junction capacitance of this diode is made very low so as to minimize other effects. The switching speed is made quite high. When the diode has high resistance it works as an open switch and it acts as a closed switch during low resistance. This transition occurs at a faster rate in switching diode, than in any ordinary one.
Applications of switching diode
These have many applications such as −
- Used in high-speed rectifying circuits
- Used in ring modulators
- Used in radio frequency receivers
- Used as reverse polarity protectors
- Used for both General purpose and high speed switching applications
Basic Electronics - Special Purpose Diodes
There are few diodes which are designed to serve some special purposes. There are many of such kinds like Transient voltage suppression diodes, Gold doped diodes, Super barrier diodes, Point contact diodes, Peltier diodes etc. But other than these, there are few prominent diodes, which have got many applications. Let us go through them.
Varactor Diode
A junction diode has two potentials on both sides where the depletion region can act as a dielectric. Hence there exists a capacitance. The Varactor diode is a special case diode that is operated in reverse bias, where the junction capacitance is varied.
The Varactor diode is also called as Vari Cap or Volt Cap. The following figure shows a Varactor diode connected in reverse bias.
If the reverse voltage applied is increased, the width of the dielectric region increases, which reduces the junction capacitance. When the reverse voltage decreases, the width of the dielectric decreases, which increases the capacitance. If this reverse voltage is completely null, then the capacitance will be at its maximum.
The following figure shows various symbols used for Varactor diode which represents its function.
Though all diodes have this junction capacitance, the Varactor diode is mainly manufactured to make use of this effect and increase the variations in this junction capacitance.
Applications of Varactor diode
This diode has many applications such as −
- It is used as a Voltage variable capacitor.
- It is used in variable LC tank circuit.
- Used as Automatic frequency control.
- Used as Frequency Modulator.
- Used as RF Phase shifter.
- Used as frequency multiplier in local oscillator circuits.
