Diodes

                            ideal diode

Ideal diode is two terminal which acts as perfect insulator when reverse biased and acts perfect conductor when forward biased.

Characteristic 

1. When ideal diode is forward biased. it acts as perfect conductor with zero resistance and zero voltage drop across it.

When forward bias voltage increases the depletion layer width is also decrease and due to decrease in depletion layer the resistance across the diodes in forward bias condition decreases.

2. When ideal diode is reversed biased it acts as perfect insulator with infinite resistance and zero current through it.

When reverse bias voltage across the diode increases the depletion layer width is increases and due to increase in depletion layer width the resistance in reverse bias condition across the diode increases.

3. Thus  Ideal diode acts as automatic switch.

4. which is closed in forward bias(i.e. acts as a closed switch).

5. it is opened when reverse biased(i.e. acts as a open switch).

There are various types of diodes : -

1. Zener Diode

2. Varactor Diode

3. Gunn diode 

4. Tunnel Diode

5. Schottky diode

Zener diode

A Zener diode not only allows current to flow from anode to cathode but also, in the reverse direction on reaching the zener voltage. due to this functionality, zener diodes are the most commonly used semiconductor diodes.

A Zener diode is a heavily doped semiconductor device that is designed to operate in the reverse direction. When the voltage across the terminals of a zener diode is reversed and the potential reaches the zener voltage(Knee voltage), the junction breakdown and the current flows in the reverse direction. This effect is knowm as the zener effect.

A zener diode operates just like a normal diode when it is forward biased-biased. However when connected in reverse biased mode, a small leakage current flows through the diode. As the reverse voltage increses to the predetermined breakdown voltage(Vz), current starts flowing through the diode. The current increses to a maximum, which is determined by series resistor, after which it stabilizes and remains constant over a wide range of applied voltage.

There are two types of breakdown for a zener diode.

(1). Avalanche Breakdown

(2). Zener Breakdown

Avalnche Breakdown

When a high value of reverse voltage is applied to the PN junction, the free electrons gain sufficient energy and accelerate at high velocities. a large number of free electrons are generated as a result of electric current in diode rapidly increses, This sudden increses in electric current may permanently destroy the normal diode, however a zener diode is designed to operate under avalanche breakdown and can sustain spike of the current. Avalanche breakdown occurs in zener diode with zener voltage greater than 6Volts.

Zener Breakdown

When the applied reverse bias voltage reaches closer to the zener Voltage, the electric field in the depletion region gets strong enough to pull electrons from their valance band. At the Zener Breakdown region, a small increase in the voltage results in the rapid increase of the electric current.

Zener effect is dominent in voltages upto 5.6 Volts.

Application

zener diode as a voltage regulator

zener in over voltage protection

zener diode in clippig circuit(for modifying AC wave form)