DIODE

WHAT IS DIODE?

A Diode is the semiconductor with two terminal typically allowing the flow of current in one direction only.A lot of diodes made with the semiconductor material such as silicon, germanium or selenium and the two terminals of the diode are called as anode and cathode.

HOW DIODE WORK?

The diode operates when a voltage signal is applied across its anode and cathode. The application of a DC voltage to make the diode operate in a circuit is called as ‘Biasing’. . The ‘ON’ state of a diode is achieved by ‘Forward biasing’ which means that positive or higher potential is applied to the anode and negative or lower potential is applied at the cathode of the diode. In other words, the ‘ON’ state of diode has the applied current in the same direction of the arrow head. The ‘OFF’ state of a diode is achieved by ‘Reverse biasing’ which means that positive or higher potential is applied to the cathode and negative or lower potential is applied at the anode of the diode. In other words, the ‘OFF’ state of diode has the applied current in the opposite direction of the arrow head.

During ‘ON’ state, the practical diode offers a resistance called as the ‘Forward resistance’.  The diode requires a forward bias voltage to switch to the ‘ON’ condition which is called Cut-in-voltage. The diode starts conducting in reverse biased mode when the reverse bias voltage exceeds its limit which is called as the Breakdown voltage. The diode remains in ‘OFF’ state when no voltage is applied across it.   

A simple p-n juction diode is fabricated by doping p and n type layers on a silicon or germanium wafer. The germanium and silicon materials are preferred for diode fabrication because:

·         They are available in high purity.  

·         Slight doping like one atom per ten million atoms of a desired impurity can change the conductivity to a considerable level.

·          The properties of these materials change on applying heat and light and hence it is important in the development of heat and light sensitive devices.                                                                                                

TYPES OF DIODES

The different types of diodes are

·         Zener diodes - This diode runs in reverse bias condition when the voltage reaches the breakdown point. A stable voltage can be achieved by placing a resistor across it to limit the current. This diode is used to provide reference voltage in power supply circuits.

·         Light emitting diodes (LED) - This is the most popular kind of diode. When it works in the forward bias condition, the current flows through the junction to produce the light.

·         Photo diodes - The electrons and holes are generated as light strikes across the p-n junction causing the current to flow. Theses diodes can work as photo detector and are used to generate electricity.

·         Constant current diodes - This diode keeps the current constant even when the voltage applied keeps changing. It consists of JFET (junction – field effect transistor) with the source shorted to the gate in order to function like a two - terminal current limiter or current source.

·         Schottky diode -  These diodes are used in RF applications and clamping circuits. This diode has lower forward voltage drop as against the silicon PN junction diodes.

·         Shockley diode - This is a four layer diode which is also known as PNPN diode. This diode is similar to thyristor where the gate is disconnected. 

·         Step recovery diodes - This semiconductor diode has the ability to generate short pulses and hence it is used in microwave applications as a pulse generator. 

·         Tunnel diodes - This diode is heavily doped in the forward bias condition that has a negative resistance at extremely low voltage and a short circuit in the negative bias direction. This diode is useful as a microwave amplifier and in oscillators. 

·         Var actor diodes - This diode works in reverse bias condition and restricts the flow of current through the junction. Depending on the amount of biasing, the width of the depletion region keeps varying. This diode comprises of two plates of a capacitor with the depletion region amidst them. The variation in capacitance depends upon the depletion region and this can varied by altering the reverse bias on the diode. 

·         PIN diodes - This diode has intrinsic semiconductor sandwiched between P- type and N- type region. Doping does not occur in this type of diode and thereby the intrinsic semiconductor increases the width of the depletion region

·         LASER diode - This diode produces laser type of light and are expensive as compared to LED. They are widely used in CD and DVD drives.  

·         Transient voltage suppression diodes - This diode is used to protect the electronics that are sensitive against voltage spikes.

·         Gold doped diodes - These diodes use gold as the do-pant and can operate at signal frequencies even if the forward voltage drop increases.

·         Super barrier diodes - These are also called as the rectifier diodes. This diodes have the property of low reverse leakage current as that of normal p-n junction diode and low forward voltage drop as that of Schottky diode with surge handling ability.

·         Point contact diodes - The construction of this diode is simpler and are used in analog applications and as a detector in radio receivers. This diode is built of n – type semiconductor and few conducting metals placed to be in contact with the semiconductor. Some metals move from towards the semiconductor to form small region of p- tpye semiconductor near the contact.

·         Peltier diodes - This diode is used as heat engine and sensor for thermoelectric cooling.

·         Gunn diode - This diode is made of materials like GaAs or InP that exhibit a negative differential resistance region.

·         Crystal diode - These are a type of point contact diodes which are also called as Cat’s whisker diode. This diode comprises of a thin sharpened metal wire which is pressed against the semiconducting crystal. The metal wire is the anode and the semiconducting crystal is the cathode. These diodes are obsolete.  

·         Avalanche diode - This diode conducts in reverse bias condition where the reverse bias voltage applied across the p-n junction creates a wave of ionization leading to the flow of large current. These didoes are designed to breakdown at specific reverse voltage in order to avoid any damage.  

·         Silicon controlled rectifier - As the name implies this diode can be controlled or triggered to the ON condition due to the application of small voltage. They belong to the family of Thyristors and is used in various fields of DC motor control, generator field regulation, lighting system control and variable frequency drive . This is three terminal device with anode, cathode and third controlled lead or gate.

·         Vacuum diodes - This diode is two electrode vacuum tube which can tolerate high inverse voltages.                                                                                                                                                      

Construction: 

A simple p-n diode is a junction where p-type and n-type layers are doped on a silicon or germanium wafer. A p-type semiconductor is formed by doping of trivalent or acceptor impurity atoms on a pure silicon or germanium thereby having an excess concentration of holes. An n-type semiconductor is formed by doping of pentavalent or donor impurity atoms on a pure silicon or germanium thereby having an excess concentration of electrons. So, holes are the majority charge carriers in a p-type region whereas electrons in the n-type region. Electron-holes pairs are thermally generated in both types which constitute the minority charge carriers. It is remarkable that a p-type material is not positively charged in spite of having excessive holes while an n-type material is not negatively charged in spite of excessive electrons.  This is because in a p-type material along with holes, the anions are generated and the total number of protons and electrons still remain the same. This is similarly observed for the n-type material.

The junction of a p-type and n-type doping on silicon or germanium wafer produces a small region of the order of micrometers which is depleted from the free charge carriers. This region is formed due to diffusion of holes from a p-type and electrons from an n-type material called as the depletion region or space charge region or transition region. The p-type region to the left of the depletion region is having acceptor negative ion layer and to the right are donor positive ion layer which induces an electric flux or potential difference across the junction. The charge concentration is positive on left of the junction and negative on the right of the junction. This potential barrier stops the holes to migrate into n-type region and electrons to migrate into p-type region as the potential rises for holes and electrons will allow migrating in to n-type and p-type regions. The charge carrier regions around the depletion regions are also called as the uncovered regions.                                                                               

Principle and Operation: 

The possible configurations for a diode are:  

1.      Open circuited

2.      Short circuited

3.      Forward biased

4.      Reverse biased

APPLICATION 

Diodes are used in various applications like rectification, clipper, clamper, voltage multiplier, comparator, sampling gates and filters.

1.      Rectification – The rectification means converting AC voltage into DC voltage. The common rectification circuits are half wave rectifier (HWR), full wave rectifier (FWR) and bridge rectifier.

·                     Half wave rectifier: This circuit rectifies either positive or negative pulse of the input AC. The figure is as shown below:   

·                     Full wave rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:

·                     Bridge rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:

2.      Clipper- Diode can be used to clip off some portion of pulse without distorting the remaining part of the waveform.:

3.      Clamper – A clamping circuit restricts the voltage levels to exceed a limit by shifting the DC level. The peak to peak is not affected by clamping. Diodes with resistors and capacitors are used to make clamping circuits. Sometimes independent DC sources can be used to provide additional shift.