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A simple Diode is the simplest two-terminal unilateral semiconductor device.  It allows current to flow only in one direction and blocks the current that flows in the opposite direction. The two terminals of the diode are called as anode and cathode. The symbol of diode symbol is as shown in the figure below. 

Symbol of a Diode

The characteristics of a diode closely match to that of a switch. An ideal switch when open does not conduct current in either directions and in closed state conducts in both directions. The characteristic of a diode is as shown in the figure below.                                                Characteristics of a Diode

Ideally, in one direction that is indicated by the arrow head diode must behave short circuited and in other one that opposite to that of the direction of arrow head must be open circuited. By ideal characteristics, the diodes is designed to meet these features theoretically but are not achieved practically. So the practical diode characteristics are only close to that of the desired.    
Diode characterstics

Working of Diode 
The diode operates when a voltage signal is applied across its terminals. The application of a DC voltage to make the diode operate in a circuit is called as ‘Biasing’. As already mentioned above the diode resembles to that of a one way switch so it can either be in a state of conduction or in a state of non conduction. 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 prefered 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 devlopment of heat and light sensetive devices.
The other variant of diodes have different construction, characteristics and applications. The different types of diodes are:
·         Small signal or Small current diode - These diodes assumes that the operating point is not affected because the signal is small.
·         Large signal diodes - The operating point in these diodes get affected as the signal is large. 
·         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 liimit 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.
·         Photodiodes - The electrons and holes are generated as light strikes across the p-n junction causing the current to flow. Theses diodes can work as photodetector 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 didoe 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 ampilifer and in oscillators. 
·         Varactor diodes - This didoe works in reverse bias condition and restricts the flow of current thorugh 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. They are used as ohtodiodes and radio frequency switches.
·         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 supression diodes - This diode is used to protect the electronics that are sensitive against voltage spikes.
·         Gold doped diodes - These diodes use gold as the dopant 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 didoe comprises of a thin sharpened metal wire which is pressed against the semiconducting crystal. The metal wire is the anode and the semconducting crystal is the cathode. These diodes are obsolete.  
·         Avalanche diode - This diode conducts in reverse bias condition where the reverse bias volage 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 Tyristors 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 controled lead or gate.
·         Vaccum diodes - This diode is two electrode vacuum tube which can tolerate high inverse voltages.
Various symbols of DiodeTypes of Diode

Generic diodes (Small signal and large signal): 
A p-n junction diode is the simplest semiconductor device. It is a two-terminal, bipolar, unilateral rectifying device that conducts only in one direction. The generic diodes are used in the following fields:
·          Rectification in power supply circuits
·         Extraction of modulation from radio signals in a radio receiver and in protection circuits where large transient currents may appear on low current transistors or ICs in interfacing with relays or other high power devices.
·         Used in series with power inputs to electronic circuits where only one of negative or positive polarity voltage is desired.
 Internal structure of p-n Diode

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. This is shown in graph below.
Graph diagram of Linear Graded diode and Step Graded diode
It is also important that the minority charge currents i.e. electron current in p-type region and hole current in n-type region decreases exponentially across the diode length.  The minority current is due to electron hole pairs generated thermally and dependent upon temperature. These currents are so small in magnitude in the order of microamperes. However in conduction state, the current through the diode crystal remains stable.  The total current is a sum of minority and majority charge currents due to bipolar nature of the diode. The majority charge currents is hole current in p-type and electron current in n-type are reduced as they migrate near junction due recombination. The minority currents is electron current in p-type and hole current in n-type are maximum near junction and reduces as they migrate away from the junction as an exponential function. The majority charge currents in their regions after crossing the junction are the diffusion currents while before junction are drift currents. 
Concept of Ohmic contacts – In addition to PN junction diode, there is a two metal semiconductor junctions originating from the leads in order to connect the device. It is assumed that the resistance of these metal semiconductor contacts remain constant despite of the magnitude and direction of current. During the diode operation, the applied voltage is solely effective for increasing or decreasing the potential barrier height of the PN junction.
Note: The use of a step graded diode can improve the performance of the diode.

Principle and Operation: 
The possible configurations for a diode are:  
1.      Open circuited
2.      Short circuited
3.      Forward biased
4.      Reverse biased
1.      Open circuited: In open circuited condition, the current that flows through the diode is zero (I = 0). The potential barrier at the PN junction remains the same as  created in the diode fabrication.
Open circuited configuration2.      Short circuited: In short circuited condition, the sum voltage in the loop must be zero. So it is assumed, that the potential barrier at the PN junction is compensated by the potential drops at the metal semiconductor junctions. The holes supplied by the n-region must be driven to the p region which is physically impossible. The similar discussion applies to the electron current in the n-region. 

Conclusion: The potential barrier height cannot be measured directly by a multimeter.
Short circuited configuration
3.      Forward bias: In forward bias condition, higher or positive potential is applied at the anode and negative or lower potential is applied at the cathode of a diode. The positive potential at anode repels the holes in p-region towards n-region while negative potential at the cathode repels electrons in n-region towards p-region. Thus, the height of the potential barrier reduces. The depletion region disappears when the applied voltage equals to the potential barrier and a large current flows through the diode. The voltage required to drive the diode into a state of conduction is called as the ‘Cut in/Offset/Threshold/Firing voltage’. The current is of considerable magnitude as it is dominantly constituted by the majority charge currents that is the hole current in the p-region and the electron current in the n-region. The current that flows from anode to cathode is limited by the crystal bulk resistance, recombination of charges and the ohmic contact resistances at the two metal semiconductor junctions.  The current is restricted to mille Amperes order.   
Forward Bias configuration
4.      Reverse Bias: In reverse bias condition,  the higher or positive potential is applied at the cathode and negative or lower potential is applied at the anode. The negative potential at anode attracts the holes in p-region that are away from the n-region while positive potential at the cathode attracts electrons in n-region that are away from the p-region. The applied voltage increases the height of the potential barrier. The current flows dominantly due to the minority charge currents that is the electron current in p-region and the hole current in n-region. Thus a constant current of negligible magnitude flows in the reverse direction which is called as the ‘Reverse saturation current’. Practically, the diode remains in a state of non – conduction. The reverse saturation current is of the order of microamperes in a germanium diode or nanoamperes in a silicon diode  If the reverse voltage exceeds the limit  of ‘breakdown/zener/Peak inverse/Peak reverse voltage’, the potential breakdown that occurs leads to a large reverse current.
Reverse Bias configuration


Characteristics of diode

The current that flows through a diode is given by the equation:
 where I - diode current. (Positive for forward and negative for reverse)
             IS - constant reverse saturation current
             V - applied voltage. (Positive for forward and negative for reverse)
 - factor dependent upon the nature of semiconductor. (1 for  
                 germanium and 2 for silicon)
            V - volt equivalent of temperature which is given by T/11600. (T is
                Temperature in Kelvin)
When a forward voltage is applied at the terminals of a diode, the diode begins to conduct. During conduction, the cut in or threshold voltage exceeds the applied forward voltage.  The threshold voltage for a germanium diode is 0.3V and for silicon diode is 0.7V. The forward current (miliampere range) initially increases linearly and then increases exponentially for high currents.
When a a reverse voltage is applied, a reverse saturation current flows through the diode. The diode continues to be in the non conducting state until the reverse voltage drops below the zener voltage. As the reverse voltage approximates the peak inverse voltage a breakdown called as the ’Avalanche breakdown’ occurs. During the breakdown, the minority charge carriers ionize the stable atoms which are followed by a chain ionization to generate a large number of free charge carriers. Thus the diode becomes short circuited and gets damaged.
Note: When diodes are connected in series their equivalent peak inverse voltage is increased while in parallel connection the current carrying capacity is increased.
As the temperature increases, the electron pairs generated thermally also increases thereby increasing the conductivity in both directions. The reverse saturation current also increases with the increase in temperature.  The change is 11% per °C for a germanium diode and 8% per °C for a silicon diode. On the other hand the diode current is doubled for every 10°C rise. With increase in voltage, the firing voltage in forward characteristics is reduced while peak reverse voltage is increased. 
Note: The peak inverse voltage can be reduced by increasing the doping level. The same concept is used to design zener diodes.
Diode resistances: The resistance associated with the diode can be evaluated in three fashions and the three types of resistances associate with a diode accordingly.
·         DC or Static resistance: It is the ratio of diode voltage to the diode current at any point of its characteristic curves. It is defined at a point on the characteristic curves.
·         AC or dynamic resistance: It is the ratio of change in diode voltage to the change in diode current. It is defined at a point on the characteristic curves over a tangent.
·         Average AC resistance: It is the ratio of change in diode voltage to the change in diode current over a straight line joining two limits of operation.
Characteristics of Diode resistance
Diode capacitances: The diode exhibits two types of capacitances transition capacitance and diffusion capacitance.
.       Transition capacitance: The capacitance which appears between positive ion layer in n-region and negative ion layer in p-region.
·         Diffusion capacitance: This capacitance originates due to diffusion of charge carriers in the opposite regions.
The transition capacitance is very small as compared to the diffusion capacitance.
In reverse bias transition, the capacitance is the dominant and is given by:
where CT  - transition capacitance
A - diode cross sectional area
             W - depletion region width
In forward bias, the diffusion capacitance is the dominant and is given by:
where CD  - diffusion capacitance
               dQ  - change in charge stored in depletion region
                V  - change in applied voltage
- time interval for change in voltage
                 g  - diode conductance
                 r  - diode resistance
The diffusion capacitance at low frequencies is given by the formula: 
The diffusion capacitance at high frequencies is inversely proportional to the frequency and is given by the formula:
Note: The variation of diffusion capacitance with applied voltage is used in the design of varactor.
Diode switching time: In AC applications, when diode is instantaneously switched from a conduction state  to a non conduction state it needs some time to return to non conduction state and behaves short circuited for a little time period in reverse direction.  This occurs because when the diode biasing is suddenly changed, the majority charge carriers migrated to other region is the minority charge carriers in the region. Specifically, holes are the minority carriers migrated from p-type to n-type in reverse bias. . These holes require some time to return back to state of non conduction which is called as the ‘Reverse recovery time’. Reverse recovery time is the sum of storage time and the transition time.
·         Storage time: The time period for which diode remains in conduction state even in the reverse direction.
·         Transition time: The time elapsed in returning back to state of non conduction.
It is desirable those diodes has minimum switching or reverse recovery time trr. Switching time of diodes is of the order of few nanoseconds to 1 microsecond. Now fast switching diodes with switching time up to few picoseconds are also available.  
Characteristics of Diode switching time

A diode is marked with a bar which indicates the cathode terminal of a diode which is as shown in the figure below:
Note: Various small signal diodes like IN4148, 0A90 and rectifying diodes like IN4001-4007,IN5400-5408,BY125-127 are available with different current, reverse saturation current and peak inverse voltage rating.

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:   
Half wave rectifier
·                     Full wave rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:
Full wave rectifier
·                     Bridge rectifier: This circuit converts the entire AC signal into DC. The figure is as shown below:
Bridge Rectifier
2.      Clipper- Diode can be used to clip off some portion of pulse without distorting the remaining part of the waveform. The figure is as shown below:
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. The figure is as shown below: 
Clamping circuit

Datasheet Analysis:
The datasheets of the diodes gives valuable stuff about their various parameters such as:
·         Peak inverse voltage,
·         Reverse saturation currents at specified reverse voltages,
·         Maximum forward current,
·         Capacitance levels,
rse recovery time,
·         Storage and operating temperatures,
·         Peak repetitive forward current,
·         Peak forward surge current,
·         Average surge current and many more. .
The graphs to represent the voltage current characteristics and temperature dependences are also supplied.

Rectifying Diodes in Market:
·         Diodes designated from IN4001 to IN4007 are available with maximum forward voltage of 1.1 V and 1A being the maximum rectifying current. The maximum reverse current are 5 uA and PIV (Peak Inverse voltage) varies from 50V to 1000V.
·         Another series of diodes is IN5400 to IN5408 with maximum forward voltage of 1.2 V and 3A being the maximum rectifying current. The maximum reverse current are 5 uA and PIV (Peak Inverse voltage) varies from 50V to 1000V.

Testing of a diode: 
A diode can be open circuited or short circuited when damaged. It can be tested using a multimeter by following the steps given below:
1.      Insert the probes into the required sockets: The digital multimeter will have several sockets for the test probes. Insert these probes and check if they are already in the correct sockets. Typically, these are labeled COM for common and the others for current or voltage. This is normally combined with the voltage measurement socket.
2.      Turn on the multimeter and select the maximum resistance range.
3.      Check resistance in forward and reverse direction. Place the red probe on diode anode and black probe on the cathode to measure the forward resistance. Place the red probe on diode cathode and black probe on anode to measure the backward resistance. The forward resistance must be very small in few ohms while backward resistance must be very high in the range of mega ohms. If forward resistance is very high the diode is open circuited and if backward resistance is very small diode will be short circuited.
4.      Another way is to select diode on the multimeter. Place the red probe on diode anode and black probe on the cathode and of the multimeter beeps then it indicates a short circuit otherwise it is open. Place the red probe on diode cathode and black probe on the anode and if the multimeter does not beep then it indicates an open circuit otherwise if it beeps the diode is short.
5.      Turn off the multimeter: Once the resistance measurement has been made, the multimeter can be turned off to preserve the batteries. It is also wise to turn the function switch to a high voltage range. In this way, if the multimeter is used again for another type of reading then no damage will be caused if it is inadvertently used without selecting the correct range and function.   

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