Zener diode

These diodes are made by heavily doped N and P type semiconductors, the quantity of doping of semiconductors is kept different so that their break down voltages are different. In that way, zener diodes different voltage levels have different voltage capacity.

Working
Zener diode operates in reverse bias and in forward bias it just operates like PN junction diode. Zener diode conducts just like an ordinary diode on forward bias. On reverse bias, leakage current flows through it. This leakage current increases with the reverse voltage. This leakage current will increase suddenly after a definite reverse voltage. This voltage is known as break down voltage of zener diode or zener voltage, and this sudden increase in current is known as zener current. For example, if a 6 volt capacity zener diode is connected in series with a 6 volt battery, then the effect of forward and reverse bias will be as follows : There will be current flow in the zener diode in the position shown in fig., but there will be no flow of current in the zener diode in the position shown in fig.. If now the battery voltage in the position of fig is raised, then there will be leakage current in the zener in the beginning. If reverse voltages are raised in steps, then on a definite reverse voltage there will be sudden rise in the current through the zener. In fig., the upper position of the graph shows the variations in forward current on forward voltag

Uses Zener diode, due to555555 breaks down in reverse bias is used in voltage regulation. For this zener diode is connected in parallel with the power supply.it consist of n p n transistor at the back of the rectifier

Graphical Symbol
The graphical symbol of these types of diodes resembles that of an ordinary diode, except the cathode has a Z shape instead of a straight line.

Construction
The diode made by connecting a very thin wire (its end is very sharp) on a semiconductor material is known as point contact diode. The wire is either slicked mechanically or electricaly. The area of point contact diode is very small.

The zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<0.000001 m) and the electric field is consequently very high (about 500000 V/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.

In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides.[2]  The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.117.203.88.183 18:00, December 15, 2012 (UTC)S.Pramodh

Special features of a diode
It depends upon the factors,*On the area of the portion joint by the wire fitted in it*On the pressure applied for connecting the wire*an the construction of the semiconductor crystal. But mostly in these types of diodes there is more leakage current, more spreading Junction Resistance and weak reverse break down.

Practically point, contact diode are made by keeping 0.005 inch wide wire on the N-type germanium layer and than large current is flew through it for a moment. As a result high temperature produces. Due to the temperature some electrons goes into the N-type germanium from the cat whisker wire and related area converts into holes. In this way, where cat whisker wire comes in contact with the N-type germanium, That area becames P-type germanium this p-type germanium area is very small, due to which there is very little capacitance in such diodes. This is the reason due to which these types of diodes are used is the microwave field for signal detection, OA70, OA79, OA85, IN34, IN4148 etc are such diodes.
 * These diodes are made in the glass and red coloured ring is printed on their cathodes.

Use of colour code system for small signal diodes:
On some small signal diodes there value in not written. To show their value colour code system is used. This colour code system is of two types. For finding the value of IN type small signal diodes : In this system there are two wide strips on the cathode side. First and second wide strip shows the code of the diodes third and fourth colour strips shows the diode number. The (Diode which keeps the features of negative resistance and which is used for the switching at the level of microwave frequency is known as tunnel diode.)
 * 1) JEDEC System:
 * In this system four digits are represented by the four colour along with the IN code.
 * Colour codes are counted from the cathode side.
 * In the colour code system, numbers represented by the colours are in the following way:
 * 1) Pro-electrons system :
 * Definition

What is tunneling?
Ans,: In any semiconductor material when any particle pass from it, whose energy is not sufficient to cross the junction resistance of the semiconductor material, than this process is known as tunnling. Means if waves made up to these particles is given at input than its maximum portion will be blocked by the junction resistance but a part of it reaches at the output. This process is known as tunneling process, The use of negative resistance property of semiconductor equipments for switching at very high frequency (IGHZ TO 10 GHZ) level is known as tunneling.

What is the property of negative resistance?
When especially structured semiconductor equipment is forward biased than in the beginning current in the output increase with voltage. But after a definite lowest point output current reduces on increasing the forward voltage. If forward voltage is continusly increased from this point than current will reduce to a definite value and than it will again start increasing.thus the condition in which output current reduces with the increase in the forward bias voltage, is known as the negative resistance property. This property of semiconductor equipment in known as negative resistance effect.

Structure
The diode made up of P and N point semiconductor material in which doping of P and N type lager is 1001) times more than the normal doping, as a result of this heavy doping the width of the junction reduces (about 6.01uM) and so some particles which do not have sufficient energy to cross the junction resistance also passes this thin junction.

Working
When ordinary diode is forward biased than it gives current in the output on a certain fixed voltage and tunnel diode conducts heavily on a voltage lower than this voltage. For example, a simple diode do not conducts upto8MV where as tunnel diode has current of about 15MA on this voltage. In the case of reverse biasing no current flows through the ordinary diode while current flows through the tunnel diode.

Uses
Due to its capability of high switching and property of negative resistance tunnel diode is used in different calculating process in amplifier, oscillator or computers. 5. Hot carrier diode or schottky diode :
 * graphical symbol

Since here a good conductor is used along with the N-type semiconductor hence here only electrons will be the current carriers.
 * construction:Just like the other diodes schottky diodes also have two layers but materials used in there layers are slightly different. These diodes have one layer of N-type semiconductor material and other of a good conductor material (gold, silver, platinum).
 * Working:When schottky barrier diode is forward biased than, negative terminal of the battery is connected at the N-type semiconductor end of the diode and positive terminal of the battery is connected at the conductor end of the diode. Due to the repulsion by the negative terminal of the battery. N-type electrons quickly crosses the junction and enters into the electron orbit of the nearer conductor atom. At the same time, positive terminal of the battery attract the electrons which comes from the N-type semiconductor. In this way, current flows through the schottky diode of hot carrier diode.But as soon as the forward biasing is stopped, flow of current in the output also stops. It reason is that such diodes do not have any minority carrier therefore there is no chance remains for leakage current.

Uses
The special feature of these diodes is getting output during forward biasing. Because of this property these diodes are used for modulation and detection of V.H.F., U.H.F. and microwave level. High level switching is also done with the help of these diodes. Example : IN5825 is a hot carrier diode (schottky diode) which gives 5A of current in the output on forward bias of just 0.38V. But as soon as this forward biasing is removed current also stops flowing thru it.


 * REVIEW:
 * Zener diodes are designed to be operated in reverse-bias mode, providing a relatively low, stable breakdown, or zener  voltage at which they begin to conduct substantial reverse current.
 * A zener diode may function as a voltage regulator by acting as an accessory load, drawing more current from the source if the voltage is too high, and less if it is too low.