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 the way, zener diodes for different voltage levels have different voltage capacity.

Working

Zener diode conducts just like a ordinary diode on forward bias. On reverse bias leakage current flows in 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 the 6 volt battery than 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 the fig. 29A but there will be no flow of current in the zener diode in the position shown in the fig,29B. If now the battery voltage in the position of fig B is raised than there will be leakage current in the zener in the beginning. If reverse voltage are raised in steps than on a definite reverse voltage there will be suddenrise in the current in the zener, In the fig. 29C, the upper position of the graph shows the variations in forward current on forward voltage and lower position shows the effect onreverse current due to reverse voltage.300 ~| | |

Uses

Zener diode, due to breaks down in reverse bias is used in voltage regulation. For this zener diode is connected in parallel with the power supply. In order to find out the regulating voltage of an unknown zener diode its known voltage is increased limits reverse biasing position which in read in voltmeter VI. This reverse voltage is increased in steps. These volts are raised until voltmeter V2 start showing volts. When voltmeter VI shows some volts than this voltage is subtracted from the voltage shown by the voltmeter VI. In this way this resultant voltage is the regulating vortage capacity of that zener diode. For example let the reading of voltmeter VI —7V and reading of voltmeter, V2=1V. then", regulating voltage capacity of zener diode -V1-V2 = 7-1 - 6V. 3- Signal diode or Cat whisker diode or Point contact diode.

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.

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,

• These diodes are made in the glass and red coloured ring is printed on their cathodes.

OA70, OA79, OA85, IN34, IN4148 etc are such diodes.

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.

1. JEDEC System:

For finding the value of IN type small signal diodes :

• 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 :

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.

• Definition

The (Jiode 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.

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 highest 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 upto8()MV 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

• 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).

Since here a good conductor is used along with the N-type semiconductor hence here only electrons will be the current carriers.

• 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 through it.