Transistor

Invention of Transistor was done by great American scientist Mr. Vardon and Mr. Bradone. in 1947. After the invention of transistor there is a treat revolution in Electronics field. It (Transistor) is totally an electronic device which is generally made of Semiconductor materials Germanium or silicon. In pure condition semiconductor is generally non conductor. By adding two types of impurities we make two types of semi conductor: By adding the P and N type semi conductor make a junction and the device called a diode. There are two junctions in a transistor, so it is called a bijunction or Bipolar transistor. In a transistor there yes indeed, are two junctions one provide a very low resistance for current flow and the other provide a very high resistance. One transistor transfers the current from low resistance towards high resistance due to this reason it called a transfer of resistor or transistor. On the Basis of construction, there are two types of transistor In both types there are three terminals, namely, emitter, base and collector. The terminal which emits the charge, called a emitter.and that which collect charge is called collector. The middle layer between the emitter and collector is called base, which makes two junctions one with emitter and other with collector, the junction between base and emitter is called emitter junction and that between base and collector is called collector junction. The function of base is to control the collector current.
 * N-type semi conductor.
 * P-type semi conductor.
 * P-N-P type
 * N-P-N type.

P-N-P Type transistor
It is made of two P-type layers and one N-type layer. In this type, we add two P-type layers, with the two sides of a N-type layer. In this way, we get a P-N junction and an other N-P junction, we can compare a P-N-P transistor with two diodes, whom N-N type semi conductors are jointed, between the two diodes, One is called emitter-base diode or emitter diode and other is called collector base or collector diode. In figure 1 (a), there are shown the two junctions of a P-N-P transistor. In fig l(b), there is a symbolic representation of P-N-P transistor and in fig l(a), there is an equivalent circuit of transistor. In symbolic representation of P-N-P transistor the direction of arrow is towards inside.

N-P-N Transistor
It is made of two N type and one P-type semiconductor layer. In this, between the two layers of N-type semiconductor, there is a layer of P-type material the properties of N-P-N transistor of that type are completely opposite to P-N-P transistor. In figLLKig 2(c) shows the diode equivalent circuit of N-P-N transistor. In the symbolic representation of N-P-N transistor the direction of arrow is towards outside.

P-N-P and N-P-N both transistors are made of silicon and Germanium low conductors. The transistors which are made of silicon semiconductor are called silicon transistor and that of Germanium semiconductor are called Germanium semiconductor. Germanium Transistors are always covered by metallic body where as silicon transistor may be both in metallic or silica body. Now a days silicon transistor are broadly used, it has many reasons. The main Reason is that the power output of Ge transistor is low than the silicon transistor. Silicon (Si) transistor can give output up to 25 watt whereas the Germanium (Ge) transistor can not give such high power. Si transistors can work on high frequency than Ge transistors. The Si transistors can work on comparatively high temperatures where as the Ge transistor become destroyed at' high temprs. The current amplification factor of si transistor is greater than the Ge transistor. In Si transistor at 30°c, the leakage current increases upto 10 times. This leakage current, increases the collector junction's temperature and may destroy it. Therefore on the basis of above reasons, si transistor is widely used than Ge one. Identify the terminals of transistor: Generally there are three terminals in transistors which are called emitter, base and collector, but in high frequency (frequencies) transistor there is an additional terminal called shield. This terminal is generally connected to the body of transistor. In each type of transistor there are different ways to identify these terminals. In some transistor, to search that terminals, there may be a guide point, by which we am know the emitter, base and collector terminals, ft some type of transistors, which are made of different companies, then the Identification method may be different. To identify the terminals of some important transistors, we follow the following points
 * 1) Now a days the transistors of BEL company are usually used. Transistors which were available of Ge material, now also available of Si material, for example AC 187 and AC 1800 are Ge transistor, now equivalent Si transistors of BEL are available in market whose numbers are BEL 187 and BEL 188. The identification of terminals of that type and other Si transistors, we do according to following figures : these transistor are - BEL 188, BEL187, BEL147, BEL148, BEL158, BEL157etc,the shapes of all that transistor are semicircular and terminal.'; are in straight line. To identify the terminals, we take transistors in hand in a way that the portion of transistor on which numbers written, remain towards us and terminals remain lower side. Then the left-most terminal is collector and right most is emitter and middle one is base. These transistors are called Si planer transistors.
 * 2) some transistors of special shapes, made by BEL company, are called Epitexial transistors. Numbers of that transistors started term BC and we identify the terminals according to fig Cl. The number of some that type of transistors are following: BC147, BC148, BC149, BC157, BC158.
 * 3) Some Epitexial transistors of BEL company whom numbers started from BF the identification of terminals of that transistors are done according to fig (5).Number of some that type transistors are BE167, BF195, BF197.
 * 4) The terminals of some planer transistor like BEL195 and BEL 194 etc. are different than other planer transistors of BEL. There the base and emitter becomes interchanged the terminals are identify according to fig (6).
 * 5) In some transistors the terminals are arranged in a triangle fashion and there is a coloured dot near a terminals. This dot shows, collector terminal. The middle one is called base and rightmost is called emitter. It is^shown in fig (7) .The numbers of some that type transistors are AC127, AC128, AC187, AC188.
 * 6) In some transistors having metallic body, there is an metallic tip, nearer to a terminal. That terminal which is nearer to tip is called emitter middle terminal is called base and leftmost terminal is called collector, as shown in fig Numbers of some that type of transistors are : BD115, 2N2905, CL100, SK100, BC109, BO150, 2SC2193, 2SC2131, 2SC1820.
 * 7) Some power transistors have special shape. Generally there are two terminals, the body of that transistors, it self work as collector. The other terminals are identify according to fig (a)Numbers of some this type of transistors are AD149, AD161, AD162 BU105, BU108, BU205, BU207, 2N3055 etc.There are two holes on the body of these transistors. Distance of pins from one hole is less than other hole. By putting the less distance hole towards us, we find that rightmost terminal is base and left one is emitter.Besides these here we are giving a table to identify terminals of some other transistors. These transistors are used in different black and white TVs.

Identification of N-P-N and P-N-P transistor
With the help of multimeter Sanwa we can identify P-N-P and N-P-N transistor, Irj"this process, by putting multimeter in 1Q range, we measure resistance between emitter-base and base-collector.We connect Black prob of multimeter to transistor's base and connect red prob to emitter and collector respectively, if needle of meter shows low resistance (i.e. gives large indication) then transistor is N-P-N transistor.When we connect red prob to base and connect black prob to emitter and collector respectively and if meter shows low resistance (means large indication) then transistor will be P-N-P transistor.Each transistor will be either P-N-P type or N-P-N type. So meter shows low resistance only for one checking.

Identification of Germanium or Si transistor
For the construction of transistors two types of semiconductors are used, namely si and Ge. Germanium transistors are generally is metallic body whereas the Si transistor may be both in metallic or silica body. In this condition it is a difficult job to identify them. By measuring the resistance between emitter and collector by multimeter sanwa-P-3 we can identify neither the transistor, is P-N-P type nor N-P-N type.For that purpose we connect black lead of multimeter to collector and Red lead to emitter. If meter shows high resistance (means niddle shows low indication), then we inter change leads of meter, i.e., black lead connected of emitter and red lead to collector. Now meter shows low resistance (means niddle shows high indication). In this way if resistance between emitter and collector is high and then come low, then the transistor will be Germanium transistor. Checking method of transistor is shown in fig-10. But if in both processes, the meter shows high resistance then the transistor will be si transistor.

Identification of Damaged transistor
The damaged transistors may be open circuit, short circuit or become leaky. Checking of that transistors is done by multimeter. To check whether the' transistor is useful or damaged, we check transistor first of all by P-N-P type. If we check an N-P-N transistor with the process of P-N-P checking and if meter shows low Resistance between base-emitter or base-collector or both. Then the transistor will be open circuited (Ckted). After above checking, we check whether the transistor is of Ge or Si. For this we measure resistance between emitter and collector. For a Si Transistor resistance between emitter and collector is very high and the niddle does not alter it's position. If niddle shows very slight (Small) indication, then transistor will be leaky. In the similar way in Ge transistor niddle should show one time high resistance and low resistance at other time between emitter and collector. But if both time, the meter shows low resistance then the transistor will be leaky. And if niddle reach upto Zero, then emitter and collector will be short. Similarly if in Ge Transistor, Meter shows high resistance both times (That is either niddle doer not more or more slightly) then the emitter and collector of transistor will be leaky and open respectively. In this way we can check the damaged transistor. We replace the damaged transistor by a new transistor of the same type. Before placing new transistor we should check it also. Many time the new transistor of same number es not available in market. In this condition we put a transistor of equivalent number. We can find the equivalent number of any transistor from "Equivalent Book" or 'Transistor Comparison Table". This book also gives some other information voltage to transistor which we can give, identification of terminals and packing. There are giving whole information about some important number's transistors. Which is given in table form in last pages of book. Biasing of Transistor: Giving the necessary supply to terminals of a transistor, is called biasing. If supply to all terminals is not proper then the transistor will not work properly. We give two types of biasing to transistor: Transistor made of two types of semiconductors P-type and N-type. If we give positive supply to P-type and negative supply to N-type, then it is called forward biasing. Forward biasing is given to base and emitter junction always. Giving negative supply to P-type and positive to N-type, is called Reverse Biasing. So in this biasing we give reverse supply to materials. Reverse bias is always applied to base collector junction. Value of reverse biasing is always larger than forward biasing. In both type of biasing base is always common, thus there present both forward and reverse bias on the base. For that reason the bias of base is called A.C. signal. Besides it, the input signal, which we want'to amplify, is also given to base. The base biasing of Transistor depends on input signal. If the base bias is not same as the input signal (wave) then the wave will not pass through transistor properly and also output will not proper. So we do biasing of base according to the input wave. That is the proper biasing of base. Now details of N-P-N and P-N-P type transistor's biasing is given. (A) Biasing of P-N-P transistor: In fig-12 biasing of a P-N-P transistor is shown. Here between base and emitter, we give forward bias i.e. We give positive supply to P-type material. Similarly, we give reverse bias to base and collector junction i.e. base is given positive supply and collector of P-type in added to negative supply. There are both positive and negative supplies connected to base, so the negative supply which causes forward bias to base, is of lower value than positive supply connected to base.
 * Forward Biasing:
 * Reverse Biasing:

Working of P-N-P transistor
According to fig-12, the emitter is forward biased and collector junction is reversed biased. Since emitter (P-type) is connected to positive supply, so the holes of emitter are repealed by the positive supply and come towards emitter junction. Due to electric pressure these holes cross the emitter junction and come into the N-type base region. The base region is very thin and made of few impurities in intrinsic semiconductor. The holes from emitter enter in base region with very high speed and cross the base region and come directly in P-region of collector. Besides it, number of electrons in the base biasing of base in-creases the forward biasing of emitter junction, then there there increases the collector current. In the same way if the base biasing increases the reverse biasing of emitter then collector current decreases and may be stop. In this way the low signal voltage applied to base controls the heavy current of collector. Practical Representation of Biasing: Practically, the supply voltages to all pins of a transistor are given from a common supply source, fig 14 shows the practical representation of forward and reverse biasing given to N-P-N and P-N-P transistor.

[Hide this message] [Show more] Avalanche breakdown From Wikipedia, the free encyclopedia • Interested in contributing to Wikipedia? • Jump to: navigation, search

Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. Contents [hide]

* 1 Explanation * 2 The avalanche process * 3 Applications * 4 See also * 5 References Explanation

Avalanche breakdown can occur within insulating or semiconducting solids, liquids, or gases when the electric field in the material is great enough to accelerate free electrons to the point that, when they strike atoms in the material, they can knock other electrons free: the number of free electrons is thus increased rapidly as newly generated particles become part of the process. This phenomenon is usefully employed in special purpose semiconductor devices such as the avalanche diode, the avalanche photodiode and the avalanche transistor, as well as in some gas filled tubes. In general purpose semiconductor devices such as common diodes, MOSFETs, transistors, it poses an upper limit on the operating voltages since the associated electric fields can start the process and cause excessive (if not unlimited) current flow and destruction of the device. When avalanche breakdown occurs within a solid insulating material it is almost always destructive. When an avalanche-like effect occurs without connecting two electrodes, it is referred to as an electron avalanche. Although there are some superficial similarities to Zener breakdown, the physical origins of the two phenomena are very different.

The avalanche process

Avalanche breakdown is a current multiplication process that occurs only in strong electric fields, which can be caused either by the presence of very high voltages, such as in electrical transmission systems, or by more moderate voltages which occur over very short distances, such as within semiconductor devices. The electric field strength necessary to achieve avalanche breakdown varies greatly between different materials: in air, 3 MV/m is typical, while in a good insulator such as some ceramics, fields in excess of 40 MV/m are required. Field strengths used in semiconductor devices that exploit the avalanche effect are often in the 20–40 MV/m range, but vary greatly according the details of the device.

Once the necessary field strength has been achieved, all that is necessary to start the avalanche effect is a free electron, and since even in the best insulators a tiny number of free electrons are always present, an avalanche will always occur. In devices that exploit the avalanche effect, the electric field is normally kept just below the threshold at which avalanche breakdown is possible, resulting in a current that is highly dependent on the generation of free electrons. In avalanche photodiodes, for example, incoming light is used to generate these free electrons.

As avalanche breakdown begins, free electrons are accelerated by the electric field to very high speeds. As these high-speed electrons move through the material they inevitably strike atoms. If their velocity is not sufficient for avalanche breakdown (because the electric field is not strong enough) they are absorbed by the atoms and the process halts. However, if their velocity is high enough, when they strike an atom, they knock an electron free from it, ionizing it (and this is referred to as impact ionization for obvious reasons). Both the original electron and the one that has just been knocked free are then accelerated by the electric field and strike other atoms, in turn knocking additional electrons free. As this process continues, the number of free electrons moving through the material increases exponentially, often reaching a maximum in just picoseconds. The avalanche can result in the flow of very large currents, limited only by the external circuitry. When all electrons reach the anode, the process stops, unless of course holes are created also.

For a bipolar junction transistor the strength of the base drive has an important impact on the avalanche voltage. If a low impedance is connected to the base then charge is quickly removed from the base which helps hold back the avalanche process, but if the base is driven by a high impedance, such as a current source, then the excess charges stay in the base and avalanche occurs at a lower electric field.

[edit] Applications

If the current is not externally limited, the process normally destroys the device where it has started, and in situations such as power line insulators, this can take the form of an explosive breakdown of the insulator. When avalanche current is externally limited, avalanche breakdown can successfully serve to several purposes. In avalanche transistors and avalanche photodiodes, this effect is used to multiply normally tiny currents, thus increasing the gain of the devices: in avalanche photodiodes, current gains of over a million can be achieved. Also, the phenomena is very fast, meaning that avalanche current quickly follows avalanche voltage variations or starting charge (number of free electrons available to start the process) variations, and this gives to avalanche transistors and avalanche photodiodes the capability of working in the microwave frequency range and in pulse circuits. In avalanche diodes, this effect is mainly used to construct over voltage protection circuits and voltage reference circuits: as a matter of fact, avalanche breakdown and Zener breakdown are jointly present in each avalanche diode, depending on breakdown voltage, which is the leading contributing process to the avalanche current.

[Hide this message] [Show more] Zener diode From Wikipedia, the free encyclopedia (Redirected from Zener breakdown) • Have questions? Find out how to ask questions and get answers. • Jump to: navigation, search Zener diode schematic symbol Zener diode schematic symbol Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the change of voltage scale between the forward biased (positive) direction and the reverse biased (negative) direction. Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the change of voltage scale between the forward biased (positive) direction and the reverse biased (negative) direction.

A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". Named for Clarence Zener, discoverer of this electrical property.

A conventional solid-state diode will not let significant current flow if it is reverse-biased below its reverse breakdown voltage. By exceeding the reverse bias breakdown voltage, a conventional diode is subject to high current flow due to avalanche breakdown. Unless this current is limited by external circuitry, the diode will be permanently damaged. In case of large forward bias (current flow in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction 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 model, this tunneling corresponds to the ionization of covalent bonds. The Zener effect was discovered by physicist Clarence Melvin Zener. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

The breakdown voltage can be controlled quite accurately in the doping process. Tolerances to within 0.05% are available though the most widely used tolerances are 5% and 10%.

Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.

In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature critical applications.

Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.

All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of 'zener diode'.

Uses

Zener diodes are widely used to regulate the voltage across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value.

In the circuit shown, resistor R provides the voltage drop between UIN and UOUT. The value of R must satisfy two conditions:

1. R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D.

For example, the common BZX79C5V6[1] device, a 5.6 V 0.5 W zener diode, has a recommended reverse current of 5 mA. If insufficient current flows through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher then nominal and could rise as high as UIN). When calculating R, allowance must be made for any current flowing through the external load, not shown in this diagram, connected across UOUT.

2. R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX, then IDVB &lt; PMAX.

A zener diode used in this way is known as a shunt voltage regulator (shunt, in this context, meaning connected in parallel, and voltage regulator being a class of circuit that produces a stable voltage across any load).

These devices are also encountered, typically in series with a base/emitter junction, in transistor stages where selective choice of a device centered around the avalanche/zener point can be used to introduce compensating temperature co-efficient balancing of the transistor PN junction. An example of this kind of use would be a DC error amplifier used in a stabilized power supply circuit feedback loop system.

Avalanche diode From Wikipedia, the free encyclopedia • Have questions? Find out how to ask questions and get answers. • Jump to: navigation, search

An avalanche diode is a diode (usually made from silicon, but can be made from another semiconductor) that is designed to break down and conduct at a specified reverse bias voltage.

The Zener diode exhibits an apparently similar effect, but its operation is caused by a different mechanism, called Zener breakdown. Both effects are actually present in any such diode, but one usually dominates the other. Zener diodes are typically restricted to a few tens of volts maximum, but silicon avalanche diodes are available with breakdown voltages of over 4000 V.[1] Contents [hide] it is all wrong * 1 Uses o 1.1 Protection o 1.2 RF noise generation * 2 See also * 3 References Uses

Protection

A common application is protecting electronic circuits against damaging high voltages. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its cathode is positive with respect to its anode. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode suffers avalanche breakdown, causing the harmful voltage to be conducted to earth. When used in this fashion they are often referred to as clamper diodes because they "clamp" the voltage to a predetermined maximum level. Avalanche diodes are normally specified for this role by their clamping voltage VBR and the maximum size of transient they can absorb, specified by either energy (in joules) or i2t. Avalanche breakdown is not destructive, as long as the diode is not allowed to overheat.

RF noise generation

Avalanche diodes generate radio frequency noise; they are commonly used as noise sources in radio equipment. For instance, they are often used as a source of RF for antenna analyzer bridges. Avalanche diodes can also be used as white noise generators.