BJT presentation report

BIPOLAR JUNCTION TRANSISTOR

MANDALIA DARSHAN [07BEC042] PRAVIN GARETA [08BEC156]

Guided By : Mr. CHINTAN BHATT

INSTITUTE OF TECHNOLOGY ,NIRMA UNIVERSITY,AHMEDABAD

CONTENTS إ

Introdoction & History of invention

Transistor Construction

Transistor Operation

Transistor Configuration

Datasheet explanation

Application & Latest Researches

References

INTRODUCTION AND HISTORY

In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much larger than the controlling (input) power, the transistor provides amplification of a signal. The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. Some transistors are packaged individually but most are found in integrated circuits.

INVENTION

The first patent for the field-effect transistor principle was filed in Canada by Austrian-Hungarian physicist Julius Edgar Lilienfeld on October 22, 1925, but Lilienfeld published no research articles about his devices, and they were ignored by industry. In 1934 German physicist Dr. Oskar Heil patented another field-effect transistor. There is no direct evidence that these devices were built, but later work in the 1990s show that one of Lilienfeld's designs worked as described and gave substantial gain. Legal papers from the Bell Labs patent show that William Shockley and a co-worker at Bell Labs, Gerald Pearson, had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles.

The first transistor

The Bell team made many attempts to build such a system with various tools, but generally failed. Setups where the contacts were close enough were invariably as fragile as the original cat's whisker detectors had been, and would work briefly, if at all. Eventually they had a practical breakthrough. A piece of gold foil was glued to the edge of a plastic wedge, and then the foil was sliced with a razor at the tip of the triangle. The result was two very closely spaced contacts of gold. When the plastic was pushed down onto the surface of a crystal and voltage applied to the other side (on the base of the crystal), current started to flow from one contact to the other as the base voltage pushed the electrons away from the base towards the other side near the contacts. The point-contact transistor had been invented.

On 15 December 1947, "When the points were, very close together got voltage amp about 2 but not power amp. This voltage amplification was independent of frequency 10 to 10,000 cycles".

On 16 December 1947, "Using this double point contact, contact was made to a germanium surface that had been anodized to 90 volts, electrolyte washed off in H2O and then had some gold spots evaporated on it. The gold contacts were pressed down on the bare surface. Both gold contacts to the surface rectified nicely... The

separation between points was about 4x10-3 cm. One point was used as a grid and the other point as a plate. The bias (D.C.) on the grid had to be positive to get amplification... power gain 1.3 voltage gain 15 on a plate bias of about 15 volts".

Brattain and H. R. Moore made a demonstration to several of their colleagues and managers at Bell Labs on the afternoon of 1947,23 December often given as the birth date of the transistor. The "PNP point-contact germanium transistor" operated as a speech amplifier with a power gain of 18 in that trial. In 1956 John Bardeen, Walter Houser Brattain, and William Bradford Shockley were honored with the Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect".

This is the replica of first working transistor.

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Origin of the term

Bell Telephone Laboratories needed a generic name for the new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode", "Crystal Triode" and "Iotatron" were all considered, but "transistor," coined by John R. Pierce, won an internal ballot.

Transistor. This is an abbreviated combination of the words "transconductance" or "transfer", and "resistor". The device logically belongs in the resistor family, and has the transconductance or transfer impedance of a device having gain, so that this combination is descriptive.

– Bell Telephone Laboratories — Technical Memorandum (May 28, 1948)

Pierce recalled the naming somewhat differently:

The way I provided the name, was to think of what the device did. And at that time, it was supposed to be the dual of the vacuum tube. The vacuum tube had transconductance, so the transistor would have 'transresistance.' And the name should fit in with the names of other devices, such as varistor and thermistor. And. . . I suggested the name 'transistor.'

The Nobel Foundation states that the term is a combination of the words "transfer" and "resistor"

- Nobelprize.org - The Transistor

FAMILY OF TRANSISTOR

Bipolar (junction) transistor   ( BJT ) BJT is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes, as opposed to unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow.

A junction transistor consists of a silicon crystal in which n type of semiconductor is sandwitched between two p type semiconductor ,is called pnp transistor. Alternatively ,transistor in which p type semiconductor is sandwiched in between n type semiconductor is called npn type bipolar junction transistor. Three portions of transistors are known as Emitter,collector and base. In which emitter is highly doped,base has the lowest impurity concentration.

Types of Transistor

Bipolor junction

Transistor

Field effect transistor

TRIAC THRISTOR

NPN PNP JFET MOSFET

Structure

Simplified cross section of a planar NPN bipolar junction transistor

A BJT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP, and ntype, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) and collector (C).

The base is physically located between the emitter and the collector and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section view of a BJT indicates that the collector–base junction has a much larger area than the emitter–base junction.

Small changes in the voltage applied across the base–emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled current sources, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base.

NPN

The symbol of an NPN Bipolar Junction Transistor.

PNP

The symbol of a PNP Bipolar Junction Transistor.

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Transistors in circuits

Structure and use of npn transistor

The diagram opposite is a schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs, but can be different depending on the current level selected for the application and the type of transistor. This applied voltage causes the lower p-n junction to 'turn-on' allowing a flow of electrons from the emitter into the base. Because of the electric field existing between base and collector (caused by VCE), the majority of these electrons cross the upper p-n junction into the collector to form the collector current,IC. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, IB . As shown in the diagram, the emitter current, IE, is the total transistor current which is the sum of the other terminal currents. That is:

In the diagram, the arrows representing current point in the direction of the electric or conventional current—the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge. The ratio of the collector current to the

base current is called the DC current gain. This gain is usually quite large and is often 100 or more.

I t should also be noted that the emitter current is related to VBE exponentially. At room temperature, increasing VBE by about 60 mV increases the emitter current by a factor of 10. The base current is approximately proportional to the emitter current, so it varies the same way.

TRANSISTOR CURRENT COMPONENTS ▐

NPN BJT with forward-biased E–B junction and reverse-biased B–C junction

An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the

Above fig indicates the water pipe analogy for transistor

region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.

The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. To ensure this, the thickness of the base is much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction.

Applying the kirchhoff's current law we get

IE = I C + IB.

and find that emitter current is the sum of collector and base current .

However collector currentcomprises two current components -minority charge carrier and majority charge carriers.

Ic = Ic(majority)+Ico

Transistor 'alpha' and 'beta'

The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by βF or hfe. It is approximately the ratio of the DC collector current to the DC base current in forward-active region, and is typically greater than 100. Another important parameter is the common-base current gain, αF. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor):

β = Ic / Ib

α = Ic / Ie

so Ic = α Ie +Icbo

and relatioshipbetween alphaand beta is

α = β/(1+ β) and β = α /(1- α)

Regions of operation ▐Bipolar transistors have five distinct regions of operation, defined mostly by applied bias:

active : The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βF, in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current flow, which corresponds to a logical "off", or an open switch.

Transistor Characteristics

Common emitter Configuration

Common emitter configuration is used mostly due to high voltage and current gain.Such circuit is indicated below.In which PNP transistor emitter is common for input and output.

Input Characteristics Output Characteristics

Active region

Transistor is operated in this region whenever it is to be used as amplifier.

In the active region emitter and collector junction are kept in forward bias.generally amplifier is used in active region only.

Here from the diagram we can write that,

Ib = - (Ic+Ib)

So we can derive that, IC = (1+β) ICO + βIb

Cut-off region

Saturation Region

typically used as a voltage amplifier. In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power supply rail

APPLICATION

Low frequency voltage amplifier

A typical example of the use of a common-emitter amplifier is shown in Figure .

Common –Base Configuration

In the common base configuration base is common for input and output .So the voltage gain depends almost exclusively on the ratio of the resistors RC / RE rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.

Characteristics Input characteristics

Output characteristics

Applications

This arrangement is not very common in low-frequency circuits, where it is usually employed for amplifiers that require an unusually low input impedance, for example to act as a preamplifier for moving-coil microphones. However, it is popular in high-frequency amplifiers, for example for VHF and UHF, because its input capacitance does not suffer from the Miller effect, which degrades the bandwidth of the common-emitter configuration, and because of the relatively high isolation between the input and output. This high isolation means that there is little feedback from the output back to the input, leading to high stability.

This configuration is also useful as a current buffer since it has a current gain of approximately unity (see formulas below). Often a common base is used in this manner, preceded by a common-emitter stage. The combination of these two form the cascode configuration, which possesses several of the benefits of each configuration, such as high input impedance and isolation.

Common collector

PNP common collector configuration

In electronics, a common-collector (also known as an emitter follower or voltage follower) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal of the transistor serves as the input, the emitter the output, and the collector is common to both (for example, it may be tied to ground reference or a power supply rail)oltage buffer

Application

Voltage buffer

The common collector circuit can be shown to have a voltage gain of almost unity. Intuitively, this behavior is understood by realizing that the base-emitter voltage in the bipolar transistor is very insensitive to bias changes, so any change in base voltage is transmitted (to good approximation) directly to the emitter. Hence the name, emitter follower. Mathematically, the gain is:

Therefore a small voltage change on the input terminal will be replicated at the output (depending slightly on the transistor's gain and the value of the load resistance; see gain formula below). This circuit is useful because it has a large input impedance, so it will not load down the previous circuit:

and a small output impedance, so it can drive low-resistance loads:

(Typically, the emitter resistor is significantly larger and can be removed from the equation):

This allows a source with a large output impedance to drive a small load impedance; it functions as a voltage buffer.

In other words, the circuit has current gain (which depends largely on the hFE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load.

One aspect of buffer action is transformation of impedances. For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thevenin resistance is reduced to only the output resistance of the voltage follower, a small resistance. That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a small load resistance and a driving stage presents a large load to the driving stage, an advantage in coupling a voltage signal to a small load.

Power amplifier

This configuration is commonly used in the output stages of class-B and class-AB amplifier — the base circuit is modified to operate the transistor in class-B or AB mode. In class-A mode, sometimes an active current source is used instead of RE to improve linearity and/or efficiency.

SOME GENERAL INFORMATION ABOUT BJT

Primary considerations when selecting a transistor are:

Voltage ratings of all three junctions of a transistor Transistor power rating and thermal resistance Transistor current handling capability and the transistor case size Leakage currents, mainly Icbo and Iebo in a transistor Transistor frequency response and /or switching times. Transistor current gain (HFE and hfe) Temperature parameter variation of a transistor Collector-emitter saturation resistance of a transistor h-parameters of a transistor

for linear applications Refer data sheet provided by company.

The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized to forward-mode operation, interchanging the collector and the emitter makes the values of a and ß in reverse operation much smaller than those found in for-ward operation; often the a of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection effi-ciency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junc-tion must come from the emitter.

Introduction to IC and History• …. Vacuum tubes, discrete components• 1947 The first integrated transistor (Bell Telephone Laboratories)• 1959 The first bipolar planar transistor• 1958 The first integrated circuit available as a monolithic chip (flip-flop)• 1965 The first op-amp• 1971 The first 4bit microprocessor (Intel 4004)• 1972 The first 8bit microprocessor (Intel 8008)• 1981 The first IBM PC

Application

a simple, low-cost, and low-power alpha-particle detection system for environmental radioactivity monitoring. The system exploits a previuosly-developed high- resistivity-silicon detector with internal amplification capability based on the bipolar-transistor (BJT) effect and readout electronics based on commercial IC's. Two-dimensional numerical device simulations are adopted to assess the feasibility of the BJT detector as an alpha-particle detector that can be operated, without losing its internal signal amplification capability, with floating base and low collector voltages, so that device technology can be kept simple, very small DC power consumption can be achieved, and a single 5-V power-supply voltage can be used for readout electronics and detector biasing. The charge amplification accomplished by the BJT detector allows a single, commercial chip to be adopted, to perform charge preamplification and 20-bit A/D conversion. The digital output is sent to a low-cost microcontroller that can be periodically interrogated through the IR port. The cost of the readout electronics is in the order of 60$ and it can operate with standard Li-ion battery for about 60 hours.

Other major applications are as follows:

Switching

Amplification

Oscillating Circuits

Sensors

DATA SHEET

REFERENCES BOOKSIntegrated Electronics : By Millman & HalkiasElectronics devices & circuits : By Robert Boystead Electronics devices & circuits Part I : By A P Godse and U A BakshiEDC BY G K MITHALUseful WESITES

Bipolar Junction Transistor : Free Wikipedia Encyclopedia

IEEE Spectrum june 1997 http://tcm.org/html/history/timeline/threads/components/index.html CEH (presently CSEM), Neuchâtel FASELEC (presently Philips) Zürich

S. Joly, Intelegis Marin

http://www.intel.com/intel/museum/25anniv/hof/hof_main.htm

http://infopad.eecs.berkeley.edu/CIC/die_photos/

http://micro.magnet.fsu.edu/chipshot.html

– Mobile Robots: Inspiration to Implementation. Jones, Seiger & Flynn.

– • Introduction to Mechatronics. Histan & Alciatore.

– • The Art of Electronics. Horowitz. (1980).

– • http://Whatis.techtarget.com