Bipolar junction transistor - Application

The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available, and because of its high transconductance and output resistance compared to MOSFETs. The BJT is also the choice for demanding analog circuits, especially for very high frequency  applications, such as radio-frequency circuits for wireless systems. Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a BiCMOS process of wafer fabrication to create circuits that take advantage of the application strengths of both types of transistor.

Bipolar junction transistor - The BJT theory

Both types of BJT function by letting a small current input to the base control an amplified output from the collector. The result is that the transistor makes a good sitch that is controlled by its base input. In the NPN in what is called active mode, the base-emitter voltage and collector-base voltage       are positive, forward biasing the emitter-base junction and reverse-biasing the collector-base junction. In the active mode of operation, electrons are injected from the forward biased n-type emitter region into the p-type base where they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in the reverse-biased collector-base junction.

Bipolar junction transistor - Types of BJT

TYPES OF BJT :

NPN :

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor between two N-doped layers. When there is a positive potential difference measured from the emitter of an NPN transistor to its base as well as positive potential difference measured from the base to the collector, the transistor becomes active. In this "on" state, current flows between the collector and emitter of the transistor.

PNP :

PNP consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base is amplified in the collector output. PNP transistor is "on" when its base is pulled low relative to the emitter.

Bipolar junction transistor - History

In December 1947 the point-contact transistor was invented at the laboratory called ‘Bell Telephone’ by Walter Brattain and John Bardeen under the direction of William Shockley. The junction version known as the bipolar junction transistor, invented by Shockley, enjoyed three decades as the device of choice in the design of discrete and integrated circuits. Nowadays, the use of the BJT has declined in favor of CMOS technology in the design of digital integrated circuits. 

Bipolar junction transistor - Introduction

INTRODUCTION :

A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the contact of two types of semiconductor for its operation. BJTs can be found either as individual discrete components, or in large numbers as parts of integrated circuits.

BJTs come in two types, PNP and NPN, based on the doping types of the three main terminal regions.

In typical operation, the base–emitter junction is forward biased, which means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base–collector junction is reverse biased. 

In an NPN transistor, when positive bias is applied to the base–emitter junction, the equilibrium is disturbed between the thermally generated carries and the repelling electric field of the n-doped emitter depletion region. The electrons injected from he emitter into the base region, diffuse through the base from the 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 makes holes the 'majority carriers' in the base.

Semiconductor diode - Shockley diode equation

The Shockley ideal diode equation ( also called the diode law) gives the I–V characteristic of an ideal diode. The Shockley ideal diode equation is:

                     
                         

        Where:

                I => is the diode current,

                I_{S =>} is the reverse bias saturation current,

                V_{D =>} is the voltage across the diode,

                V_{T =>} is the termal voltage,

                N => is the ideality factor, depends on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted). By setting n = 1 above, the equation reduces to the Shockley ideal diode equation.

The thermal voltage V_T ( approximately 25.85 mV at 300 K, a temperature close to "room temperature") is commonly used in device simulation software. At any temperature it is a known constant defined by:

        

                              
           where 

                 k => is the Boltzmann constant,

                 T => is the absolute temperature of the p–n junction 
                 q => is the magnitude of charge of an electron. 

Semiconductor diode - types of semiconductor diode

There are different types of p – n junctions diodes, depending often on its physical aspect of a diode.

Normal (p–n) diodes,(usually made of doped silicon or germanium) are found in CMOS integrated circuits, which include two diodes per pin and many other internal diodes.

Here i’m going to explain some of them:

Avalanche diodes:

An avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization.

Cat’s whisker or cristal diodes:

A cat's-whisker detector is an antique electronic component consisting of a thin wire that lightly touches a crystal of semiconducting mineral to make a crude point-contact rectifier.

Constant current diodes:

Constant-current diode they allow a current through them to rise to a certain value, and then level off at a specific value. These devices keep the current flowing through them unchanged when the voltage changes.

Gunn diodes:

Gunn diodes are similar to tunnel diodes (made of materials such as GaAs or InP) that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscilators to be built.

Light-emiting diodes (LEDs):

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting..

Thermal diodes:

Thermal diodes are used for conventional p–n diodes used to monitor temperature due to their varying forward voltage with temperature.

Zener diodes:

A Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "zener knee voltage", "zener voltage" or "avalanche point".

Semiconducor diode - 'p-n' juncion diode

The most basic property of a junction diode is that it conducts an electric current in one direction and blocks it in the other. This behaviour arises from the electrical characteristics of a junction, called a ‘p – n juncion’, fabricated within a semiconductor crystal. The most commonly used semiconductor material is silicon. The junction diode is useful in a wide variety of applications including the rectification of ac signals, the detection of radio signals, the conversion of solar power to electricity, and in the generation and detection of light. It also finds use in a variety of electronic circuits as a switch, as a voltage reference or even as a tunable capacitor. The p-n junction is also the basic building block of a host of other electronic devices, of which the most well-known is the junction transistor. For this reason, a study of the properties and behaviour of the p-n junction is important.

Semiconductor diodes - Electronic symblos

To diference each type of diode we use symbols. There are alternate symbols for some types of diodes, though the differences are minor.

        => Diode                  =>Light Emiting Diode (LED)
         => Photodiode           => Schottky Diode
          => Transient Voltage => Tunnel Diode
                                            Suppression (TVS)

       => Varicap                 => Zener Diode

Seiconductor diodes - Introduction

INTRODUCTION :
A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.

However, diodes have a more complicated behavior than a simple on–off action. Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction. The voltage drop across a forward-biased diode varies only a little with the current, and is a function of temperature.

Semiconductor diodes' nonlinear current-voltage characteristic can be tailored by varying the semiconductor materials and doping, introducing impurities into the materials. These are exploited in special-purpose diodes that perform many different function (eg: diodes are used to regulate voltage, to generate radio frequency oscillations, and to produce light). Tunnel diodes exhibit negative resistance, which makes them useful in some types of circuits.

Diodes were the first semiconductor electronic devices. The first semiconductor diodes (made in 1906) were made of mineral crystals such as galena, and nowadays they are made of sillicon.

Integrated circuit - Developments

Programmable logic devices, developed in the 1980, contain circuits whose logical function and connectivity can be programmed by the user. This allows a single chip to be programmed to implement different LSI-type functions such as logic gates, adders and registers.  

In the past, radios could not be fabricated in the same low-cost processes as microprocessors. But since 1998, a large number of radio chips have been developed using CMOS processes.  

Since the 2000s, the integration of optical functionality into silicon chips has been actively pursued in: academic research and industry, resulting like this in the successful commercialization of silicon based integrated optical transceivers combining optical devices with CMOS based electronics.

Integrated circuits - Classification

HOW CAN ICs BE CLASSIFIED?

Integrated circuits (ICs) can be classified into analog, digital and mixed-signals.

- Digital ICs are further sub-categorized as logic ICs, memory chips, interface ICs, Power Management ICs, and    programmable devides.

- Analog ICs are further sub-categorized as linear ICs and RF ICs.

- Mixed-signals ICs are further sub-categorized as data acquisition ICs and clock/timing ICs.

Integrated circuits - The fabrication of 'Integrated circuits'

Here i lend you a video of how an integrated circuit is made.

Integrated circuits - Kilby's invention

The idea of the integrated circuit was conceived by a radar scientist working for the Royal Radar Establishment of the British Ministre of Defence, Geoffrey W.A Dummer. Dummer presented the idea to the public at the Symposium on Progress in Quality Electronic Components in Washington, D.C. on 7 May 1952. He gave many symposia publicly to propagate his ideas, and unsuccessfully attempted to build such a circuit in 1956.

A precursor idea to the IC was to create small ceramic squares (wafers), each one containing a single miniaturized component. Components could then be integrated and wired into a bidimensional or tridimensional compact grid. This idea, which looked very promising in 1957, was proposed to the US Army by Jack Kilby, and led to the short-lived Micromodule Program. However, as the project was gaining momentum, Kilby came up with a new, revolutionary design: the IC.
Kilby won the 2000 Nobel Prize in Physics for his part of the invention of the integrated circuit.

Integrated circuits - Introduction

INTRODUCTION:

ICs were made possible by experimental discoveries showing that semiconductor devices could perform the functions of vacuum tubes. The integration of large numbers of tiny transistor into a small chip was an enormous improvement over the manual assembly of circuits using discrete electronic components. The integrated circuit's mass production capability, reliability, and building-block approach to circuit design ensured the rapid adoption of standardized integrated circuits in place of designs using discrete transistors.

There are two main advantages of IC’s over discrete circuits:

-Cost is low because the chips, with all their components, are printed as a unit by photolithography rather than being constructed one transistor at a time.

-Performance is high because the components switch quickly and consume little power as a result small size and close proximity of the components.