Transistors: Definition, Types and Working

• The study notes cover the topic of Transistors

Objectives of this study

• Understand the history and the definition of a transistor
• Learn about the different types and its basic construction
• Understand how a Bipolar Junction Transistor (BJT) and a Field Effect Transistor (FET) work

Introduction to Transistors

• The invention of the transistor initiated a technological revolution that is still relevant today
• Complex electronic devices and systems are based on early developments in semiconductor transistors
• There was a need for a device that was small, robust, reliable, energy efficient, and cheap to manufacture

Importance of Transistors

• The transistor is considered the most important invention of the 20th century
• Transistors replaced vacuum tubes
• They are integral to the integrated circuit which makes the information age electronics possible

What is a Transistor?

• The term "transistor" is a shortened form of "current-transferring resistor"
• The transistor is a three-layer semiconductor device
• It typically contains two n-type layers and one p-type layer, or two p-type layers and one n-type layer of material
• Transistors amplify, control, and generate electrical signals

Purpose

• The purpose of a transistor is to amplify and switch electronic signals on or off (high or low)

Applications

• Modern electronics which includes microprocessors, cell phones, and motor controllers use transistors

Vacuum Tubes

• Vacuum tubes used to be for signal amplifiers and switches

Advantages of Vacuum Tubes

• Operate at high power and high frequency
• Can be used in higher voltage system
• Less vulnerable to electromagnetic pulses

Disadvantages of Vacuum Tubes

• Bulky and fragile
• Energy inefficient
• Expensive

History of Transistors: Key Milestones

1874

• Ferdinand Braun discovered rectification
• He discovered crystals that can only conduct current in one direction under certain conditions

1883

• Edison discovered the Edison effect, or thermionic emission
• The flow of electrons from metals is caused by thermal vibration energy (heat) that overcomes the electrostatic forces holding electrons to the surface

1895

• Guglielmo Marconi sent a radio signal over a distance of more than one mile.

1895

• John Ambrose Fleming developed the vacuum tube
• He created a device that can modify a signal by controlling the movement of electrons in an evacuated space
• Electron flow occurs only from the filament to the plate, creating a diode, which is a device that conducts current in one direction only

1898

• Thomson discovered the electron

1906

• Lee De Forest invented the Triode vacuum tube
• A "Triode amplifier" allows for farther telephone conversations
• The triode was unreliable and used a lot of power

1907

• Bell telephone patents expire
• AT&T (Bell's company) bought De Forest's triode patent
• This resulted in transcontinental telephone service

1928

• The first patents for the transistor principle were registered in Germany by Julius Edgar Lilienfield
• He proposed the basic principle behind the MOS field-effect transistor

1934

• German Physicist Dr. Oskar Heil patented the field effect transistor

1936

• Mervin Kelly, Bell Lab's director of research, believed that a better amplifier was needed for the best phone service
• He formed a department dedicated to solid state science.

1945

• Bill Shockley was the team leader of the solid state department (Hell's Bell Lab) hired Walter Brattain and John Bardeen
• He designed the first semiconductor amplifier, relying on the field effect
• The device was a small cylinder coated thinly with silicon, mounted close to a small, metal plate
• The device didn't work, and Shockley assigned Bardeen and Brattain to find out why

1947

• Bardeen and Brattain built the point contact transistor
• It was made of strips of gold foil on a plastic triangle, pushed down into contact with a slab of germanium

1947 cont.

• Shockley created the Junction transistor which served as a "sandwich"
• This transistor was more practical and easier to fabricate than the point contact transistor
• The Junction Transistor became the central device of the electronic age

1948

• Bells Lab unveiled the transistor
• It was decided to name it a transistor instead of a Point-contact solid state amplifier
• John Pierce invented the name, combining transresistance with the ending common to devices, like varistor and thermistor

The 1950s

• Sony received a license from Bell Labs to build transistors
• In 1946, Sony produced products for radio repair
• In 1950, Sony decided to build something for the mass consumption; the transistor radio
• In the United States, transistors were primarily used for computers and military applications

1955

• The Shockley Semiconductor foundation helped sow the seeds of silicon valley

1957

• The "traitorous eight" abandoned Shockley establishing Fairchild Semiconductor.

1958

• Jack Kilby of Texas Instruments invented the Integrated Circuit (IC)
• It occurred to him that all parts of a circuit could be made out of the same piece of silicon
• The entire circuit could be built out of a single crystal
• It reduced the size and made it easier to produce electric components

1958 cont.

• The integrated circuit is a single device that contains an interconnected array of elements like transistors, resistors, capacitors, and electrical circuits contained in a silicon wafer

1968

• Bob Noyce and Gordon Moore, two of the traitorous eight together with Andy Grove, form Intel Corporation

Moore's Law

• An observation made by Gordon E. Moore, predicted that the number of transistors inside an Integrated Circuit would double every 24 months
• It also noted at the density that the cost of a transistor was also minimized

Transistor Problems

• Power density increased
• Device variability
• Reliability
• Complexity
• Leakage
• Power dissipation limits device density
• Transistors will operate near ultimate limits of size and quality – eventually, no transistor can be fundamentally better

The Future of Transistors

• Molecular electronics
• Carbon nanotubes transistors
• Nanowire transistors
• Quantum computing
• CMOS devices will add functionality to CMOS non-volatile memory, opto-electronics, sensing...
• CMOS technology will address new markets macroelectronics, bio-medical devices,
• Biology may provide inspiration for new technologies bottom-up assembly, human intelligence

General Applications

• Application of the transistor include general applications such as microphones, robotics and laptops

How a Transistor Works

• A transistor can function as an insulator or a conductor
• Transistor's fluctuate between these two states, allowing switching or amplification
• While transistors have multiple applications, their main are functions are switching and amplification
• The transistor works like a dimmer
  • Pushing the dimmer knob turns the light on and off, creating a switch
  • Rotating the knob back and forth adjusts the light's brightness, which involves a modulator

How a Transistor Works Cont.

• Both the dimmer and the transistor can control current flow
• Both can act as a switch and as a modulator/amplifier
• Transistors are millions of times faster than a “hand” operated device like a light dimmer

Components

• Transistors are made of semi-conductors such as silicon and gallium arsenide
• These materials carry electricity not well enough to be called conductors; nor badly enough to be called insulators
• They are hence named semiconductor
• The transistor controls its own semi conductance, acting like a conductor when needed, or as an insulator (nonconductor) when that is needed

Transistor's Analogy

• You can compare a transistor to an ordinary faucet
• The water that enters the faucet would be like a source in the Transtistor
• The water then leaves the faucet into the sink, working as a drain in the Transistor
• The water tap controls the amount of water acting as the gate in the Transistor
• With a small force current can be controlled, similar to the charge of the gate

Transistors are Made of Silicon

• Silicon is a grey colored element with crystalline structure
• It is the second most abundant element in the earth's crust, after oxygen
• Silicon is always found in combined form in nature, often with oxygen as quartz, and is found in rocks and silica sand
• Silicon needs to be in a very pure form to be used as a semiconductor
• If there is more than one impure particle in a million, the silicon can not be used
• Silicon is the most frequently used semiconducting material today

Doping

• Doping is the process of introducing impure elements (dopants) into semiconductor wafers to form regions of differing electrical conductivity

Doping Effects

• P-type semiconductors create positive charges, where electrons have been removed, in the lattice structure
• N-type semiconductors add unbound electrons to create a negative charge in the lattice structure
• A P-N junction results from this combination

P-N Junctions

• P-N junctions control current flow via external voltage.
• Two P-N junctions create the bipolar junction transistor (BJT)
• They control the current flow and amplify the current flow

Transistor Categories

• Semiconductor material
• Structure
• Polarity
• Maximum power rating
• Maximum operating frequency
• Application
• Physical packaging
• Amplification factor

Types of Transistors

• There types of transistors include BJTs and FETs

Bipolar Junction Transistor (BJT)

• Bipolar Junction Transistors (BJT) consist of three “sandwiched" semiconductor layers connected to collector (C), emitter (E), and base (B) pins
• Current supplied to the base controls the amount of current that flows through the collector and emitter

Bipolar Junction Transistor Schematic

• BJTs include NPN & PNP configuration and different bias types

BJT Characteristics Curves

• Characteristic curves show transistor parameters
• The slope of ICE / IBE is called the Transfer Characteristic (β)

BJT Characteristic Curves of Input

• The measure of conductivity of a transistor can be indicated in a graph with the Base-Emitter current against the Base-Emitter voltage

BJT output characteristics

• The collector current (Ic) is nearly independent of the collector-emitter voltage (VCE), and instead depends on the base current (IB).

BJT Operating Regions

• BJT Operating Regions can be in a cut off, linear or saturated region

BJT Applications

• BJTs can be used as a switch and offers a lower cost and substantial reliability over conventional mechanical relays
• Transistors operates purely in a saturated or cutoff state (on/off)
• Can be used in digital applications when configured as a switch

BJT Applications as an Amplifier

• BJTs can be used as a 1 watt audio amplifier in many electrical designs

Field Effect Transistors (FET)

• FETs also amplify of switch electrical currents but works differently to a BJT

FET Basics

• FETs operate using an electric field and Voltage Control
• It includes three distinct pieces which are the Drain, Source and Gate

FET versus BJT?

• Similarities include applications as an amplifier, a switch etc and relies of PNP or NPN junctions to allow current flow
• Differences include Voltage vs Current Input which is unipolar vs Bipolar and has a higher noise and lower gain bandwidth

Types of Field-Effect Transistors