Transistors: Introduction and Types

Unit 02: Transistors

• Introduction and Types
• BJT Construction, Types, and Regions of Operations
• CB and CE configurations with characteristics and current relationships
• BJT used as a Switch and DC Load Line analysis
• Voltage Divider Bias Circuit, Single Stage CE Amplifier
• Enhancement MOSFET: Types, Construction, Operation, and Characteristics

Unit 02-01: Transistors: Introduction and Types

• Transistors are found in any modern electrical device
• Transistors are the basic building blocks of modern-day computing
• They combine to create logic gates that enable computation
• The invention of the transistor in 1947 opened the door to the modern information age
• Before transistors, computers used thermionic valves or vacuum tubes
• The first electronic computer ENIAC weighed more than 27 tons
• It also took up 1800 square feet

Vacuum Tubes

• The basic working principle is thermionic emission--heating a metal to release electrons
• In 1904, John Ambrose Fleming created the first vacuum tube device, called an oscillation valve
• Fleming's device consisted of two electrodes: a cathode and an anode, inside a glass tube
• Heating the cathode releases electrons via thermionic emission
• Applying a positive voltage to the anode attracts the electrons across the gap
• Removing air creates a vacuum that allows electrons to flow from the cathode to the anode
• A vacuum tube with two electrodes is called a diode
• Diodes allow electric current to flow in only one direction
• They are still used today, but are semiconductor based today

Triode Vacuum Tube

• In 1907, Lee de Forest added a third electrode called the control grid
• This enabled the vacuum tube to be used not just as a rectifier but as an amplifier
• The control grid is placed between the cathode and anode
• It is shaped like a mesh to allow electron flow
• Adjusting the voltage applied to the grid controlled the number of electrons flowing from the cathode to the anode
• A strong negative grid voltage repels electrons and chokes the current flow
• Increasing the grid voltage allows more electrons to pass through, increasing current
• The triode can serve as an ON/OFF switch or a signal amplifier

Vacuum Tubes vs Transistors

• Vacuum tubes are bulky and made of glass, which makes them delicate
• Vacuum tubes require high voltages, consume more power and generate more heat
• The failure rate of vacuum tubes is high

Transistors

• In 1947, William Shockley, Walter Brattain, and John Bardeen created the first transistor
• Transistors replicate the functions of tubes (switching and amplification) but are semiconductor based
• The three physicists won the Nobel Prize in Physics in 1956 for their discovery
• A transistor is a semiconductor device to amplify signals or act as an electrically controlled switch
• A transistor is a three terminal device where a small current/voltage at one terminal controls a larger current between the other two
• Transistors are essential components in almost every electronic circuit
• The word "transistor" is a combination of "transfer" and "resistance"
• The first generation of computers depended upon vacuum tubes
• The second generation used transistors
• The third generation used integrated circuits
• The fourth generation came about with the invention of the microprocessor

Transistors Replacing Vacuum Tubes

• Vacuum tubes are not completely extinct
• They remain useful in high-power RF transmitters
• These devices can generate more power than modern semiconductor equivalents
• Vacuum tubes are still found in particle accelerators, MRI scanners, and microwave ovens

Vacuum Tubes and Transistors Compared

• Vacuum Tube Advantages
• Highly linear without negative feedback, especially for some small-signal types
• Smooth clipping, considered more musical than transistors
• Tolerant of overloads and voltage spikes
• Characteristics are highly independent of temperature
• Simpler circuit designs than semiconductor equivalents
• Easier maintenance due to user-replaceable tubes
• Vacuum Tube Disadvantages
• Bulky and less suitable for portable products
• Require high operating voltages and high power consumption with heater supplies
• Generate lots of waste heat
• Lower power efficiency than transistors in small-signal circuits
• Low-cost glass tubes are physically fragile
• Shorter lifetimes (typically 1-5 years for power tubes)
• Higher cost than equivalent transistors
• Transistor Advantages
• Usually lower cost and smaller than tubes, especially in small-signal circuits
• Can be combined in one die to make an integrated circuit
• Lower power consumption and less waste heat than equivalent tubes
• Can operate on low-voltage supplies
• Usually more physically rugged than tubes
• Transistor Disadvantages
• Tendency toward higher distortion than equivalent tubes
• Complex circuits and considerable negative feedback are required for low distortion
• Sharp clipping, considered non-musical
• Device parameters vary considerably with temperature
• Less tolerant of overloads and voltage spikes than tubes
• More difficult maintenance; devices not easily replaced by the user

Types of Transistors

• Transistors are classified into Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET)
• BJTs are classified into NPN and PNP transistors,
• FETs are classified into JFET and MOSFET.
• JFETs are further classified into N-Channel JFET and P-Channel JFET
• MOSFETs are classified into Depletion Mode MOSFET and Enhancement Mode MOSFET
• Depletion and Enhancement mode MOSFETs are further classified into respective N-Channel and P-Channel MOSFETs
• Major families of transistors are BJT and FET
• All transistors use specific arrangements of semiconductor materials like Silicon, Germanium and Gallium-Arsenide
• Transistors are classified based on their structure
• Each type has different characteristics, advantages, and disadvantages

BJTs vs FETs

• A BJT has current flow due to both electrons and holes making it a bipolar device
• A FET has current flow is due to only majority charge carriers making it a unipolar device
• BJTs have three terminals: Emitter (E), Base (B) and Collector (C)
• There are two PN junctions in a BJT
• BJTs are classified into NPN and PNP transistors depending on construction
• BJTs are current-controlled devices; a small current through the Base causes a large current to flow between Collector and Emitter
• FETs have three terminals: Source (S), Gate (G) and Drain (D)
• FETs are voltage-controlled devices
• Voltage Gate-Source controls the Drain-Source current
• FET has greater switching speed and thermal stability than BJT

Junction Field Effect Transistors (JFETs)

• They are the earliest and simplest type of FET
• In N-Channel JFETs, the current flow is due to electrons
• Applying voltage between Gate and Source forms a channel between the Source and Drain
• In P-Channel JFETs, the current flow is due to holes

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

• MOSFETs have a metal gate separated from the semiconductor region by a thin layer of metal oxide (SiO2)
• MOSFETs are also known as Insulated Gate FETs because the Gate region is insulated from the Source and Drain region
• MOSFETs can also include a fourth terminal called the Substrate (SS) for grounding
• MOSFETs offer high input impedance and low output impedance
• MOSFETs available in Depletion and Enhancement types
• Depletion MOSFETs have a permanently present channel between Drain and Source
• Enhancement MOSFETs the channel is created temporarily after applying Gate to Source voltage

Unit 02-02: BJT: Construction, Types, and Regions of Operations

• The BJT construction involves three doped semiconductor regions
• Topics include the construction of BJT
• Two diode analogy of BJT
• The terminals of BJT
• The different types of transistor packages
• The different regions of operation and applications of a BJT based on biasing of its two PN junctions
• The structure of a BJT is like a sandwich with two PN junctions connected back-to-back
• DMM can be used to test a BJT

BJT Construction

• The BJT is constructed with three doped semiconductor regions separated by two PN junctions
• The three regions are called Emitter, Base, and Collector,
• NPN type consists of a P region sandwiched between two N regions
• PNP type consists of a N region sandwiched between two P regions

BJT Carrier Types

• The term bipolar refers to the use of both holes and electrons as current carriers
• Electrons are the majority charge carriers in NPN types
• Holes are the majority charge carriers in PNP type
• NPN transistors are widely used because the mobility of electrons is higher than holes

BJT Junctions

• The PN junction joining the Base and Emitter regions is the Base-Emitter or Emitter junction.
• The PN junction joining the Base and Collector regions is the Base-Collector or Collector junction
• A single PN junction is equivalent to a diode, therefore, a BJT acts like two diodes connected back-to-back
• These leads are labeled E, B and C for Emitter, Base and Collector respectively.
• The Base region is lightly doped and very thin compared, the heavily doped Emitter and the moderately doped Collector regions.

Transistor Packages

• Transistor packages house transistor components, provide connections, protection, and may remove heat
• Transistor package standards include "TO" (transistor outline) and "SOT" (small-outline transistor) for surface mount devices
• Low power transistor packages use plastic with a flat surface
• High power transistors use plastic, metal, or a mix of both with a metal tab for attaching a heat sink

Unit 02-03: CB and CE Configurations with Their Characteristics and Current Relationships

• BJT can be used with different configurations
• Topics covered are the current flow in the BJT
• The current relationships in the BJT
• CB and CE configurations of the BJT
• The region of operation of the BJT
• A three terminal BJT is connected in a circuit having four terminals
• The BJT can be used as a constant current source
• BJT also has alpha (a) and beta (ẞ) associated with it

BJT Terminal Configurations

• BJTs have three terminals: Emitter (E), Base (B) and Collector (C)
• Circuit connections need four terminals: two for input and two for output
• This is solved by using one terminal in common for both input and output sides
• BJT circuits under DC operating conditions are called BJT configurations
• The three main configurations of the BJT:
  • Common Base (CB) configuration
  • Common Emitter (CE) configuration
  • Common Collector (CC) configuration

Common Base Configuration Characteristics

• In CB configuration, the Base is the common terminal between input and output
• Input is applied between Emitter and Base
• Output is taken between Collector and Base
• The B-E junction is forward biased while the B-C transistor is reversed biased
• The input current is Emitter current (IE) while the output current is Collector current (Ic)

Base Current with BJT

• In BJT, the Base current (IB) is very small because the Base region is lightly doped and very thin
• Emitter current (IE) is always equal to Collector current (Ic) plus Base current (IB).
• IE = IC + IB
• The current gain or current amplification factor is defined as the ratio of the output current to the input current
• The current gain of CB configuration is denoted by symbol alpha (ɑ)
• It is calculated as: ɑ = Ic/Ie
• Alpha is always less than unity, with a value from 0.95 to 0.995
• Input characteristic of CB configuration is a graph input current on Y-axis versus input voltage on X-axis at constant output voltage.
• In CB configuration, input is applied between Emitter and Base terminals and the B-E junction is forward biased.
• Output characteristic is the graph of output current on Y-axis verses output voltage on X-axis at constant input current
• There are three regions of operation of the BJT in these characteristics
• The BJT acts as a constant current source within this region

Common Emitter Configuration

• In CE configuration, the Emitter is the common terminal between input and output sides
• Input is applied between Base and Emitter terminals
• Output is taken between Collector and Emitter terminals
• B-E junction is forward biased
• B-C junction is reverse biased
• Input current is Base current (IB)
• Output current is Collector current (Ic)
• The current gain of CE configuration is denoted by symbol beta (β)
• It is calculating as: β= Ic/Ie
• ẞ ranges from 20 to 200 or even higher
• Decreasing effective Base width increases the Base current and decreases beta

Unit 02-04: BJT as Switch, DC Load Line

• Aims to describe BJT switching operation
• Also to give a basic application of a BJT switching circuit and the purpose of DC bias
• Discuss and determine the Q-point / DC operating point of a BJT circuit
• Draw a DC load line for a given biased BJT circuit
• Discuss the reasons for output waveform distortion
• The computer can't operate without a transistor

BJT Switching

• The transistor operates in the cut-off region because the Base-Emitter junction is biased
• In this condition, the transistor acts as an open switch between the Collector and Emitter
• In the saturation region, the Base-Emitter and Base-Collector junctions are forward biased
• The Base current is large enough for the Collector current to reach saturation
• In this condition, the transistor acts as a short circuit between Collector and Emitter

Troubleshooting a Biased BJT

• Several faults can occur in a simple transistor bias circuit
• Possible faults include open bias resistors, open or resistive connections, shorted connections, and opens or shorts internal to the transistor itself
• BJT can used in circuits of basic faults
• If transistor circuit not operating correctly, verify supply is connected and operating
• Look at the Collector to see present voltage, if saturation voltage is 0
• If it is cut off, the Collector Voltage will equal the source voltage

DC Load Line

• DC bias establishes the DC operating point (Q-point) for proper linear operation of an amplifier
• If an amplifier isn't biased with correct DC voltages on the input and output, it can go into saturation or cutoff when an input signal is applied
• The output signal shows an amplified replica of the input signal except that it is inverted
• In proper biasing can cause distortion in the output signal
• A DC load line is drawn on output characteristics of CE configuration of transistor
• Draw a straight line of collector from saturation and cuttoff values to form the Q point

Unit 02-05: Voltage Divider Bias Circuit, Single Stage CE Amplifier

• Also called: Unit 02-05: Voltage Bias circuit;CE amp Bias
• Objectives cover the linear DC operating
• Explain the Voltage Divider Bias Circuit
• Also explain a CE amplifier to a circuit , and what is applied.
• Voltage Bias circuit has one source
• The purpose of biasing a circuit is to establish a proper stable DC operating point

Overview of Volts, Bipolar. Junc.

• Used to provide steady, or known operating current
• Allows current and voltage without any incoming signals
• Voltage Divider Bias is a biasing for a transitor operation using one source ressistance
• Helps replace battery, keeps schematic simular and simpler

Unit 02-06: Enhancement MOSFET: Types, Construction, Operation and Characteristics

• Not detailed in this set of Text