Transistors Different types and sizes BJT (PNP) Electrical Diagram First Transistor Modern Electronics FET and BJT Transistor

Purpose ▫To amplify and switch electronic signals on or off (high or low) Modern Electronics Microprocessor Cell Phones Motor Controllers

Vacuum tubes Purpose ▫Used as signal amplifiers and switches ▫Advantages  High power and frequency operation  Operation at higher voltages  Less vulnerable to electromagnetic pulses ▫Disadvantages  Very large and fragile  Energy inefficient  Expensive

Invention Evolution of electronics ▫In need of a device that was small, robust, reliable, energy efficient and cheap to manufacture 1947 ▫John Bardeen, Walter Brattain and William Schockly invented transistor Transistor Effect ▫“when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.”

General Applications

Doping Process of introducing impure elements (dopants) into semiconductor wafers to form regions of differing electrical conductivity Negatively charged SemiconductorPositively charged semiconductor

Doping Effects P-type semiconductors ▫Created positive charges, where electrons have been removed, in lattice structure N-type semiconductors ▫Added unbound electrons create negative charge in lattice structure Resulting material ▫P-N junction

P-N junction Forward Biasing Reverse Biasing

P-N junction ▫Controls current flow via external voltage Two P-N junctions (bipolar junction transistor, BJT) ▫Controls current flow and amplifies the current flow

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

Types of Transistors Bipolar Junction Transistor (BJT) Field Effect Transistors (FET) Power Transistors

BJT Introduction Bipolar Junction Transistors (BJT) consists of three “sandwiched” semiconductor layers The three layers are 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

BJT Schematic NPN ▫BE forward bias ▫BC reverse bias PNP ▫BE reverse bias ▫BC forward bias NPN PNP

BJT Characteristic Curves Transfer Characteristic Characteristic curves can be drawn to show other useful parameters of the transistor The slope of I CE / I BE is called the Transfer Characteristic (β)

BJT Characteristic Curves Input Characteristic The Input Characteristic is the base emitter current I BE against base emitter voltage V BE I BE /V BE shows the input Conductance of the transistor. The increase in slope of when the V BE is above 1 volt shows that the input conductance is rising There is a large increase in current for a very small increase in V BE.

BJT Characteristic Curves Output Characteristic collector current (I C ) is nearly independent of the collector-emitter voltage (V CE ), and instead depends on the base current (I B ) I B1 I B2 I B3 I B4

BJT Operating Regions Operating Region ParametersMode Cut Off V BE < V cut-in V CE > V supply I B = I C = 0 Switch OFF Linear V BE = V cut-in V sat < V CE < V supply I C = β*I B Amplification Saturated V BE = V cut-in, V CE < V sat I B > I C,max, I C,max > 0 Switch ON

BJT Applications BJT Switch Offer lower cost and substantial reliability over conventional mechanical relays. Transistor operates purely in a saturated or cutoff state (on/off) This can prove very useful for digital applications (small current controls a larger current)

BJT Applications BJT Amplifier

BJT Applications BJT Amplifier

Field Effect Transistors (FET) Chase Thompson

FET Basics Electric Field Voltage Controlled FET includes three distinct pieces ▫Drain ▫Source ▫Gate

FET versus BJT? Same: Applications: amplifier, switch, etc. Relies on PNP or NPN junctions to allow current flow Difference: Voltage vs Current Input Unipolar vs Bipolar Noise Higher input impedance Fragile and low gain bandwidth

Types of Field-Effect Transistors TypeFunction Junction Field-Effect Transistor (JFET) Uses reversed biased p-n junction to separate gate from body Metal-Oxide-Semiconductor FET (MOSFET) Uses insulator (usu. SiO 2 ) between gate and body Insulated Gate Bipolar Transistor (IGBT) Similar to MOSFET, but different main channel Organic Field-Effect Transistor (OFET) Uses organic semiconductor in its channel Nanoparticle Organic Memory FET (NOMFET) Combines the organic transistor and gold nanoparticles

JFET Reverse Biased PN- junction Depletion mode devices ▫Creates a potential gradient to restrict current flow. (Increases overall resistance)

JFET N-channel JFET P-channel JFET uses same principles but ▫Channel current is positive due to holes instead of electron donors ▫Polarity of biasing voltage must be reversed

N-Type Characteristics

Characteristics and Applications of FETs JFETs Simplest type of FET – easy to make High input impedance and resistance Low Capacitance Slower speed in switching Uses? – Displacement sensor – High input impedance amplifier – Low-noise amplifier – Analog switch – Voltage controlled resistor

MOSFET Similar to JFET ▫A single channel of single doped SC material with terminals at end ▫Gate surrounds channel with doping that is opposite of the channel, making the PNP or NPN type ▫BUT, the MOSFET uses an insulator to separate gate from body, while JFET uses a reverse-bias p-n junction p-channel n-channel MOSFET enhanced mode MOSFET depleted mode

How does a MOSFET work? Simplified Notation No current flow “Short” allows current flow No Voltage to Gate Voltage to Gate Source Drain n n

MOSFET Triode Mode/Linear Region V GS > V th and V DS < ( V GS - V th ) V GS : Voltage at the gate V th : Threshold voltage V DS : Voltage from drain to source μ n : charge-carrier effective mobility W: gate width L: gate length C ox : gate oxide capacitance per unit area λ : channel-length modulation parameter Saturation/Active Mode V GS > V th and V DS > ( V GS - V th )

Characteristics and Applications of FETs MOSFETs Oxide layer prevents DC current from flowing through gate Reduces power consumption High input impedance Rapid switching More noise than JFET Uses? Again, switches and amplifiers in general The MOSFET is used in digital CMOS logic, which uses p- and n- channel MOSFETs as building blocks To aid in negating effects that cause discharge of batteries Use of MOSFET in battery protection circuit

Power Transistors  Concerned with delivering high power  Used in high voltage and high current application In general Fabrication process different in order to:  Dissipate more heat  Avoid breakdown Different types: Power BJTs, power MOSFETS, etc.

Comparison PropertyBJTMOSFETJFET GmBestWorstMedium SpeedHighMediumLow NoiseModerateWorstBest Good Switch NoYes High-Z GateNoYes ESD Sensitivity LessMoreLess