Engineering Module - Semester I - Transistors PDF

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Transistors Dr Nevan Bermingham What is a transistor? A transistor is a 3 terminal electronic device made of semiconductor material. Transistor...

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Transistors Dr Nevan Bermingham What is a transistor? A transistor is a 3 terminal electronic device made of semiconductor material. Transistors have many uses, including amplification, switching, voltage regulation, and the modulation of signals  History  Before transistors were invented, circuits used vacuum tubes:  Fragile  large in size  Heavy  generate large quantities of heat  require a large amount of power History The first transistors were created at Bell Telephone Laboratories in 1947  William Shockley, John Bardeen, and Walter Brattain created the transistors in and effort to develop a technology that would overcome the problems of tubes  The first patents for the principle of a field effect transistor were registered in 1928 by Julius Lillenfield.  Shockley, Bardeen, and Brattain had referenced this material in their work The word “transistor” is a combination of the terms “transconductance” and “variable resistor” Today an advanced microprocessor can have as many as 1.7 billion transistors. Doping (Revision) Foreign elements are added to the semiconductor to make it electropositive or electronegative P-type semiconductor (postive type)  Dopants include Boron, Aluminum, Gallium, Indium, and Thallium  Ex: Silicon doped with Boron  The boron atom will be involved in covalent bonds with three of the four neighboring Si atoms. The fourth bond will be missing and electron, giving the atom a “hole” that can accept an electron Doping N-type semiconductor (negative type)  Dopants include Nitrogen, Phosphorous, Arsenic, Antimony, and Bismuth Ex: Silicon doped with Phosphorous  The Phosphorous atom will contribute and additional electron to the Silicon giving it an excess negative charge Diodes (Revision) Forward Bias  Current flows from P to N Reverse Bias  No Current flows  Excessive heat can cause dopants in a semiconductor device to migrate in either direction over time, degrading diode https://www.youtube.com/watch?v=IcrBqCFLHIY What is a Transistor?  Bipolar Junction Transistors  NPN Transistor Most Common Configuration  Base, Collector, and Emitter  Base is a very thin region with less dopants  Base collector junction reversed biased  Base emitter junction forward biased Fluid flow analogy:  If fluid flows into the base, a much larger fluid can flow from the collector to the emitter  If a signal to be amplified is applied as a current to the base, a valve between the collector and emitter opens and closes in response to signal fluctuations Operation – Water Analogy What to read some more, see this webpage:- Click Here for More! Operation – Water Analogy What to read some more, see this webpage:- Click Here for More! Symbols How to remember which is which? NPN: Not Pointing iN C: Collector B: Base What to read some more, see this webpage:- E: Emitter Click Here for More! Operation Electricity is considered C to flow Positive to Negative – This is the OPPOSITE to Electron flow. C: Collector B B: Base E: Emitter E What to read some more, see this webpage:- Click Here for More! Animation BJT Transistors BJT (Bipolar Junction Transistor) npn  Base is energized to allow current flow pnp  Base is connected to a lower potential to allow current flow npn BJT Transistors High potential at collector Low potential at emitter Allows current flow when the base is given a high potential (+) pnp BJT Transistors High potential at emitter Low potential at collector Allows current flow when base is connected to a low potential (-) Transistors as signal amplifiers Direct Current (DC) Power Supply The higher Voltage DC power supply is shaped exactly as the input signal at the Base. Low Voltage/Current Signal The output is the same Controls the Base shape as the Input Wave, but Amplified Transistors as signal amplifiers Darlington Transistors Allow for much greater gain in a circuit Field Effect Transistors (FET) Analogous to BJT BJT FET Transistors Collector Drain FET Transistors Base Gate switch by voltage Emitter Source rather than by N/A Body current D G S FET Transistors FET (Field Effect Transistors) MOSFET (Metal-Oxide-Semiconductor Field- Effect Transistor) JFET (Junction Field-Effect Transistor) MESFET (metal–semiconductor field-effect transistor) is a field-effect transistor semiconductor device similar to a JFET) MODFET (modulation-doped field-effect transistor) Most common are the n-type MOSFET or JFET FET Transistors – Circuit Symbols MOSFET In practice the body D D and source leads are B G G B almost always connected S S Most packages have these leads already JFET connected D G S  FET & MOSFET Transistors JFET MOSFET gate gate drain source P N N drain N source P MOSFET Transistors MOSFET Transistors As we know from our studies about diodes, at a PN junction, a depletion region is naturally created even when there are no electric fields. This is the natural state when the gate voltage is 0 (at the GATE) MOSFET Transistors If you increase the gate voltage, that positive voltage will attract electrons in the substrate up to the area between the source and drain, an area called the channel region, this time a physical region. As the electrons gather in the channel, the depletion region expands and eventually there is a depletion region across the entire channel. MOSFET Transistors However, as the gate voltage continues to increase, increasing the electric field, and finally passes the threshold voltage, electrons from the source and drain flow in and form an inversion layer of electrons that connect the source and drain regions. Now that they’re connected, if a voltage is applied across the source and drain, a current will flow. The MOSFET is now operating in the linear (or triode) region. MOSFET Transistors The channel length, L - the distance between the source and drain. Second is the channel width, W - which is how long the source and drain are. These two features are very important when it comes to designing a MOSFET Click Here for a Video on MOSF ETs https://youtu.be/Bfvyj88Hs_o types Bipolar Junction Transistors versus Field Effect Transistors The basic construction of a BJT is two PN junctions producing three terminals. Depending on the type of junctions, the BJT can be a PNP type or an NPN type. The three terminals are identified as the Emitter or E, the Base or B and the Collector or C. BJTs usually function as current controlling switches. types Bipolar Junction Transistors versus Field Effect Transistors The FET construction does not have a PN junction in its main current carrying path Can be made from an N-type or a P-type semiconductor material with high resistivity. A PN junction is formed on the main current carrying path, also called the channel, and this can be made of either a P-type or an N-type material. types Bipolar Junction Transistors versus Field Effect Transistors The three leads of a FET are the Source (S), Drain (D) and Gate (G), with Source and Drain forming the ends of the channel and the Gate controlling the channel conductivity. Unlike the BJT, the FET is a unipolar device since it functions with the conduction of electrons alone for the N-channel type or on holes alone for a P-channel type. types Bipolar Junction Transistors versus Field Effect Transistors FET’s have very high input resistances so very little or no current flows into the input terminal making them ideal for use as electronic switches. When no voltage is applied to the gate of an enhancement FET the transistor is in the “OFF” state similar to an “open switch”. The depletion FET is inherently conductive and in the “ON” state when no voltage is applied to the gate similar to a “closed switch”. Differences between types JFET versus MOSFET Power Transistors Additional material for current handling and heat dissipation Can handle high current and voltage Functionally the same as normal transistors Transistor Uses Switching Amplification Variable Resistor Problem… Problem… Direct Current Source Resistor NPN Bipolar Junction Transistors (NPN BJT) Light Emitting Diode LED Battery Which one will light the LED? Any Questions?