Student Lecture by: Giangiacomo Groppi Joel Cassell Pierre Berthelot September 28 th 2004
Lecture outline Historical introduction Semiconductor devices overview Bipolar Junction Transistor (BJT) Field Effect Transistors (FET) Power Transistors
Transistor History Invention: 1947,at Bell Laboratories. John Bardeen, Walter Brattain, and William Schockly developed the first model of transistor (a Three Points transistor, made with Germanium) They received Nobel Prize in Physics in 1956 "for their researches on semiconductors and their discovery of the transistor effect" First model of Transistor First application: replacing vacuum tubes (big & inefficient). Today: millions of Transistors are built on a single silicon wafer in most common electronic devices
What is a transistor? The Transistor is a three-terminal, semiconductor device. It s possible to control electric current or voltage between two of the terminals (by applying an electric current or voltage to the third terminal). The transistor is an active component. With the Transistor we can make amplification devices or electric switch. Configuration of circuit determines whether the transistor will work as switch or amplifier As a miniature electronic switch, it has two operating positions: on and off. This switching capability allows binary functionality and permits to process information in a microprocessor.
Semiconductors Most used semiconductor: Silicon Basic building material of most integrated circuits Has four valence electrons, in its lattice there are 4 covalent bonds. Silicon crystal itself is an insulator: no free electrons Intrinsic concentration (n i ) of charge carriers: function of Temperature (at room temp. 300K n i = 10 10 /cm 3 )
Semiconductors 2 Electric conductibility in the Silicon crystal is increased by rising the temperature (not useful for our scope) and by doping. Doping consists in adding small amounts of neighbor elements.
Semiconductors 3: Doping Two Dopant Types 1. N-type (Negative) Donor impurities (from Group V) added to the Si crystal lattice. Dominant mobile charge carrier: negative electrons. Group V elements such as Phosphorous, Arsenic, and Antimony. 2. P-type (Positive) Acceptor impurities (from Group III) added to the Si crystal lattice. Dominant mobile charge carrier: positive holes. Group III elements such as Boron, Aluminum, and Gallium. N-type P-type
The simplest example: p-n junction It s also called Junction Diode Allows current to flow from P to N only. Because of the density gradient, electrons diffuse to the p region, holes to the n region. Because of the recombination, the region near the junction is depleted of mobile charges. Two types of behavior: Forward and Reverse biased.
Forward bias Forward biasing: The external Voltage lowers the potential barrier at the junction. The p-n junction drives holes (from the p-type material) and electrons (from the n-type material) to the junction. A current of electrons to the left and a current of holes to the right: the total current is the sum of these two currents.
Reverse bias Reverse biasing: Reverse voltage increases the potential barrier at the junction. There will be a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion region equals the applied voltage, the current will cease except for the small thermal current. It s called reverse saturation current and is due to hole-electrons pairs generated by thermal energy.
Diode characteristics Forward biased (on)- Current flows It needs about 0.7 V to start conduction (V d ) Reversed biased (off)- Diode blocks current Ideal: Current flow = 0 Real: I flow = 10-6 Amps (reverse saturation current) V threshold
Bipolar Junction Transistor (BJT) 3 adjacent regions of doped Si (each connected to a lead): Base. (thin layer,less doped). Collector. Emitter. 2 types of BJT: npn. pnp. Most common: npn (focus on it). npn bipolar junction transistor Developed by Shockley (1949) pnp bipolar junction transistor
BJT npn Transistor 1 thin layer of p-type, sandwiched between 2 layers of n-type. N-type of emitter: more heavily doped than collector. With V C >V B >V E : Base-Emitter junction forward biased, Base-Collector reverse biased. Electrons diffuse from Emitter to Base (from n to p). There s a depletion layer on the Base-Collector junction no flow of e - allowed. BUT the Base is thin and Emitter region is n + (heavily doped) electrons have enough momentum to cross the Base into the Collector. The small base current I B controls a large current I C
BJT characteristics Current Gain: α is the fraction of electrons that diffuse across the narrow Base region 1- α is the fraction of electrons that recombine with holes in the Base region to create base current The current Gain is expressed in terms of the β (beta) of the transistor (often called h fe by manufacturers). β (beta) is Temperature and Voltage dependent. It can vary a lot among transistors (common values for signal BJT: 20-200). I I C B β = = αi = (1 α) I I I C B E E α = 1 α
npn Common Emitter circuit Emitter is grounded. Base-Emitter starts to conduct with V BE =0.6V,I C flows and it s I C =β I B. Increasing I B, V BE slowly increases to 0.7V but I C rises exponentially. As I C rises,voltage drop across R C increases and V CE drops toward ground. (transistor in saturation, no more linear relation between I C and I B )
Common Emitter characteristics Collector current controlled by the collector circuit. (Switch behavior) In full saturation V CE =0.2V. Collector current proportional to Base current The avalanche multiplication of current through collector junction occurs: to be avoided No current flows
BJT as Switch V in (High) BE junction forward biased (V BE =0.7V) Saturation region V CE small (~0.2 V for saturated BJT) V out = small I B = (V in -V B )/R B V out = Low V in (Low ) < 0.7 V BE junction not forward biased Cutoff region No current flows V out = V CE = V cc V out = High
BJT as Switch 2 Basis of digital logic circuits Input to transistor gate can be analog or digital Building blocks for TTL Transistor Transistor Logic Guidelines for designing a transistor switch: V C >V B >V E V BE = 0.7 V I C independent from I B (in saturation). Min. I B estimated from by (I Bmin I C /β). Input resistance such that I B > 5-10 times I Bmin because β varies among components, with temperature and voltage and R B may change when current flows. Calculate the max I C and I B not to overcome device specifications.
Operation point of BJT Every I B has a corresponding I-V curve. Selecting I B and V CE, we can find the operating point, or Q point. Applying Kirchoff laws around the base-emitter and collector circuits, we have : I B = (V BB -V BE )/R B V CE = V cc I C *R C I C = V R CC C V R CE C
Operation point of BJT 2 I C = V R CC C V R CE C Q Load-line curve
BJT as amplifier Common emitter mode Linear Active Region Significant current Gain Example: Let Gain, β = 100 Assume to be in active region -> V BE =0.7V Find if it s in active region
BJT as amplifier 2 V I I I V BE E B C CB = = 0.7V I B B = β * I = V + I CC B C = 100*0.0107= 1.07mA I = ( β + 1) I VBB VBE = = R + R *101 E C * R C I B 5 0.7 402 E * R E = 0.0107mA V = 10 (3)(1.07) (2)(101*0.0107) 0.7 = = 3.93V BE = V CB >0 so the BJT is in active region
Operation region summary Operation Region Cutoff Saturation Active Linear Breakdown I B or V CE Char. I B = Very small V CE = Small V CE = Moderate V CE = Large BC and BE Junctions Reverse & Reverse Forward & Forward Reverse & Forward Beyond Limits Mode Open Switch Closed Switch Linear Amplifier Overload
Field Effect Transistors 1955 : the first Field effect transistor works Increasingly important in mechatronics. Similar to the BJT: Three terminals, Control the output current BJT Terminal Base Collector Emitter FET Terminal Gate Drain Source
Field Effect Transistors Three Types of Field Effect Transistors MOSFET (metal-oxide-semiconductor field-effect transistors) Enhancement mode Depletion mode JFET (Junction Field-effect transistors) Each in p-channel or n-channel The more used one is the n-channel enhancement mode MOSFET, also called NMOS
MOSFET (enhancement mode n-channel) Symbols (base connected to the source or not) Enhancement mode N-channel => Source and Drain are n type Enhancement mode => Depletion mode The arrow head indicates the direction of the pn substratechannel junction Increase VGS to make the travel from D to S easier for the electrons
NMOS Behavior VGS < Vth IDS=0 V GS > Vth : 0 < VDS < VPinch off Depletion mode (or active region), gate holes are repelled. variable resistor (controled by VGS) VDS > VPinch off Inversion mode (or saturation region), IDS constant. VDS > VBreakdown IDS increases quickly Should be avoided
NMOS Characteristic For VDS > VPinchoff, the base current is a function of VGS Active region Pinchoff Point Saturation region
NMOS Vs PMOS Symbols:
NMOS Vs PMOS VGS > Vth Vth < 0 IDS=0 V GS < Vth : 0 < VDS < VPinch off Depletion mode (or active region), gate holes are repelled. variable resistor (controled by VGS) VDS > VPinch off Inversion mode (or saturation region), IDS constant. VDS > VBreakdown IDS increases quickly Should be avoided Analogous to the pnp BJT
NMOS uses High-current voltage-controlled switches Analog switches Drive DC and stepper motor Current sources Chips and Microprocessors CMOS: Complementary fabrication
NMOS Example For V pinchoff < V DS < 0 And V GS > V TH
JFET overview The circuit symbols: JFET design:
Can be used with V G =0 JFET Behavior
Can be used with V G < 0 JFET Behavior
JFET Behavior VGS > Vth IDS=0 V GS < Vth : 0 < VDS < VPinch off Depletion mode (or active region), gate holes are repelled. variable resistor (controled by VGS) VDS > VPinch off Inversion mode (or saturation region), IDS constant. VDS > VBreakdown IDS increases quickly Should be avoided Analogous to the pnp BJT
JFET uses Small Signal Amplifier Voltage Controlled Resistor Switch
General: Signal Amplifiers Switches FET Summary JFET: For Small signals Low noise signals Behind a high impedence system Inside a good Op-Ampl. MOSFET: Quick Voltage Controlled Resistors RDS can be really low : 10 mohms
Power Transistors In General Fabrication is different in order to: Dissipate more heat Avoid breakdown So Lower gain than signal transistors BJT essentially the same as a signal level BJT Power BJT cannot be driven directly by HC11 MOSFET base (flyback) diode Large current requirements
References Introduction to Mechatronics and Measurement Systems by D.G. Alciatore, McGraw-Hill Microelectronics by J. Millman, McGraw-Hill Several Images from Internet: some websites are: http://www.engr.colostate.edu/~dga/mechatronics/figures/ http://www.ecse.rpi.edu/~schubert/course-ecse-6290 SDM-2/ http://hyperphysics.phy-astr.gsu.edu/hbase/solids/diod.html