Transistors Student Lecture by: Giangiacomo Groppi Joel Cassell Pierre Berthelot September 28th 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 application: replacing vacuum tubes (big & inefficient). Today: millions of Transistors are built on a single silicon wafer in most common electronic devices First model of Transistor

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 (ni) of charge carriers: function of Temperature (at room temp. 300K ni = 1010 /cm3)

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 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. 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. P-type N-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 (Vd ) Reversed biased (off)- Diode blocks current Ideal: Current flow = 0 Real : Iflow= 10-6 Amps (reverse saturation current) V threshold

BJT

Bipolar Junction Transistor (BJT) npn bipolar junction transistor pnp bipolar junction transistor 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). Developed by Shockley (1949)

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 VC>VB>VE: 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 IB controls a large current IC

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 hfe by manufacturers). β (beta) is Temperature and Voltage dependent. It can vary a lot among transistors (common values for signal BJT: 20 - 200).

npn Common Emitter circuit Emitter is grounded. Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=b*IB. Increasing IB, VBE slowly increases to 0.7V but IC rises exponentially. As IC rises ,voltage drop across RC increases and VCE drops toward ground. (transistor in saturation, no more linear relation between IC and IB)

Common Emitter characteristics Collector current controlled by the collector circuit. (Switch behavior) In full saturation VCE=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 Vin(Low ) < 0.7 V BE junction not forward biased Cutoff region No current flows Vout = VCE = Vcc Vout = High Vin(High) BE junction forward biased (VBE=0.7V) Saturation region VCE small (~0.2 V for saturated BJT) Vout = small IB = (Vin-VB)/RB Vout = Low

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: VC>VB>VE VBE= 0.7 V IC independent from IB (in saturation). Min. IB estimated from by (IBmin» IC/b). Input resistance such that IB > 5-10 times IBmin because b varies among components, with temperature and voltage and RB may change when current flows. Calculate the max IC and IB not to overcome device specifications.

Operation point of BJT Every IB has a corresponding I-V curve. Selecting IB and VCE, we can find the operating point, or Q point. Applying Kirchoff laws around the base-emitter and collector circuits, we have : IB = (VBB-VBE)/RB VCE = Vcc – IC*RC

Operation point of BJT 2 Load-line curve Q

BJT as amplifier Common emitter mode Linear Active Region Significant current Gain Example: Let Gain, b = 100 Assume to be in active region -> VBE=0.7V Find if it’s in active region

BJT as amplifier 2 VCB>0 so the BJT is in active region

Operation region summary IB or VCE Char. BC and BE Junctions Mode Cutoff IB = Very small Reverse & Reverse Open Switch Saturation VCE = Small Forward & Forward Closed Switch Active Linear VCE = Moderate Reverse & Forward Linear Amplifier Break-down VCE = Large Beyond Limits Overload

FET

Field Effect Transistors 1955 : the first Field effect transistor works Increasingly important in mechatronics. BJT Terminal FET Terminal Base Gate Collector Drain Emitter Source Similar to the BJT: Three terminals, Control the output current

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) Depletion mode Symbols (base connected to the source or not) The arrow head indicates the direction of the pn substrate-channel junction N-channel => Source and Drain are n type Enhancement mode => Increase VGS to make the travel from D to S easier for the electrons

NMOS Behavior VGS < Vth VGS > Vth : 0 < VDS < VPinch off IDS=0 VGS > 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 Active region Saturation region For VDS > VPinchoff , the base current is a function of VGS Pinchoff Point

NMOS Vs PMOS Symbols:

NMOS Vs PMOS VGS > Vth Vth < 0 VGS < Vth : IDS=0 VGS < 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. Analogous to the pnp BJT VDS > VBreakdown IDS increases quickly Should be avoided

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 Vpinchoff < VDS < 0 And VGS > VTH

JFET overview The circuit symbols: JFET design:

JFET Behavior Can be used with VG=0

JFET Behavior Can be used with VG < 0

JFET Behavior VGS > Vth VGS < Vth : 0 < VDS < VPinch off IDS=0 VGS < 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. Analogous to the pnp BJT VDS > VBreakdown IDS increases quickly Should be avoided

JFET uses Small Signal Amplifier Voltage Controlled Resistor Switch

FET Summary General: Signal Amplifiers Switches 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 BJT MOSFET 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