September 28 th 2004

1. Transistors September 28th 2004 • Student Lecture by: • Giangiacomo Groppi • Joel Cassell • Pierre Berthelot

2. Lecture outline • Historical introduction • Semiconductor devices overview • Bipolar Junction Transistor (BJT) • Field Effect Transistors (FET) • Power Transistors

3. 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

4. What is a transistor ? • The Transistor is a three-terminal, semiconductor device. • It’s possible to control electric current or voltagebetweentwo 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.

5. 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)

6. 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.

7. N-type P-type 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.

8. 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.

9. 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.

10. 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.

11. V threshold 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)

12. BJT

13. npn bipolar junction transistor pnp bipolar junction transistor 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). Developed by Shockley (1949)

14. 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-Collectorreverse 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

15. 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).

16. 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)

17. 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

18. 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

19. 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 IBminbecause 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.

20. 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

21. Operation point of BJT 2 Load-line curve Q

22. 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

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

24. Operation region summary

25. FET

26. Field Effect Transistors • Similar to the BJT: • Three terminals, • Control the output current • 1955 : the first Field effect transistor works • Increasingly important in mechatronics.

27. 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

28. Depletion mode Enhancement mode Symbols (base connected to the source or not) MOSFET (enhancement mode n-channel) 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

29. NMOS Behavior 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. • VDS > VBreakdown • IDS increases quickly • Should be avoided

30. For VDS > VPinchoff , the base current is a function of VGS Active region Saturation region Pinchoff Point NMOS Characteristic

31. NMOS Vs PMOS • Symbols:

32. NMOS Vs PMOS VGS > Vth Vth < 0 • 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

33. NMOS uses • High-current voltage-controlled switches • Analog switches • Drive DC and stepper motor • Current sources • Chips and Microprocessors • CMOS: Complementary fabrication

34. For Vpinchoff < VDS < 0 And VGS > VTH NMOS Example

35. The circuit symbols: JFET design: JFET overview

36. JFET Behavior Can be used with VG=0

37. JFET Behavior Can be used with VG < 0

38. JFET Behavior 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

39. JFET uses • Small Signal Amplifier • Voltage Controlled Resistor • Switch

40. 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

41. 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

42. 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