ME 4447 / 6405 Student Lecture. Transistors. Abiodun Otolorin Michael Abraham Waqas Majeed

Transcription

1 ME 4447 / 6405 Student Lecture Transistors Abiodun Otolorin Michael Abraham Waqas Majeed

2 Lecture Overview Transistor? History Underlying Science Properties Types of transistors Bipolar Junction Transistors (BJT) Field Effect Transistors (FET) Power Transistors Applications

3 Transistor? Transistor = transconductance + varistor 3 terminal electronics device. Solid state Semiconductor material. Part and parcel of almost every circuit (digital / analog)

4 Transistor? Two main categories: Through-Hole Surface-Mount Packaging Materials: Plastic, Glass, metal or ceramics * Power transistors packages clamped to heat sinks.

5 History Vacuum Tube (Predecessors) Edison effect in the Light Bulb ( ). John Fleming implements it in a diode. Lee DeForest extends it using third electrode. This triode was used as both an amplifier and a switch (1906). Diode Triode

6 First Transistor William Shockley, John Bardeen, and Walter Bell Labs in 1947 Replaced vacuum tubes Smaller, robust, durable, no warm ups Solid state 1956 Nobel Prize Current Transistors semiconductor

7 Current Transistor The world's first single chip microprocessor 2,300 MOS (metal oxide semiconductor) transistors Equivalent 18,000 vacuum tubes contained in 3,000 ft 3 Nov the Intel 4004 Today s microprocessor has 2B transistors. SD cards has 50B transistors. Brought revolution like IC, Chips.

8 Underlying Science FREE ELECTRONS are required for current flow. Conductors have abundant of them. Insulators & semiconductors have scarcity. TEMPERATURE increase produces free electrons but not possible. SMALLER BAND GAPS of semiconductors than insulators help yield free electrons. Hence, DOPING is done.

9 Underlying Science 8 electrons for stable valence shell. GIII > P-dopant has 3 valence shell electrons (e.g. B, Ga). GIV > Semiconductor has 4 valence shell electrons (e.g. Si, Ge). GV > N-dopant has 5 valence shell electrons (e.g. P, As)

10 Underlying Science Covalent bonding/pair sharing. GIII + GIV => -1 electron => Hole > P-type GV + GIV => +1 electron => Electron > N-type Dopants Semiconductor

11 Properties Forward Bias (on) Current flows from P to N. V d = 0.7 V to start conduction. Reverse Bias (off) No Current flows (ideally). Excessive voltage or heat can cause degradation to diode.

12 Properties V threshold

13 Properties NPN Transistor PNP Transistor

14 Properties Collector current controlled by the collector circuit (Switching) Collector current I C proportional to Base current I B (Amplification + Regulation) In full saturation V CE =0.2V (Variable resistor) No current flows

15 Properties Collector current controlled by the collector circuit (Switching) Collector current I C proportional to Base current I B (Amplification + Regulation) In full saturation V CE =0.2V (Variable resistor) No current flows

16 Bipolar Junction Transistor (BJT) 3 layers of Semiconductor 3 Terminal (Base, Collector & Emitter) Each terminal may act as Input, Output or Common 3 possible configurations Common Emitter Both Current and Voltage gain Common Base Voltage gain but no current gain Common Collector Current gain but no voltage gain NPN Common Emitter

17 BJT Types NPN Base is energized to allow current flow High potential at collector Low potential at emitter Allows current flow when the base is given a high potential PNP Base is connected to a lower potential to allow current flow High potential at emitter Low potential at collector Allows current flow when base is connected to a low potential

18 Cut-off Region: V BE < V TH, I B =0 off switch Active Linear Region: V BE =V TH, I B 0, I C =βi B current amplifier Current gain (β) 100 for most transistors Voltage Gain = β(i C /I B ) Saturation Region: V BE =V TH, I B >I C,max / β on switch BJT Characteristics

19 Operation region overview Operation Region Cutoff I B or V CE Char. I B = Very small BC and BE Junctions Reverse & Reverse Saturation V CE = Small Forward & Forward Active Linear V CE = Moderate Reverse & Forward Break-down V CE = Large Beyond Limits Mode Open Switch Closed Switch Linear Amplifier Overload

20 Types of BJT Circuits Major BJT Circuits Transistor Switch Common-Emitter Amplifier Emitter Follower Current Source

21 NPN Transistor Switch V in (Low ) < 0.7 V BE junction is not forward biased Cutoff region No current flows V out = V CE = V cc V out = High 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

22 NPN Common Emitter Amplifier Linear Active Region Significant current Gain Example: Let Gain, β = 100 Assume to be in active region -> V BE =0.7V Is device in active region?

23 NPN Common Emitter Amplifier 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 E * R E = mA V = 10 (3)(1.07) (2)(101*0.0107) 0.7 = = 3.93V BE = V CB >0 so the BJT is in active region

24 Power Across BJT P BJT = V CE * I CE Should be below the rated transistor power Should be kept in mind when considering heat dissipation Reducing power increases efficiency

25 Darlington Transistors Two BJT devices combined BE junction in series Allow for much greater gain in a circuit β = β 1 * β 2 V BE =V BE1 + V BE2

26 Field Effect Transistors (FET) Analogous to BJTs FETs switch by voltage rather than by current FETs have 4 terminals (except for J-FET which have 3) BJT Collector Base Emitter N/A FET Drain Gate Source Body D G B S

27 Plate Semiconductor FET: The Basic Idea Current flow is controlled via the field effect Electrons gather when a field is formed Current flows when an electron bridge is created

28 Types of FETs MOSFET (Metal-Oxide-Semiconductor) JFET (Junction Gate) MESFET (Metal-Semiconductor) MODFET (Modulated-Doping) HFET (Hetero-structure) HEMT (High Electron Mobility Transistor) Boba Fett (Most common are MOSFET and JFET)

29 The Two Most Common Types MOSFET 2 varieties: enhancement mode (shown) JFET depletion mode gate gate drain N N source P P drain N source Both MOSFETs and JFETs can be n-channel or p-channel depending upon the doping of the drain and source

30 Junction Gate FET (JFET) Source and Drain are connected to n-doped material (or p-doped material for p-channel) Gate is connected to p-doped material When a negative biased voltage is applied to the Gate, the size of the depletion layer increases and impedes current flow from the source to the drain. JFETs can thus behave as voltage-controlled variable resistors JFETs work by depletion p-doped n-doped depletion layer

31 MOSFET Depletion Mode Enhancement Mode

32 n-channel Enhanced MOSFET n-channel because source and drain are connected to n-doped regions With no voltage applied, the MOSFET behaves as shown (no channel is formed between the source and drain) Because a channel is being created where none existed, this is an enhancement mode MOSFET

33 n-channel Enhanced MOSFET When a voltage is applied, the resulting field causes electrons from the p-doped region to collect near the gate and a channel is created that connects the source and the drain. The size and shape of the channel vary depending upon the amount of voltage applied.

34 Circuit Symbols MOSFET In practice the body and source leads are almost always connected G D B G D B Most packages have these leads already connected S JFET S D G S

35 Performance Characteristics D Current flow G B S

36 Performance Regions Region Criteria Effect on Current Cut-off V GS <V th I DS =0 D Current flow Linear V GS >V th And V DS <V GS -V th Transistor acts like a variable resistor, controlled by V gs G B Saturation V GS >V th And Essentially constant current S V DS >V GS -V th

37 JFET vs MOSFET MOSFET High switching speed Can have very low R DS JFET Will operate at V G <0 Better suited for low signal amplification G D B Current flow Susceptible to ESD S More commonly used as a power transistor

38 Power Transistors Additional material for current handling and heat dissipation Can handle high current and voltage Functionally the same as normal transistors

39 Applications Switching Amplification (Voltage / Current) Variable Resistor (VDR) Voltage Regulation

40 Switching

41 Switching Power to motor is proportional to duty cycle MOSFET transistor is ideal for this use DC motor

42 Amplification Analog signal (e.g. sensor, audio, etc.). Op-amps does same but suitable for voltage amplification and this is for current/power amplification.

43 Transistors can be used in series to produce a very high current gain Amplification

44 Variable Resistor FET operating in Linear or Ohmic Mode Lesser usage

45 Voltage Regulation Transistors can be used in regulating voltage for high power devices. Inefficient Power supplies

46 Application Examples Digital logic circuits Microprocessors, microcontrollers, chips (TTL) Photo-transistors Replaces normal switches, mechanical relays. ADC Opamp Encoders Multiplexers Power supplies

47 References function/p-type.html 3/dtob.gif

48 References (contd.) Sabri Cetinkunt; Mechatronics John Wiley and sons; 2007 Mechatronics by D. Bradley Mechatronics System Design by Shetty Mechatronics : Principles and Application by Onwubolu Applied Mechatronis by A. Smaili et al

49 Questions?