Basic Electronics: Diodes and Transistors. October 14, 2005 ME 435

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1 Basic Electronics: Diodes and Transistors Eşref Eşkinat E October 14, 2005 ME 435

2 Electric lectricity ity to Electronic lectronics Electric circuits are connections of conductive wires and other devices whereby the uniform flow of electrons occurs. Electronic circuits add a new dimension to electric circuits in that some means of control is exerted over the flow of electrons by another electrical signal, either a flow of electrons by another electrical signal, either a voltage or a current. Electronics technology experienced a revolution in 1948 with the invention of the transistor. Transistors control the flow of electrons through solid semiconductor substances. T solid-state state electronics.. Transistor technology is often referred to as

3 Active versus passive devices Active versus passive devices An active device is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic,, it must contain at least one active device. Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all passive devices. Active devices include, vacuum tubes, transistors, silicon-controlled rectifiers ( controlled rectifiers (SCRs SCRs), and TRIAC s

4 Amplifiers The practical benefit of active devices is their amplifying ability. An n active device allows a small amount of electricity to control a large amount of electricity. Devices utilizing a voltage as the controlling signal are called voltage- controlled devices. Devices working on the principle of one current controlling another current are known as current-controlled controlled devices.

Semiconductors Semiconductors have had a monumental impact. You find semiconductors at the heart of microprocessor chips as well as transistors.

5 Semiconductors Semiconductors have had a monumental impact. You find semiconductors at the heart of microprocessor chips as well as transistors. Anything that's computerized or uses radio waves depends on semiconductors. Today, most semiconductor chips and transistors are created with silicon.. You may have heard expressions like "Silicon Valley" and the "silicon economy," and that's why -- silicon is the heart of any electronic device. Clockwise from top: A chip, an LED and a transistor are all made from semiconductor material.

6 Understanding Silicon Understanding Silicon Silicon is a very common element.. It I t is the main element in sand. Silicon ilicon" " in the periodic table, is next to Aluminum luminum,, below carbon and above germanium. Carbon, silicon and germanium have a unique property in their electron structure -- each has four electrons in its outer orbital.. This allows them to form nice crystals. The four electrons form perfect covalent bonds with four neighboring atoms, creating a lattice.. In carbon, we know the crystalline form as diamond. In silicon, the crystalline form is a silvery, metallic- looking substance. Metals tend to be good conductors of electricity because they usually have "free electrons" that can move easily between atoms, and electricity involves the flow of electrons. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds,, so they can't move around. A pure silicon crystal is nearly an insulator

7 Doping Silicon Doping Silicon You can change the behavior of silicon and turn it into a conductor by doping an impurity into the silicon crystal. N-type - In N-type N doping, phosphorus or arsenic is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons. The fifth electron has nothing to bond to, so it's free to move around. N-type N silicon is a good conductor. Electrons have a negative charge, hence the name N-type. doping it. In doping, you mix a small amount of P-type - In P-type P doping, boron or gallium is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. P Holes can conduct current. A hole accepts an electron from a neighbor, moving the hole over a space. P-type P silicon is a good conductor. N-type and P-type P silicon are not that amazing by themselves; but when you put them together, you get some very interesting behavior at the junction.

8 P-N N Junction The free electrons in the N-type silicon are repelled by the negative terminal of the battery. The holes in the P-type silicon are repelled by the positive terminal. At the junction between the N-type and P- type silicon, holes and free electrons meet. The electrons fill the holes. Those holes and free electrons cease to exist, and new holes and electrons spring up to take their place. The effect is that current flows through the junction. The negative electrons in the N-type silicon get attracted to the positive terminal of the battery. The positive holes in the P-type silicon get attracted to the negative terminal of the battery. No current flows across the junction because the holes and the electrons are each moving in the wrong direction.

9 Junction Diodes Junction Diodes A PN junction passes current only in one direction. A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other.

10 Diode Operation Diode Operation When the diode is forward- biased and conducting current, there is a small voltage dropped across it, leaving most of the battery voltage dropped across the lamp. When the battery's polarity is reversed and the diode becomes reverse- biased, it drops all of the battery's voltage and leaves none for the lamp.

11 Diode Characteristic Curve Diode Characteristic Curve A real diode requires about 0.7 V to enable significant current flow.

12 Review A diode is an electrical component acting as a one-way valve for current. When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward forward-biased biased. When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased biased. The voltage dropped across a conducting, forward-biased diode is called the forward voltage.. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed principally by the chemical composition of the P-N P junction. Silicon diodes have a forward voltage of approximately 0.7 volts

13 Half Wave Rectifier Circuit Half Wave Rectifier Circuit Application : Light dimming When V i is positive, diode is reverse biased and is equivalent to open circuit. Therefore V 0 =V i. When Vi is negative, diode is forward biased and is equivalent to short circuit. Therefore V 0 =0 V.

14 Full Wave Rectifier Full Wave Rectifier If we need to rectify AC power so as to obtain the full use of both half-cycles of the sine wave, a different rectifier circuit configuration must be used. Such a circuit is called a full-wave rectifier.

15 Full rectifier Full rectifier

16 Inductive Kick Inductive Kick The switch attempts to change the current instantaneously. Inductors generate voltage to oppose current changes. The diode allows the voltage to dissipate through the resistor.

17 without Flyback diode Flyback diode with

18 Peak Detector Peak Detector When a time varying signal V in is applied at the input, the output V out retains the maximum positive value of the input signal. In the actual circuit there is some decrease of Vout in time because of the capacitor leakage.

19 Zener Diodes Zener Diodes When a diode is reverse biased with large voltage, it allows a large reverse current to flow : Breakdown. Special diodes with well defined breakdown voltages maintain constant voltage over a wide range of currents. They are Zener diodes and used as voltage regulators. Zener diode should be reverse biased with a voltage in excess of its zener voltage V z. Typically : V z = V.

20 Zener Diode as Voltage regulator Zener Diode as Voltage regulator In the above circuit, output voltage of the source, V z, is kept relatively constant, because of the breakdown characteristic of the Zener diode. Zener diodes are often rated by their power dissipation, which is: P Zmax = i Zmax V Z

21 Optoelectronic diodes Optoelectronic diodes Light emitting diode : LED Photodiode light detector circuit.

22 LED (Light Emitting Diodes) LED (Light Emitting Diodes) While all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outward. of photons outward. LED LED s form the numbers on digital clocks,, transmit information from remote controls,, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.

23 Diode Video Diode Video

24 Transistors and Chips Transistors and Chips A transistor is created by using three layers rather than the two layers used in a diode. You can create either an NPN or a PNP sandwich. A transistor can act as a switch or an amplifier you apply a small current to the center layer of the sandwich, a much larger current can flow through the sandwich as a whole. This gives a transistor its switching behavior. A small current A transistor can act as a switch or an amplifier. When can turn a larger current on and off. A silicon chip is a piece of silicon that can hold thousands of transistors. With transistors acting as switches, you can create Boolean gates,, and with Boolean gates you can create microprocessor chips. The natural progression from silicon to doped silicon to transistors to chips is what has made microprocessors and other electronic devices so inexpensive. The fundamental principles are surprisingly simple. Today oday,, tens of millions of transistors can be inexpensively formed onto a single chip.

25 Transistors Transistors consist of multiple layers of Transistors consist of multiple layers of n- and p- silicon, examples are npn configuration or a pnp configuration in the bipolar junction transistor (BJT). A small amount of current introduced at the base (center) of the transistor will cause the overall device to be forward biased and allow current rent to flow from collector to emitter October 14, 2005 ME 435

27 BJT Transistor (NPN) BJT Transistor (NPN) I E =I C +I B V BE = V B -V E V CE = V C -V E V C > V B > V E I C = β I B Typically β 100.

28 Transistor Video Transistor Video

29 Simple Transistor Application Simple Transistor Application +10V Mechanical Switch Lamp 1K 10V 0.1A

30 Example Applications Example Applications

31 Characteristics of a common emitter Characteristics of a common emitter NPN BJT Cutoff : V BE < 0.7 V, i B =0 i C 0, V CE > 0. Active : V BE = 0.7 V i C =βi B, V CE > 0.7 V Saturation : i B >i C /β and V BE = 0.7 V V CE = V SAT = 0.2 V.

32 Power dissipation in Transistors Power dissipation in Transistors

33 The common emitter transistor circuit for a npn transistor. The transistor is forward biased when the base-to to-emitter voltage is V. When designing a transistor switch, the transistor must be in saturation when it is on (otherwise it will heat up and might fail) For BJT s,, the V CE at saturation is about 0.2V.2V. I C R C I B + - V BE - + V CE

34 Guaranteeing that the transistor is in saturation Guaranteeing that the transistor is in saturation Given a typical signal transistor, 2N3904 specifications : Max. collector current, I C (max) = 200mA, V CE at saturation = 0.2V, β= 100 Find the necessary input voltage to ensure saturation: Since V CE is 0.2 V, I C =10-0.2/10kΩ = 9.8 ma. Thus, the base current must be at least 9.8/β or ma and I B =(Vin-.7)/10kΏ : Vin = = 1.68 V. Remember these guidelines for a transistor switch: The base-to-emitter voltage must be 0.7 V to be on. Maximum values of I C, I B and V CE must be observed and maintained. There must be sufficient base current to ensure saturation (I B > I C /β, and V CE = 0.2 V)) Collector current is independent of base current at saturation.

35 This is a very poor amplifier. Clips the negative values of I B

36 We shall not study amplifiers in this course. We shall not study amplifiers in this course.

37 Darlington Pair Darlington Pair

38 And Gate And Gate

39 OR gate OR gate

FIELD EFFECT TRANSISTORS FIELD EFFECT TRANSISTORS n Three terminals : Gate, Drain, Source. BJT is current controlled amplifier, FET is voltage (V D ) controlled.

40 FIELD EFFECT TRANSISTORS FIELD EFFECT TRANSISTORS n Three terminals : Gate, Drain, Source. BJT is current controlled amplifier, FET is voltage (V D ) controlled. i D ~ V G FET s have very high input impedance, therefore : I G = 0. This simplifies circuit design FET s require less operating power compared to BJT. Have better high frequency characteristics than BJT : up to 200kHz. However, they are sensitice to static electricity. Therefore, elctrical insulation is very important for FET s.

41 JFET s With no voltage applied between gate and source, the channel is an a open path for electrons to flow If f a voltage is applied between gate and source of such polarity that it reverse- biases the PN junction, the flow between source and drain connections becomes limited. This behavior is due to the depletion region of the PN junction. This action may be likened to reducing the flow of a liquid through a flexible hose by squeezing it: with enough force, the hose will be constricted enough to completely block the flow.

42 Behavior of FET s Saturation Cutoff : V GS < V T, i G =0 i C 0, V DS V DD. V T = 1-2 V. Active (Ohmic Region) : V GS > V T i D ~ (V GS -V T ) 2, V DS > V GS -V T Saturation : V GS >> V T i D = V DD /R D : constant V DS i D R ON V GS

43 I o 10mA 5mA MOSFET Characteristics MOSFET Characteristics V GS = 5V 4V 3V 2V V os V GS = 5V 30V 150 µa 4V R os = 1/g m I o 100 µa 3V 50 µa 2V V os (V)

44 Applications of MOSFET s When V G -V T V DD, the mosfet enters into saturation, resulting nearly full voltage V DD across the load (R on is small). If the load is inductive, a flyback diode is necessary to prevent damage to the mosfet, when switched off. Mosfet power switch circuit.

Diode conducts when V anode > V cathode. Positive current flow. Diodes (and transistors) are non-linear device: V IR!

Diode conducts when V anode > V cathode. Positive current flow. Diodes (and transistors) are non-linear device: V IR! Diodes: What do we use diodes for? Lecture 5: Diodes and Transistors protect circuits by limiting the voltage (clipping and clamping) turn AC into DC (voltage rectifier) voltage multipliers (e.g. double