ENGG1310_L1_SEMI_S7.pdf

Microelectronics Semiconductors Transistors: BJT & MOS Optoelectronics ENGG1310 Reference book: Fundamentals of Microelectronics, Behzad Razavi (any edition)  Lenses Operating system Artificial Intelligence (AI) Co...

Microelectronics Semiconductors Transistors: BJT & MOS Optoelectronics ENGG1310 Reference book: Fundamentals of Microelectronics, Behzad Razavi (any edition)  Lenses Operating system Artificial Intelligence (AI) Computer system Sensors MemoryMicroprocessor Communi Power cations manage module ment ICs Battery Camera AMOLED display Wireless charging From design and manufacturing…. To applications…… A few types of circuits Circuit components Resistors, capacitors, inductors, sources etc  Example: Find the branch currents I1, I2 and I3 using mesh analysis Step1. Identify the meshes and assign variables Step2. Apply KVL to each mesh For mesh 1, -15+5i1+10(i1-i2)+10=0 For mesh 2, 6i2 +4i2 +10(i2-i1)-10=0 Simplifying the equations, we have 3i1 − 2i2 = 1 (1)   i1 − 2i2 = −1 (2) Step3. Solving the equations Method 1. By Elimination. (1) minus (2) gives 2i1=2 => i1=1A. Put this result back into (2) gives i2=1A. From these results, we have I1 =i1=1A, I2 =i2=1A and I3 = i1 -i2 = 0A Method 2. By Cramer’s rule. Write the system of equations in matrix form 3 -2  i1  1 -1 2  i  = 1   2   The determinants required by the Cramer’s rule are 3 -2 1 -2 3 1 Δ= = 4, Δ1 = = 4, Δ 2 = =4 -1 2 1 2 -1 1 This gives i1=Δ1/Δ=1A, i2=Δ2/Δ=1A and the results for I1, I2 and I3 follows Circuits comprising Passive Devices Components such as resistors, capacitors and inductors, which are passive devices, cannot control current by means of another electrical signal Active Devices Active devices has the ability to electrically control electric charge flow Vacuum tubes and transistors are examples of active devices Transistors (BJTs and MOSFETs) are built from semiconductor pn junctions Semiconductor Fundamentals Semiconductor devices serve as heart of microelectronics. PN junction is the most fundamental semiconductor device. Semicondutors Semiconductors have conductivities between those of metals and insulators The conductivity can be varied over several orders of magnitude by adding controlled amounts of impurity atoms The ability to control and change the conductivity of semiconductors allows for the design of a variety of semiconductor devices Charge Carriers in Semiconductor To understand PN junction’s IV characteristics, it is important to understand charge carriers’ behavior in solids, how to modify carrier densities, and different mechanisms of charge flow. Periodic Table This abridged table contains elements with three to five valence electrons, with Si being the most important.  a valence electron is an electron in the outer Silicon shell associated with an atom Si has four valence electrons. Therefore, it can form covalent bonds with four of its neighbors. When temperature goes up, electrons in the covalent bond can become free. Electron-Hole Pair Interaction With free electrons breaking off covalent bonds, holes are generated. Holes can be filled by absorbing other free electrons, so effectively there is a flow of charge carriers. https://www.youtube.com/watch?v=N8MuD_xu6L4 Energy band Energy Electrons in a solid exhibit different energy levels. The grouping of these different energy levels is know as the energy band  Metals In metals the bands either overlap or are partially filled Electrons and empty energy states are intermixed within the bands so that electrons can move freely under the influence of an electric field Electrons in the conduction band contribute to the conduction process Semiconductors Semiconductors A band gap is the distance between the valence band of electrons and the conduction band Essentially, the band gap represents the minimum energy that is required to excite an electron up to a state in the conduction band where it can participate in conduction The difference between semiconductors and insulators is the much smaller size of the bandgap For example Si has a bandgap of ~1.1 eV compared with ~5eV for diamond This allows for excitation of electrons from the valence band to the conduction band by reasonable amounts of thermal or optical energy Insulators Insulators have a filled valence band separated from an empty conduction band by a bandgap containing no allowed energy states There can be no charge transport within the valence band since no empty states are available into which electrons can move Doping (N type) Pure Si can be doped with other elements to change its electrical properties. For example, if Si is doped with P (phosphorous), then it has more electrons, or becomes type N (electron) Concentration of donor atoms: ND / cm3 Doping (P type) If Si is doped with B (boron), then it has more holes, or becomes type P Concentration of acceptor atoms : NA / cm3 Summary of Charge Carriers Properties of semiconductor types in silicon N-type (negative) P-type (positive) Dopant Group V (e.g. Group III (e.g. Boron) Phosphorus) Bonds Excess Electrons Missing Electrons (Holes) Majority Carriers Electrons Holes Minority Carriers Holes Electrons  Silicon is the most common semiconductor. Is it possible to use other elements in the Periodic Table as semiconductors? PN Junction Why do we need a PN junction? From computers to logic gates to pn junctions Boolean algebra Boolean algebra was formulated by George Boole, an English mathematician (1815-1864) who described propositions whose outcome would be either true or false In computer work it is used in addition to describe circuits whose state can be either 1 (true) or 0 (false) Examples of Boolean algebra Boolean algebra can be physically realized by transistor circuits Any operation that can be described in boolean algebra can be turned into a simple transistor circuit called a gate Gates are the building blocks of computers OR gate simple logic gate An OR gate implements the PR operation from boolean algebra BUT how is it physically implemented? PN Junction (Diode) When N-type and P-type dopants are introduced side-by-side in a semiconductor, a PN junction or a diode is formed. Diode’s Three Operation Regions In order to understand the operation of a diode, it is necessary to study its three operation regions: equilibrium, reverse bias, and forward bias. Diode in Reverse Bias When the N-type region of a diode is connected to a higher potential than the P-type region, the diode is under reverse bias There is a built-in electric field across the junction that blocks current flow Behaves as an open circuit under Diode in Forward Bias When the N-type region of a diode is at a lower potential than the P-type region, the diode is in forward bias. The built-in electric field decreased In forward bias, a current flows through the pn junction which depends on the forward bias voltage VF Water flow analogy 38 IV Characteristic of PN Junction input-output characteristics VD I D = I S (exp − 1) VT 𝑘𝑇 𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 = ≅ 26𝑚𝑉 𝑞 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝐼 ≈ 10 𝐴 The current and voltage relationship of a PN junction is exponential in forward bias region, and relatively constant in reverse bias region. What is the forward biased voltage VD of a pn junction diode if the diode current ID is 1 mA, given that Is = 1 x 10-12 A and VT = 25 mV? 40 For a forward-biased bipolar junction transistor (BJT), the collector current IC is 1 mA when the base-emitter voltage VBE is 0.7 V. What is IC if VBE is increased by 10%? (VT = 26 mV) Logic gates: the building blocks of digital circuits Diode Logic: OR GATE Diodes together with resistors can be used to implement digital logic functions Diodes connected to 5V inputs (logic 1) will conduct (forward biased) Current from source flow to resistor, so vY=5V and keep the diodes whose inputs are low (logic 0) in reverse bias Thus Y=A OR B OR C Diode logic can only implement OR and AND, because inverters (NOT gates) require an active device What’s another common use of pn-junction diodes? What’s the difference between a resistor and a diode? More complex logic circuits  Require logic gates beyond OR and AND gates Active devices needed-> transistors Bipolar Transistors Active Devices Active devices has the ability to electrically control electric charge flow Vacuum tubes and transistors are examples of active devices Transistors (BJTs and MOSFETs) are built from semiconductor pn junctions Structure and Symbol of Bipolar Transistor Bipolar transistor can be thought of as a sandwich of three doped Si regions. The outer two regions are doped with the same polarity, while the middle region is doped with opposite polarity. Forward Active Region Forward active region: VBE > 0, VBC < 0. Input-Output Characteristics of Bipolar Transistor When an “input” signal is applied at the base (VBE), an “output” current flows at the collector (IC) 52 Water tap analogy from internet resources Transconductance “gain” Transconductance, gm shows a measure of how well the transistor converts voltage to current It is one of the most important parameters in circuit design Non-linear! gm can be visualized as the slope of IC versus VBE A large IC has a large slope and therefore a large gm How do we use a BJT as a voltage amplifier? PNP transistors With the polarities of emitter, collector, and base reversed, a PNP transistor is formed All the principles that applied to NPN's also apply to PNP’s, with the exception that emitter is at a higher potential than base and base at a higher potential than collector 56 What’s the difference between a diode and a BJT? example: NOT Logic Gate When input is LOW, the transistor Q1 turns OFF. Current flows through R1 to the output, thus output is HIGH When input is HIGH, the transistor Q1 turns ON. Current flows through collector-emitter junction to GND, causing output to go LOW Another type or transistor: MOS Transistors Parallel-plate Capacitor The capacitor consists of 2 conducting plates separated with a dielectric (nonconducting) materials Parallel-plate Capacitor The charges are stored on the plates, thereby maintaining an electric field between the plates This field therefore stores energy, and thus the capacitor can store energy Metal-Oxide-Semiconductor (MOS) Capacitor The MOS structure can be thought of as a parallel-plate capacitor, with the top plate being the positive plate, oxide being the dielectric, and Si substrate being the negative plate. (We are assuming P-substrate.) Structure and Symbol of n-MOSFET This device is symmetric, so either of the n+ regions can be source or drain. Formation of Channel in nMOS First, a positive potential is applied to the metal gate and a negative potential to the n-doped Drain and Source Formation of Channel in nMOS Next, the holes are repelled by the positive gate voltage, leaving behind negative ions and forming a depletion region. Formation of Channel in nMOS Finally, electrons are attracted to the interface, creating a channel (“inversion layer”)  MOSFET Characteristics A potential is applied between the Drain and Source VD to generate an output drain current ID The output drain current ID varies with a varying input voltage VG while keeping VD constant Non-linear! PMOS Transistor It is possible to create a MOS device where holes are the dominant carriers. It is called the PMOS transistor. It behaves like an NMOS device with all the polarities reversed. CMOS CMOS stands for “Complementary Metal Oxide Semiconductor” In CMOS technology, both N-type and P-type transistors are used to design logic functions. The same signal which turns ON a transistor of one type is used to turn OFF a transistor of the other type This is the dominant semiconductor technology for microprocessors, microcontroller chips, memories like RAM, ROM, EEPROM and application- specific integrated circuits (ASICs)  Why CMOS? In a 1963 conference paper C. T. Sah and Frank Wanlass of the Fairchild R & D Laboratory showed that logic circuits combining p-channel and n- channel MOS transistors in a complementary symmetry circuit configuration drew close to zero power in standby mode. Wanlass patented the idea that today is called CMOS. CMOS Technology It possible to grow an n-well inside a p-substrate to create a technology where both NMOS and PMOS can coexist. It is known as CMOS, or “Complementary MOS”. Comparison of Bipolar and MOS Transistors Bipolar devices have a higher gm than MOSFETs for a given bias current due to its exponential IV characteristics CMOS devices have low static power utilization, huge noise immunity- allow for integrating logic functions with high density on an integrated circuit Why are smaller transistors better? Moore’s Law Moore's law is a term used to refer to the observation made by Gordon Moore in 1965 that the number of transistors in a dense integrated circuit (IC) doubles about every two years The Future of Moore's Law "It can't continue forever“ Eventually miniaturization will lead to atomic level At that point the law cannot be sustained Optoelectronics Revisiting the bandgap Optical absorption The excitation of an electron from the valence band to the conduction band requires a minimum energy of Eg When an incident photon of energy >Eg interacts with an electron in the valence band, the electron absorbs the incident photon and gains sufficient energy to surmount the energy bandgap to reach the conduction band  Consequently a free electron in the conduction band and a hole, corresponding to a missing electron in the valence band, are created Photodetector: application of absorption Photodetectors and arrays Recombination and luminescence When an electron and hole recombine, the electron drops from an energy level in the conduction band to an energy level in the valence band, resulting in the generation of photons (light) or phonons (lattice vibrations) LEDs: Application of (electro)luminescence Light-emitting diodes (LEDs) are p-n junction devices The junction in a LED is forward biased and when electrons cross the junction from the n- to the p-type material, the electron-hole recombination process produces some photons in the IR or visible in a process called electroluminescence. An exposed semiconductor surface can then emit light. How do we change the colour of emission? White light + colour filters LCD displays- colour filtering How do we change the colour of emission? White LEDs White is a mixture of colours Applications of LEDs