Lecture 16


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Lecture 16

  1. 1. Today’s Objectives- Semiconductor Devices <ul><li>Band diagrams for intrinsic vs. extrinsic semiconductors </li></ul><ul><li>Conductivity for intrinsic vs. extrinsic semiconductors </li></ul><ul><li>Band diagrams for equilibrium, forward and reverse biased p-n junctions </li></ul><ul><li>Diodes (all sorts…) </li></ul><ul><li>Transistors </li></ul><ul><li>Ionic conductors as sensors </li></ul><ul><li>Dominant ionic carriers </li></ul>
  2. 2. Band diagram summary <ul><li>All materials can be described by band diagrams, though they are especially useful for understanding semiconductor devices: </li></ul><ul><ul><li>E f hints at the dominant type of dopants. </li></ul></ul><ul><ul><li>E g tells about the ionic character of the material. </li></ul></ul><ul><ul><li>E g suggests the magnitude of the intrinsic conductivity. </li></ul></ul>1.29 InP 0.33 InAs 0.16 InSb (“III-V” compound) 1.14 Si 0.67 Ge E g (eV) at 273 K Semiconductor Filled (deep valence) E f Insulator (Al 2 O 3 ) Filled (valence) Empty (conduction) Band gap Band gap Intrinsic conductivity decreases P type
  3. 3. Summary: Intrinsic vs. Extrinsic (n or p) • Intrinsic : # electrons = # holes (n = p) --case for pure Si • Extrinsic : --n ≠ p --occurs when DOPANTS are added with a different # valence electrons than the host (e.g., Si atoms) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n)
  4. 4. Conductivity Summary • Conductors, semiconductors, and insulators... --difference is whether there are accessible energy states for conductance electrons. • For metals, conductivity increases with less scattering: --reducing deformation. --reducing imperfections. --decreasing temperature. • For intrinsic semiconductors, conductivity increased by --increasing temperature. • For doped semiconductors --increasing temperature doesn’t make a difference until T increases sufficiently for intrinsic carriers to dominate. --with increased dopants, mobility decreases (more scattering, same as metals).
  5. 5. Semiconductor Properties Intrinsic SCs: Conductivity only via thermally induced jumps from the VB to the CB across the band gap. Extrinsic SCs: Conductivity due to thermally induced much smaller jumps from the donor level E D to the CB (n-SC) or from the VB to the acceptor level E A (p-SC). SCs (intrinsic or extrinsic) are lousy conductors (compared to metals). SCs (intrinsic or extrinsic) are lousy electrical insulators (compared to ceramics and polymers). Typical conductivities: Si (intrinsic) 10 -5 per ohm.cm, Si (doped), 10 -1 to 10 2 , Cu 10 6 , Quartz 10 -19 .
  6. 6. Semiconductor Properties Intrinsic/Extrinsic SCs cannot be used as electrical wires, too much power loss… Intrinsic/Extrinsic SCs cannot be used in charge storage or electrical insulation, too much charge leakage… What are they good for???? Absolutely nothing on their own… But, how can a multi-billion $ industry be based on such materials? SC devices exploit asymmetry in the band diagrams at the junction (interface) when differently doped SCs are brought together.
  7. 7. Semiconductor Junctions When materials with dissimilar electrical properties are brought in contact, a depletion layer forms at the junction (interface). e- h+
  8. 8. What if we combine p and n SCs? DIODE <ul><li>When a p -type semiconductor and an n -type semiconductor are joined, holes dominate in the p -region , while electrons dominate in the n -region . </li></ul>There is an asymmetry in the band diagram at the junction. This asymmetry becomes more pronounced when there is a potential across the semiconductor junction.
  9. 9. Band structures: n and p type semiconductors <ul><li>The only difference in the band diagrams for n and p type semiconductors is where the Fermi level lies within the bandgap (just shift the diagrams). </li></ul>Energy Filled (deep valence) E f p-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap Filled (deep valence) E f n-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap
  10. 10. Simplified Band Structures <ul><li>Band diagrams can be simplified to 3 lines each: Ec, Ef, and Ev. </li></ul>Energy Filled (deep valence) E f n-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap Filled (deep valence) E f p-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap E f n-type E c E v p-type E v E c E f
  11. 11. Contacting semiconductors <ul><li>As semiconductors come into contact, the Fermi level must be flat so the bands shift to accommodate it. </li></ul>Energy Filled (deep valence) E f n-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap E f n-type E c E v Filled (deep valence) E f p-type Semiconductor Filled (valence) Empty (conduction) Band gap Band gap p-type E v E c E f
  12. 12. Rules for band diagrams <ul><li>Valence states are filled with electrons, conduction bands are partially or completely empty. </li></ul><ul><li>Outer electrons from donor or acceptor dopants usually exist in isolated states within the bandgap (near the conduction or valence bands, respectively). </li></ul><ul><li>Electrons fall “down” in energy, holes ‘fall’ up . </li></ul><ul><li>For joined materials, the Fermi level is always flat. </li></ul><ul><li>For joined materials, the band edges shift and/or bend any way that is necessary to accommodate a flat Fermi level. </li></ul><ul><li>Applied biases shift the Fermi level, and the conduction and valence bands, on the side with the bias. </li></ul><ul><li>A positive applied bias moves the bands down; a negative bias moves them up (or the other side of a device down). </li></ul><ul><li>Again, band bending occurs near the interfaces to accommodate applied biases. </li></ul>
  13. 13. Bands in Biased Diodes <ul><li>Equilibrium </li></ul><ul><ul><li>Electrons ‘roll’ downhill, so they mostly stuck in the n side. </li></ul></ul><ul><ul><li>Holes ‘roll’ uphill, so they are mostly stuck in the p side. </li></ul></ul><ul><li>pn: Forward Bias </li></ul><ul><ul><li>Some electrons and holes can begin to jump the barrier. </li></ul></ul><ul><li>pn: Reverse Bias </li></ul><ul><ul><li>No electrons jump the barrier. </li></ul></ul>
  14. 14. Forward biased p-n diode <ul><li>When a battery or power supply is attached to the diode with the positive bias connected to the p side, this is termed forward bias . </li></ul><ul><li>Free electrons now move toward the positive terminal, and holes toward the negative terminal. Some might make it all the way, but most will meet each other near the junction, recombine (or annihilate), and give off energy (often as photons, i.e., light ). </li></ul><ul><li>New electrons are provided by the electrodes. </li></ul><ul><li>Current is conducted. </li></ul>
  15. 15. Reverse Biased p-n diode <ul><li>When a negative bias is applied to the p side, free holes are attracted to the negative electrode, but before long there are no more holes available ( the sample is depleted ). </li></ul><ul><li>Similarly, free electrons in the n side are attracted to the positive electrode, leaving no free carriers for sustaining a current. </li></ul><ul><li>The electrodes are consuming electrons and holes, not supplying them. And the sample has a limited supply. </li></ul><ul><li>No current is conducted. </li></ul>
  16. 16. p-n diode as a switch <ul><li>Since current can conduct in the forward direction, but not in the reverse direction, this is an excellent solid state switch with a built-in logic . </li></ul>
  17. 17. pn diode as a rectifier <ul><li>If we apply an AC field to the diode, it acts as a rectifier . </li></ul><ul><li>Convert ac signal centered on 0V to a DC current (with an ac component). </li></ul><ul><li>Attach to a big capacitor and the ac component averages out, leaving a non-zero DC voltage. </li></ul><ul><li>Chargers for Cell phones, laptops, etc. </li></ul>
  18. 18. Zener Diode <ul><li>For extreme reverse biasing, breakdown occurs whereby electrons tunnel laterally across the band gap directly from the n to the p region. </li></ul><ul><li>Also, avalanche breakdown can occur where carriers get so much energy from the electric field that as they interact with atoms they strip electrons away, creating more free carriers that themselves strip new electrons from other atoms, etc... </li></ul>Used in protection of circuits
  19. 19. Photodiode A photodiode is an n-p junction . When light of sufficient photon energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent . basic design more efficient design
  20. 20. Photodiode Boeing’s 31% efficient photovoltaic cell. Farming sunlight…
  21. 21. Light-Emitting Diodes (LEDs) A light-emitting diode ( L E D ) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction . Like a normal diode, L E D consists of a p-n junction . As in other diodes, current flows easily from the p-side to the n-side in the forward but not in the reverse direction. Charge-carriers — electrons and holes — flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore, its c o l o r , depends on the band gap energy of the materials forming the p-n junction .
  22. 22. Light-Emitting Diodes (LEDs) AlGaAs - red and IR AlGaP - green AlGaInP - high-brightness orange-red , orange, yellow, and green GaAsP - red , orange , and yellow GaP - red , yellow and green GaN - green , blue , white InGaN - near UV , bluish-green and blue SiC as substrate - blue Sapphire (Al 2 O 3 ) as substrate - blue ZnSe - blue Diamond (C) - UV
  23. 23. Light-Emitting Diodes (LEDs) Applications: traffic lights/signals, motorcycle lights, flashlights, light bars on emergency vehicles, fiber optics, optical computer mice, display panels… A large LED screen at the University of Arkansas stadium.
  24. 24. Transistors Transistors are back-to-back p-n or n-p diodes. This one is a bipolar junction transistor (BJT) . There are many other kinds of transistors. The transistor is used in a wide variety of digital and analog functions, including signal amplification , switching (logic) , voltage regulation , and signal modulation ,
  25. 25. Transistors – Band Diagrams A transistor looks like two diodes back-to-back and the band diagram is symmetric . No current could flow through a transistor because back-to-back diodes would block current both ways . However, when you apply a small current to the BASE transistor, a much larger current can flow through it as a whole. This gives a transistor its switching behavior and also amplifies the incoming signal. A small current can turn a larger current on and off.
  26. 26. Transistors – Signal Amplification
  27. 27. Transistors – Switch with a Built-in Logic Metal Oxide Semiconductor Field Effect Transistor (MOSFET) This is a depletion MOSFET that is “ normally on .” You need to apply reverse bias to pinch off the conduction from the p -channel . There are MOSFETs that are normally off as well (called enhancement type MOSFETs).
  28. 28. Hall Effect <ul><li>For any SC device it is CRUCIAL to know the type of carriers, their concentration, and their mobility . </li></ul><ul><li>This is NOT an easy test considering that the doping of Si is at the ppm level. </li></ul><ul><li>All these quantities can be determined via the Hall Effect . </li></ul><ul><li>An applied magnetic field B Z results in bending of the electron paths of a SC across which there is a current j x . </li></ul><ul><li>This sets up an electric field E y . </li></ul><ul><li>The accurate measurement of this field and the resistivity of the sample yield the type of carriers ( e- or h+ ), their concentration, and mobility. </li></ul>
  29. 29. Clean Rooms <ul><li>Clean rooms are crucial as all the properties of SC devices strongly depend on the dopant/impurity content at the ppm level!!! </li></ul><ul><li>Special clothes, boots, gloves, face mask…. </li></ul><ul><li>Maybe even a breathing apparatus. </li></ul><ul><li>Class 1000 (1000 specs of dust per cubic foot) is typical for a university research lab. </li></ul><ul><li>The pros use class 10 or 100 in the primary facility. </li></ul><ul><li>Mask fabrication is even cleaner. </li></ul><ul><li>Samples are transferred in ultraclean, portable ‘pods.’ </li></ul>The modern computer has over 200 MILLION transistors.
  30. 30. April 1972 Name of Processor: 8008 Clock speed: 200 kilohertz Number of transistors: 3,500 September 1978 Name of Processor: 8086 Clock speed: 10 MHz Number of transistors: 29,000 February 1982 Name of Processor: 286 Clock speed: 12 MHz Number of transistors: 134,000 October 1985 Name of Processor: 386 Clock speed: 16 MHz Number of transistors: 275,000 June 1991 Name of Processor: 486 Clock speed: 50 MHz Number of transistors: 1,200,000 January 1996 Name of Processor: Pentium Clock speed: 166 MHz Number of transistors: 3.3 million August 1998 Name of Processor: Pentium II Clock speed: 450 MHz Number of transistors: 7.5 million March 2000 Name of Processor: Pentium III Clock speed: 1.0 GHz Number of transistors: 28 million Nov 2002 Name of Processor: Pentium 4 Clock speed: 3.0 GHz Number of transistors: 55 million
  31. 31. IBM 650 that &quot;became the most popular medium-sized computer in America in the 1950's - rental cost was $5000 per month - 1500 were installed - able to read punched cards or magnetic tape - used rotating magnetic drum main memory unit that could store 4000 words!!! Nokia 9210 Communicator is part of the latest wave of web cell phones Dual 2GHz PowerPC G5 8 GB RAM
  32. 32. Processing Steps <ul><li>Finished devices have as many as 30 distinct layers. </li></ul>
  33. 33. Ionic Conductivity <ul><li>In addition to electrons and holes contributing to conduction, anions and/or cations might also move and thus transfer charge (conduct). </li></ul><ul><ul><li>Usually only considered for ionic materials where electronic conductivity is limited (E g is huge). </li></ul></ul><ul><ul><li>Dominant ionic carriers are: </li></ul></ul><ul><ul><ul><li>Interstitial cations (Li, H) </li></ul></ul></ul><ul><ul><ul><li>Vacancies (oxygen) </li></ul></ul></ul><ul><ul><li>Generally unimportant except at high temperatures. </li></ul></ul><ul><ul><ul><li>Dominant carriers in many gas sensors, fuel cell systems, etc. </li></ul></ul></ul>
  34. 34. APPLICATION: SENSORS • Ex: Oxygen sensor: ZrO 2 • A concentration gradient causes diffusion of Oxygen through the ceramic. • This creates a measurable voltage. In order to make sensor response more rapid, increase diffusion rate by introducing more vacancies. • Add Ca impurity, requiring extra e - : --create Ca interstitials (not likely in this case) --create O 2- vacancies (dominant) ∆ V  =  –  e k T   ·  ln  c 1 / c 2
  35. 35. P-N RECTIFYING JUNCTION REVIEW • Allows flow of electrons in one direction only (e.g., useful to convert alternating current to direct current. --No applied potential: no net current flow. --Forward bias: carrier flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. --Reverse bias: carriers flow away from p-n junction; carriers depleted; little current flow.
  36. 36. Popular culture
  37. 37. SUMMARY Next Class: Review for Test 2 <ul><li>Band diagrams for intrinsic vs. extrinsic semiconductors </li></ul><ul><li>Conductivity for intrinsic vs. extrinsic semiconductors </li></ul><ul><li>Band diagrams for equilibrium, forward and reverse biased pn junctions </li></ul><ul><li>I/V response for a pn junction and how this leads to rectification </li></ul><ul><li>Ionic conductivity vs electronic conductivity </li></ul><ul><li>Dominant ionic carriers </li></ul>E f n-type E c E v p-type E v E c E f