Semiconductor physics


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Semiconductor physics

  2. 2. OVERVIEW <ul><li>Semiconductor fundamentals </li></ul><ul><li>Doping </li></ul><ul><li>PN Junction </li></ul><ul><li>Diode Equation </li></ul><ul><li>Zener Diodes </li></ul><ul><li>LEDs </li></ul>
  3. 3. SEMICONDUCTOR: AN INTRODUCTION <ul><li>Conductors: Allow Electric </li></ul><ul><li>current to flow through them </li></ul><ul><li>Insulators: Do not Allow Electric current to flow through them </li></ul><ul><li>Semiconductors: Materials whose conductivity lies in between of Conductors and Semiconductor </li></ul>
  4. 4. SEMICONDUCTORS <ul><li>A material whose properties are such that it is not quite a conductor, not quite an insulator </li></ul><ul><li>Some common semiconductors </li></ul><ul><ul><li>elemental </li></ul></ul><ul><ul><ul><li>Si - Silicon (most common) </li></ul></ul></ul><ul><ul><ul><li>Ge - Germanium </li></ul></ul></ul><ul><ul><li>compound </li></ul></ul><ul><ul><ul><li>GaAs - Gallium arsenide </li></ul></ul></ul><ul><ul><ul><li>GaP - Gallium phosphide </li></ul></ul></ul><ul><ul><ul><li>AlAs - Aluminum arsenide </li></ul></ul></ul><ul><ul><ul><li>AlP - Aluminum phosphide </li></ul></ul></ul><ul><ul><ul><li>InP - Indium Phosphide </li></ul></ul></ul>
  5. 5. CRYSTALLINE SOLIDS <ul><li>In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal </li></ul><ul><li>Silicon crystal has a diamond lattice </li></ul>
  6. 6. CRYSTALLINE NATURE OF SILICON <ul><li>Silicon as utilized in integrated circuits is crystalline in nature </li></ul><ul><li>As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell </li></ul><ul><li>For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron </li></ul>
  7. 7. INTRINSIC NATURE OF SILICON <ul><li>Silicon that is free of doping impurities is called intrinsic </li></ul><ul><li>Silicon has a valence of 4 and forms covalent bonds with four other neighboring silicon atoms </li></ul>
  8. 8. CRYSTALLINE STRUCTURE OF SEMICONDUCTOR <ul><li>Semiconductors have a regular crystalline structure </li></ul><ul><ul><li>for monocrystal, extends through entire structure </li></ul></ul><ul><ul><li>for polycrystal, structure is interrupted at irregular boundaries </li></ul></ul><ul><li>Monocrystal has uniform 3-dimensional structure </li></ul><ul><li>Atoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium </li></ul>
  9. 9. CRYSTALLINE STRUCTURE OF SEMICONDUCTOR <ul><li>Silicon atoms have 4 electrons in outer shell </li></ul><ul><ul><li>inner electrons are very closely bound to atom </li></ul></ul><ul><li>These electrons are shared with neighbor atoms on both sides to “fill” the shell </li></ul><ul><ul><li>resulting structure is very stable </li></ul></ul><ul><ul><li>electrons are fairly tightly bound </li></ul></ul><ul><ul><ul><li>no “loose” electrons </li></ul></ul></ul><ul><ul><li>at room temperature, if battery applied, very little electric current flows </li></ul></ul>
  10. 10. CONDUCTION IN CRYSTAL LATTICES <ul><li>Semiconductors (Si and Ge) have 4 electrons in their outer shell </li></ul><ul><ul><li>2 in the s subshell </li></ul></ul><ul><ul><li>2 in the p subshell </li></ul></ul><ul><li>As the distance between atoms decreases the discrete subshells spread out into bands </li></ul><ul><li>As the distance decreases further, the bands overlap and then separate </li></ul><ul><ul><li>the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom </li></ul></ul><ul><ul><li>4 possible electrons in the lower band ( valence band ) </li></ul></ul><ul><ul><li>4 possible electrons in the upper band ( conduction band ) </li></ul></ul>
  11. 11. ENERGY BANDS IN SEMICONDUCTORS <ul><li>The space between the bands is the energy gap , or forbidden band </li></ul>
  12. 12. INSULATORS, SEMICONDUCTORS , AND METALS: COMPARISON <ul><li>This separation of the valence and conduction bands determines the electrical properties of the material </li></ul><ul><li>Insulators have a large energy gap </li></ul><ul><ul><li>electrons can’t jump from valence to conduction bands </li></ul></ul><ul><ul><li>no current flows </li></ul></ul><ul><li>Conductors (metals) have a very small (or nonexistent) energy gap </li></ul><ul><ul><li>electrons easily jump to conduction bands due to thermal excitation </li></ul></ul><ul><ul><li>current flows easily </li></ul></ul><ul><li>Semiconductors have a moderate energy gap </li></ul><ul><ul><li>only a few electrons can jump to the conduction band </li></ul></ul><ul><ul><ul><li>leaving “ holes ” </li></ul></ul></ul><ul><ul><li>only a little current can flow </li></ul></ul>
  13. 13. INSULATORS, SEMICONDUCTORS, AND METALS (CONTINUED) Conduction Band Valence Band Conductor Semiconductor Insulator Overlap Band Gap More Band Gap
  14. 14. ELECTRON-HOLE PAIRS <ul><li>Sometimes thermal energy is enough to cause an electron to jump from the valence band to the conduction band </li></ul><ul><li>Electrons also “fall” back out of the conduction band into the valence band, combining with a hole </li></ul>pair elimination hole electron pair creation
  15. 15. DOPING AND CONDUCTION <ul><li>To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes </li></ul><ul><ul><li>elements with 5 outer electrons contribute an extra electron to the lattice ( donor dopant) </li></ul></ul><ul><ul><li>elements with 3 outer electrons accept an electron from the silicon ( acceptor dopant) </li></ul></ul>
  16. 16. DOPING AND CONDUCTION CONTINUED... <ul><li>Phosphorus and arsenic are donor dopants </li></ul><ul><ul><li>if phosphorus is introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current </li></ul></ul><ul><ul><ul><li>very loosely bound to atom and can easily jump to conduction band </li></ul></ul></ul><ul><ul><li>produces n type silicon </li></ul></ul><ul><ul><ul><li>sometimes use + symbol to indicate heavier doping, so n+ silicon </li></ul></ul></ul><ul><ul><li>phosphorus becomes positive ion after giving up electron </li></ul></ul>
  17. 17. DOPING AND CONDUCTION CONTINUED… <ul><li>Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom </li></ul><ul><ul><li>boron is an acceptor dopant </li></ul></ul><ul><ul><li>yields p type silicon </li></ul></ul><ul><ul><li>boron becomes negative ion after accepting an electron </li></ul></ul>
  18. 18. DIFFUSION OF DOPANTS <ul><li>It is also possible to introduce dopants into silicon by heating them so they diffuse into the silicon </li></ul><ul><ul><li>no new silicon is added </li></ul></ul><ul><ul><li>high heat causes diffusion </li></ul></ul><ul><li>Can be done with constant concentration in atmosphere </li></ul><ul><ul><li>close to straight line concentration gradient </li></ul></ul><ul><li>Or with constant number of atoms per unit area </li></ul><ul><ul><li>predeposition </li></ul></ul><ul><ul><li>bell-shaped gradient </li></ul></ul><ul><li>Diffusion causes spreading of doped areas </li></ul>Top view Side view
  19. 19. DIFFUSION OF DOPANTS (CONTINUED) Concentration of dopant in surrounding atmosphere kept constant per unit volume Dopant deposited on surface - constant amount per unit area
  20. 20. ION IMPLANTATION OF DOPANTS <ul><li>One way to reduce the spreading found with diffusion is to use ion implantation </li></ul><ul><ul><li>also gives better uniformity of dopant </li></ul></ul><ul><ul><li>yields faster devices </li></ul></ul><ul><ul><li>lower temperature process </li></ul></ul><ul><li>Ions are accelerated from 5 Kev to 10 Mev and directed at silicon </li></ul><ul><ul><li>higher energy gives greater depth penetration </li></ul></ul><ul><ul><li>total dose is measured by flux </li></ul></ul><ul><ul><ul><li>number of ions per cm 2 </li></ul></ul></ul><ul><ul><ul><li>typically 10 12 per cm 2 - 10 16 per cm 2 </li></ul></ul></ul><ul><li>Flux is over entire surface of silicon </li></ul><ul><ul><li>use masks to cover areas where implantation is not wanted </li></ul></ul><ul><li>Heat afterward to work into crystal lattice </li></ul>
  21. 21. HOLE AND ELECTRON CONCENTRATIONS <ul><li>To produce reasonable levels of conduction doesn’t require much doping </li></ul><ul><ul><li>silicon has about 5 x 10 22 atoms/cm 3 </li></ul></ul><ul><ul><li>typical dopant levels are about 10 15 atoms/cm 3 </li></ul></ul><ul><li>In undoped (intrinsic) silicon, the number of holes and number of free electrons is equal, and their product equals a constant </li></ul><ul><ul><li>actually, n i increases with increasing temperature </li></ul></ul><ul><li>This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the number of holes </li></ul>
  22. 22. DOPING <ul><li>The N in N-type stands for negative. </li></ul><ul><li>A column V ion is inserted. </li></ul><ul><li>The extra valence electron is free to move about the lattice </li></ul>There are two types of doping N-type and P-type. <ul><li>The P in P-type stands for positive. </li></ul><ul><li>A column III ion is inserted. </li></ul><ul><li>Electrons from the surrounding Silicon move to fill the “hole.” </li></ul>
  23. 23. ENERGY-BAND DIAGRAM <ul><li>A very important concept in the study of semiconductors is the energy-band diagram </li></ul><ul><li>It is used to represent the range of energy a valence electron can have </li></ul><ul><li>For semiconductors the electrons can have any one value of a continuous range of energy levels while they occupy the valence shell of the atom </li></ul><ul><ul><li>That band of energy levels is called the valence band </li></ul></ul><ul><li>Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy levels </li></ul><ul><ul><li>This is the conduction band </li></ul></ul>
  24. 24. BAND GAP <ul><li>Between the valence and the conduction band is a range of energy levels where there are no allowed states for an electron </li></ul><ul><li>This is the band gap </li></ul><ul><li>In silicon at room temperature [in electron volts]: </li></ul><ul><li>Electron volt is an atomic measurement unit, 1 eV energy is necessary to decrease of the potential of the electron with 1 V. </li></ul>
  25. 25. COUNTER DOPING <ul><li>Insert more than one type of Ion </li></ul><ul><li>The extra electron and the extra hole cancel out </li></ul>
  26. 26. P-N JUNCTION <ul><li>Also known as a diode </li></ul><ul><li>One of the basics of semiconductor technology - </li></ul><ul><li>Created by placing n-type and p-type material in close contact </li></ul><ul><li>Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type </li></ul>
  27. 27. P-N JUNCTION <ul><li>Region of charges left behind (dopants fixed in crystal lattice) </li></ul><ul><ul><li>Group III in p-type (one less proton than Si- negative charge) </li></ul></ul><ul><ul><li>Group IV in n-type (one more proton than Si - positive charge) </li></ul></ul><ul><li>Region is totally depleted of mobile charges - “depletion region” </li></ul><ul><ul><li>Electric field forms due to fixed charges in the depletion region </li></ul></ul><ul><ul><li>Depletion region has high resistance due to lack of mobile charges </li></ul></ul>
  28. 28. THE P-N JUNCTION Direction of Current
  29. 29. DEPLETION LAYER FORMATION  The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region
  30. 30. BIASING THE P-N DIODE Forward Bias Applies - voltage to the n region and + voltage to the p region CURRENT! Reverse Bias Applies + voltage to n region and – voltage to p region NO CURRENT DIODES CAN BE CONSIDERED AS SWITCH
  31. 31. P-N JUNCTION – REVERSE BIAS <ul><li>positive voltage placed on n-type material </li></ul><ul><li>electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal </li></ul><ul><li>width of depletion region increases </li></ul><ul><li>allowed current is essentially zero (small “drift” current) </li></ul>No current Flow Depletion layer width Increses
  32. 32. P-N JUNCTION – FORWARD BIAS <ul><li>positive voltage placed on p-type material </li></ul><ul><li>holes in p-type move away from positive terminal, electrons in n-type move further from negative terminal </li></ul><ul><li>depletion region becomes smaller - resistance of device decreases </li></ul><ul><li>voltage increased until critical voltage is reached, depletion region disappears, current can flow freely </li></ul>
  33. 33. P-N JUNCTION - V-I CHARACTERISTICS <ul><li>Voltage-Current relationship for a p-n junction (diode) </li></ul>
  34. 34. CURRENT-VOLTAGE CHARACTERISTICS THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE
  36. 36. SEMICONDUCTOR DIODE - OPENED REGION <ul><li>The p-side is the cathode, the n-side is the anode </li></ul><ul><li>The dropped voltage, V D is measured from the cathode to the anode </li></ul><ul><li>Opened: V D  V F : </li></ul><ul><li>V D = V F </li></ul><ul><li>I D = circuit limited, in our model the V D cannot exceed V F </li></ul>
  37. 37. SEMICONDUCTOR DIODE - CUT-OFF REGION <ul><li>Cut-off: 0 < V D < V F : </li></ul><ul><li>I D  0 mA </li></ul>
  38. 38. SEMICONDUCTOR DIODE - CLOSED REGION <ul><li>Closed: V F < V D  0: </li></ul><ul><ul><li>V D is determined by the circuit, I D = 0 mA </li></ul></ul><ul><li>Typical values of V F : 0.5 ¸ 0.7 V </li></ul>
  39. 39. ZENER EFFECT <ul><li>Zener break down: V D <= V Z : </li></ul><ul><li>V D = V Z , I D is determined by the circuit. </li></ul><ul><li>In case of standard diode the typical values of the break down voltage V Z of the Zener effect -20 ... -100 V </li></ul><ul><li>Zener diode </li></ul><ul><ul><li>Utilization of the Zener effect </li></ul></ul><ul><ul><li>Typical break down values of V Z : -4.5 ... -15 V </li></ul></ul>
  40. 40. LED <ul><li>Light emitting diode, made from GaAs </li></ul><ul><ul><li>V F =1.6 V </li></ul></ul><ul><ul><li>I F >= 6 mA </li></ul></ul>
  41. 41.