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Introduction

Introduction. Semiconductor Physics. Introduction: Semiconductor Physics. Silicon bond model: electrons and holes Generation and recombination Thermal equilibrium Intrinsic semiconductor Extrinsic semiconductor. Silicon Bond Model: Electrons and Holes .

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Introduction

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  1. Introduction Semiconductor Physics

  2. Introduction: Semiconductor Physics • Silicon bond model: electrons and holes • Generation and recombination • Thermal equilibrium • Intrinsic semiconductor • Extrinsic semiconductor

  3. Silicon Bond Model: Electrons and Holes Si is in column IV of periodic table

  4. Silicon Bond Model: Electrons and Holes • Electronic structure of Si atom: • 10 core electrons (tightly bound) • 4 valence electrons (loosely bound, responsible for most chemical properties) • Other semiconductors: • Ge, C (diamond form), SiGe • GaAs, InP, InGaAs, ZnSe, CdTe (on average, 4 valenceelectrons per atom)

  5. Silicon Bond Model: Electrons and Holes • Silicon crystal structure • Silicon is a crystalline material • Long range atomic arrangement • Diamond lattice: • Atoms tetrahedrally bonded by sharing valence electrons (covalent bonding) • Each atom shares 8 electrons • In low energy and stable situation • Si atomic density: 5 x 1022 cm-3

  6. Simple “flattened model of Si crystal:

  7. Silicon Bond Model: Electrons and Holes • At 0K • All bonds satisfied  all valence electrons engaged in bonding • No free electrons • At finite temperature • Finite thermal energy • Some bonds are broken • “free” electrons (mobile negative charge, -1.6x 10-19 C) • “free” holes (mobile positive charge , 1.6x 10-19 C)

  8. Silicon Bond Model: Electrons and Holes

  9. Silicon Bond Model: Electrons and Holes “Free” electrons & holes are called carriers • Mobile charged particles • “electron” means free electron • not concerned with bonding electrons or core electrons n = (free) electron concentration [cm -3] p = hole concentration [cm -3]

  10. Generation and Recombination • Generation = break up of covalent bond to form electron and hole • Requires energy from thermal or optical sources (or other external sources) • Recombination = formation of bond by bringing together electron and hole • Releases energy in thermal or optical form • A recombination event requires 1 electron + 1 hole Generation & recombination most likely at surfaces where periodic crystalline is broken.

  11. Thermal Equilibrium • Thermal Equilibrium= steady state + absence of external energy sources • Important consequence: • In thermal equilibrium and for a given semiconductor,np product is a constant that depends only on temperature!

  12. Intrinsic Semiconductor • When a bond breaks, an electron and a hole are produced: n0= p0 Also: n0p0 = ni2 Then: n0= p0 = ni ni = intrinsic carrier concentration [cm -3 ] In Si at 300K (room temperature): ni = 1x 1010 cm -3

  13. Extrinsic Semiconductor • Doping • introduction of foreign atoms to engineer semiconductor electrical properties • Donors • Introduce electron to the semiconductor • For Si, group-V atoms with 5 valence electrons (As, P, Sb) • 4 electrons of donor atom participate in bonding • 5th electron is easy to release • Donor site become positively charged

  14. Periodic Table Source: http://www.chemicool.com/

  15. Valence Electrons • The valence electrons are the electrons in the last shell or energy level of an atom. • The valence electrons increase in number as you go across a period • The number of valence electrons stays the same as you go up or down a group, but they increase as you go from left to right across the periodic table

  16. Extrinsic Semiconductor: Donor • Nd = donor concentration [cm -3 ] • IfNd<< ni , doping irrelevant • Intrinsic semiconductor  n0= p0 = ni • IfNd>> ni , doping controls carrier concentrations • Extrinsic semiconductor  n0= Nd , p0 = ni2 /Nd Note: n0 >> p0 n-type semiconductor

  17. Extrinsic Semiconductor: Donor group-V atoms with 5 valence electrons

  18. Extrinsic Semiconductor: Acceptor • Acceptors • Introduce holes to the semiconductor • For Si, group-III atoms with 3 valence electrons (B) • 3 electrons used in bonding to neighboring Si atoms • 1 bonding site “unsatisfied” • Easy to accept neighboring bonding electron to complete all bonds • At room temperature, each acceptor releases 1 hole that is available to conduction • Acceptor site become negatively charged

  19. Extrinsic Semiconductor: Acceptor • Na = acceptor concentration [cm -3 ] • If Na << ni , doping irrelevant • Intrinsic semiconductor  n0= p0 = ni • If Na >> ni , doping controls carrier concentrations • Extrinsic semiconductor  p0= Na , n0 = ni2 /Na Note: p0 >> n0 p-type semiconductor

  20. Extrinsic Semiconductor: Acceptor • group-III atoms with 3 valence electrons

  21. Extrinsic Semiconductor Trivalent impurities - impurity atoms with 3 valence electrons - produce p-type semiconductors by producing a "hole" or electron deficiency • Pentavalent impurities • impurity atoms with 5 valence electrons • produce n-type semiconductors by contributing extra electrons

  22. Extrinsic SemiconductorP-type & N-type Semiconductor

  23. Summary • In a semiconductor, there are two types of “carriers”: electrons and holes • In thermal equilibrium and for a given semiconductor n0p0 is a constant that only depends on temperature: n0p0 = ni2 • For Si at room temperature: ni = 1x 1010 cm -3 • Intrinsic semiconductor: pure semiconductor n0= p0 = ni

  24. Summary • Carrier concentration can be engineered by addition of “dopants” (selected foreign atoms): • Pentavalent impurities (P, As, Sb)  n-type semiconductor: n0= Nd , p0 = ni2 /Nd • Trivalent impurities (B, Al, Ga)  p-type semiconductor: p0= Na , n0 = ni2 /Na

  25. Video Links from Youtube AMD MICROPROCESSOR http://www.youtube.com/watch?v=-GQmtITMdas&feature=related From Sand to Silicon http://www.youtube.com/watch?v=Q5paWn7bFg4 CH IP MANUFACTURING PROCESS http://www.youtube.com/watch?v=9rCyu8B0tYs&feature=related Semiconductor Electronics Theory Lesson 1 Segment 1 - Semiconductor Atoms http://www.youtube.com/watch?v=qkjCe0r5-cw Introduction to Semiconductor Materials (2) http://www.youtube.com/watch?v=AgkQrCeJF1Y&NR=1 Semiconductors Theory 1 Segment 2A - Doped Silicon Crystal http://www.youtube.com/watch?v=U8daujO20nM&feature=related

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