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 Si is in column IV of periodic table 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) 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 • Simple “flattened model of Si crystal: 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) Silicon Bond Model: Electrons and Holes 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] 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. 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! Intrinsic Semiconductor • When a bond breaks, an electron and a hole are produced: n0 = p0 Also: n0p0 = ni 2 Then: n0 = p0 = ni ni = intrinsic carrier concentration [cm -3 ] In Si at 300K (room temperature): ni = 1x 1010 cm -3 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 Periodic Table Source: http://www.chemicool.com/ 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 Extrinsic Semiconductor: Donor • Nd = donor concentration [cm -3 ] • If Nd << ni , doping irrelevant – Intrinsic semiconductor n0 = p0 = ni • If Nd >> ni , doping controls carrier concentrations – Extrinsic semiconductor n0 = Nd , p0 = ni 2 /Nd Note: n0 >> p0 n-type semiconductor Extrinsic Semiconductor: Donor group-V atoms with 5 valence electrons 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 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 = ni 2 /Na Note: p0 >> n0 p-type semiconductor Extrinsic Semiconductor: Acceptor group-III atoms with 3 valence electrons Extrinsic Semiconductor Pentavalent impurities - impurity atoms with 5 valence electrons - produce n-type semiconductors by contributing extra electrons Trivalent impurities - impurity atoms with 3 valence electrons - produce p-type semiconductors by producing a "hole" or electron deficiency Extrinsic Semiconductor P-type & N-type Semiconductor 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 = ni 2 • For Si at room temperature: ni = 1x 1010 cm -3 • Intrinsic semiconductor: pure semiconductor n0 = p0 = ni Summary • Carrier concentration can be engineered by addition of “dopants” (selected foreign atoms): – Pentavalent impurities (P, As, Sb) n-type semiconductor: n0 = Nd , p0 = ni 2 /Nd – Trivalent impurities (B, Al, Ga) p-type semiconductor: p0 = Na , n0 = ni 2 /Na 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