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EE 4345 - Semiconductor Electronics Design Project Spring 2002 - Lecture 01

Professor Ronald L. Carterronc@uta.edu

http://www.uta.edu/ronc/

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Silicon Covalent Bond (2D Repr)

• Each Si atom has 4 nearest neighbors

• Si atom: 4 valence elec and 4+ ion core

• 8 bond sites / atom• All bond sites filled• Bonding electrons

shared 50/50_ = Bonding electron

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Si Energy BandStructure at 0 K

• Every valence site is occupied by an electron

• No electrons allowed in band gap

• No electrons with enough energy to populate the conduction band

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Si Bond ModelAbove Zero Kelvin

• Enough therm energy ~kT(k=8.62E-5eV/K) to break some bonds

• Free electron and broken bond separate

• One electron for every “hole” (absent electron of broken bond)

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Band Model forthermal carriers• Thermal energy ~kT

generates electron-hole pairs

• At 300K Eg(Si) = 1.124 eV

>> kT = 25.86 meV,Nc = 2.8E19/cm3

> Nv = 1.04E19/cm3>> ni = 1E10/cm3

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Donor: cond. electr.due to phosphorous

• P atom: 5 valence elec and 5+ ion core

• 5th valence electr has no avail bond

• Each extra free el, -q, has one +q ion

• # P atoms = # free elect, so neutral

• H atom-like orbits

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Band Model fordonor electrons• Ionization energy

of donor Ei = Ec-Ed ~ 40 meV

• Since Ec-Ed ~ kT, all donors are ionized, so ND ~ n

• Electron “freeze-out” when kT is too small

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Acceptor: Holedue to boron

• B atom: 3 valence elec and 3+ ion core

• 4th bond site has no avail el (=> hole)

• Each hole adds -(-q) and has one -q ion

• #B atoms = #holes, so neutral

• H atom-like orbits

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Classes ofsemiconductors• Intrinsic: no = po = ni, since Na&Nd << ni

=[NcNvexp(Eg/kT)]1/2,(not easy to get)

• n-type: no > po, since Nd > Na

• p-type: no < po, since Nd < Na

• Compensated: no=po=ni, w/ Na- = Nd

+ > 0

• Note: n-type and p-type are usually partially compensated since there are usually some opposite- type dopants