1 E84 Lecture 3/26/14 K. Candler Agenda o Semiconductor ...
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1 E84 Lecture 3/26/14 K. Candler Agenda o Semiconductor Materials o Crystal Growth o Intrinsic Semiconductors o Extrinsic Semiconductors Introduction – A semiconductor is a material that has electrical conductivity to a degree that is between that of a conductor (such as copper, silver, gold) and an insulator (such as glass). – Semiconductors are the foundation of modern electronics, e.g., o Transistors o Solar cells o Light-emitting diodes (LEDs) o Photodiodes o Digital and analog ICs Semiconductor Materials – Si, Ge, GaAs, SiC – The bonding model: (Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley) o 4 valence electrons o Covalent bonds o Si is a very poor conductor at room temperature…no free electrons – Purity o Purity of semiconductors needs to be very carefully controlled. o Modern semiconductors are some of the purist solid materials that exist. In silicon: Unintentional dopant atoms < 1 per 10 9 Si atoms (like finding 25 apple trees in a forest of pine trees planted coast to coast at 50 ft centers across the U.S.)
Transcript of 1 E84 Lecture 3/26/14 K. Candler Agenda o Semiconductor ...
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E84 Lecture 3/26/14 K. Candler Agenda o Semiconductor Materials o Crystal Growth o Intrinsic Semiconductors o Extrinsic Semiconductors Introduction – A semiconductor is a material that has electrical conductivity to a degree that is
between that of a conductor (such as copper, silver, gold) and an insulator (such as glass).
– Semiconductors are the foundation of modern electronics, e.g.,
o Transistors o Solar cells o Light-emitting diodes (LEDs) o Photodiodes o Digital and analog ICs
Semiconductor Materials – Si, Ge, GaAs, SiC – The bonding model:
(Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley)
o 4 valence electrons o Covalent bonds o Si is a very poor conductor at room temperature…no free electrons
– Purity
o Purity of semiconductors needs to be very carefully controlled. o Modern semiconductors are some of the purist solid materials that exist. In
silicon: Unintentional dopant atoms < 1 per 109 Si atoms (like finding 25 apple trees in a forest of pine trees planted coast to coast at 50 ft centers across the U.S.)
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– Structure
Amorphous Polycrystalline Crystalline
(Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley)
o Amorphous: No recognizable long-range order o Polycrystalline: Completely ordered in segments o Crystalline: Entire solid is made up of atoms in orderly array
Crystal Growth – Obtaining Ultrapure Polycrystalline Si
(Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley)
– Obtaining Single-Crystal Si
o Invented in 1916 by a Polish scientist, Jan Czochralski o A seed crystal is dipped into a crucible of molten silicon and withdrawn
slowly, pulling a cylindrical single crystal as the silicon crystallizes on the seed.
o Show video (up to 4 min mark): http://www.youtube.com/watch?v=aWVywhzuHnQ
Intrinsic Semiconductors – No impurities and lattice defects in its crystal structure – If an electron gains enough energy (from thermal or optical excitation), it can break
the covalent bond and become a free carrier. o E > Eg ; Eg = bandgap energy (the energy needed for an electron to break a
bond) Eg = 1.12 eV (Si) Eg = 1.42 eV (GaAs)
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Eg = 0.66 eV (Ge) Eg (metal) << Eg (semiconductor) << Eg (insulator)
– When a bond is broken, two mobile charge carriers are created: electrons (negative
charge) and holes (positive charge)
(Figure from Howe & Sodini, Microelectronics, Prentice Hall)
– no = po ≡ ni (at thermal equilibrium)
o no = electron concentration at thermal equilibrium [cm-3] o po = hole concentration at thermal equilibrium [cm-3] o ni = intrinsic carrier concentration (ni = 1.5 x 1010 cm-3 in Si at T = 300 K)
– Exercise: How many bonds are broken in Si at room temperature? (Hint: silicon atom density = 5 x 1022 Si atoms/cm3)
o Total possible bonds = 5 x 1022 Si atoms/cm-3 x 4 bonds/atom = 2 x 1023
bonds/cm-3 o # broken bonds at room temp = ni = 1.5 x 1010 cm-3 o # broken bonds/total possible bonds = 1.5 x 1010/2 x 1023 ~ 0.7 x 10-13 less
than one bond in 1013 is broken in Si at room temperature! – Main point: At room temperature, relatively few electrons gain enough energy to
become free electrons, the overall conductivity of semiconductors is low, thereby their name semiconductors.
– Increasing temperature leads to better or worse conductivity? Extrinsic Semiconductors – Contain impurity atoms, which contribute extra electrons and holes (improve
conductivity) – Impurities are introduced through doping. – Dopants are Group III (B, Ga, In, Al) or V (P, As, Sb). – Doping with Group V Elements (Donors)
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(Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley)
Extra electrons: N-type semiconductor Majority carrier: electron Minority carrier: hole
– Doping with Group III Elements (Acceptors):
(Figure from Pierret, Semiconductor Device Fundamentals, Addison Wesley)
Extra holes: P-type semiconductor Majority carrier: hole Minority carrier: electron
– How to calculate # electrons and holes (mobile carriers) in doped Si?
o Mass Action Law:
o rate of electron-hole pair generation = rate of recombination no charge buildup inside Si in thermal equilibrium (no heat flow)
o N-type case
(one electron per donor)
no ≅ Nd
po =ni
2
no=
ni2
Nd
�
no ⋅ po = ni2
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o P-type case (one hole per donor)
o Example: A silicon sample is doped with 1017 As atoms per cm3. What are the carrier concentrations in the Si sample at 300 K? As is n-type, Nd = 1017 cm-3 - no = Nd = 1017 cm-3 - po = ni
2/ no = 1020/1017 = 103 cm-3
o Main point: The majority carriers outnumber the minority carriers by many orders of magnitude!
po ≅ Na
no =ni
2
po=
ni2
Na
