Lecture 15

Today’s objectives- Semiconductors and Integrated Circuits ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Semiconductor Industry in 2003 ,[object Object],[object Object],[object Object],[object Object],http://www.infras.com/Tutorial/sld001.htm

Typical Semiconductors GaAs ZnS (Zinc Blende) Structure 4 Ga atoms at (0,0,0)+ FCC translations 4 As atoms at ( ¼,¼,¼)+FCC translations Bonding: covalent, partially ionic Silicon Diamond Cubic Structure 4 atoms at (0,0,0)+ FCC translations 4 atoms at ( ¼,¼,¼)+FCC translations Bonding: covalent

Band structures for semiconductors and insulators ,[object Object],[object Object],E f , Fermi level Metal (Cu) partially filled 4s (conduction) filled 3p, 2p, 2s, 1p, 1s (valence) Empty 4p (conduction) Band gap Band gap Energy Filled (deep valence) E f Insulator (Al 2 O 3 ) Filled (valence) Empty (conduction) Band gap Band gap Filled (deep valence) E f Semiconductor (Si) Filled (valence) Empty (conduction) Band gap Band gap E c E v

Electron Conductivity ,[object Object],[object Object],[object Object],[object Object],n =electrons/m 3 (10 16 for Si) metal

Electrical Conduction in Intrinsic SCs Schematic Band Diagram “ Real” Band Diagram (empty at T=OK) (full at T=OK) h + e - ,[object Object],e - jumping to CB via thermal excitation at T>OK

Electron and hole conductivity How can we think of conductivity carried by a hole, something that isn’t there? • Total Electrical Conductivity thus given by: # electrons/m 3 electron mobility # holes/m 3 hole mobility • In a semiconductor, there can be electrons and holes:

Intrinsic carriers ,[object Object],[object Object],[object Object],[object Object],μ h is ~ 20% of μ e

Analogy to metals ,[object Object],[object Object],[object Object],[object Object],[object Object]

Extrinsic SCs P in Si donates an extra electron to the crystal. This electron exists in (or near) the conduction band. The electron thus may be able to carry current in an E field.

Typical Donor and Acceptor Dopants for Si ,[object Object],[object Object],[object Object],[object Object],[object Object]

Donor electrons ,[object Object],[object Object],[object Object],E f =E donor = E c -0.05eV

Extrinsic conductivity—p type ,[object Object],[object Object],[object Object]

Acceptor vs. donor doped extrinsic semiconductors E f =E donor = E c -0.05eV E f =E acceptor = E v +0.05eV ,[object Object]

Summary: Intrinsic vs. Extrinsic (n or p) • Intrinsic : # electrons = # holes (n = p) --case for pure Si • Extrinsic : --n ≠ p --occurs when DOPANTS are added with a different # valence electrons than the host (e.g., Si atoms) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n)

Intrinsic vs. Extrinsic— charge concentration vs. Temperature ,[object Object],• Comparison: intrinsic vs extrinsic conduction... For an extrinsic doping level of: 10 21 /m 3 of a n-type donor impurity (such as P). --for T < 100K: " freeze-out” thermal energy only sufficient to excite a very few electrons. --for 150K < T < 450K: "extrinsic" --for T >> 450K: "intrinsic" Adapted from Fig. 18.16, Callister 6e . (Fig. 18.16 from S.M. Sze, Semiconductor Devices, Physics, and Technology , Bell Telephone Laboratories, Inc., 1985.)

Actual Conductivity vs. Temperature ,[object Object],[object Object],Adapted from Fig. 19.15, Callister 5e . (Fig. 19.15 adapted from G.L. Pearson and J. Bardeen, Phys. Rev. 75 , p. 865, 1949.) Why the decrease?

Carrier mobility vs. dopant concentration ,[object Object],[object Object]

SUMMARY ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Lecture 15

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Lecture 15