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Integrated Circuit Devices

Integrated Circuit Devices. Professor Ali Javey Summer 2009. MS Junctions Reading: Chapter14. Metal-Semiconductor Contacts. Two kinds of metal-semiconductor contacts : metal on lightly doped silicon – rectifying Schottky diodes metal on heavily doped silicon –

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Integrated Circuit Devices

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  1. Integrated Circuit Devices Professor Ali Javey Summer 2009 MS Junctions Reading: Chapter14

  2. Metal-Semiconductor Contacts • Two kinds of metal-semiconductor contacts: • metal on lightly doped silicon – • rectifying Schottky diodes • metal on heavily doped silicon – • low-resistance ohmic contacts

  3. : electron affinity

  4. Metal Work Function The work function of metals depend strongly on the “environment.” • Workfunction, M is an invariant property of metal. It is the minimum energy required to free up electrons from metal. Question: how can you change S ?

  5. Ideal MS Contact • Assumptions: • M and S are in intimate contact, on atomic scale • No oxides or charges at the interface • No intermixing at the interface

  6. Question: how are Vbi and FBn related?

  7. Schottky Barrier Heights: Metal on Si fBntends to increase with increasing metal work function

  8. Schottky Barrier Heights: metal silicide on Si Silicide-Si interfaces are more stable than metal-silicon interfaces. After metal is deposited on Si, an annealing step is applied to form a silicide-Si contact. The term metal-silicon contact includes silicide-Si contacts.

  9. Depletion Width: Does this depletion width equation look familiar?

  10. Question How can you measure FBn and FBp?

  11. Using CV Data to Determine fB qVbi FBn Ec Ef Ev Question: How should we plot the CV data to extract fB? FBn q(Vbi + V) qV Ec Ef Ev

  12. Using CV Data to Determine fB 2 1 /C V - V bi Once Vbi is known, FBcan be determined using:

  13. Carrier Injection at the MS Contacts For each MS junction, 3 components contribute to the overall injection current: • Thermionic emission current • Tunneling current • Thermally activated tunneling current What parameters affect each component?

  14. Is the net current zero? Why?

  15. Question: How do p+n junctions differ from MS junctions under a forward bias?

  16. Question • How does the band diagram look for a MS junction with a Schottky barrier height of zero?

  17. Applications of Schottky Diodes I Schottky f PN junction B V • I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on fB . • A Schottky diode is the preferred rectifier in low voltage, high current applications.

  18. Ohmic MS Contacts Two ways to achieve ohmic MS contacts: • Reduce the Schottky barrier height. How??? • Reduce the Schottky barrier width (depletion width). How???? How would each approach give you an ohmic contact?

  19. Schottky Barriers and Fermi Level Pinning In actual fabricated metal-Si junctions, Fermi level pinning prevents us from ever getting zero Schottky barrier height. Two tricks for reducing Fermi level pinning: 1. thin interfacial oxide/nitride 2. 1D semiconductors 1D nanotube Diameter ~ 1 nm

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