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

4. : electron affinity

5. 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 ?

6. 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

7. Question: how are V bi and  Bn related?

10. Schottky Barrier Heights: Metal on Si  Bn tends to increase with increasing metal work function

11. 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.

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

14. Question How can you measure  Bn and  Bp ?

15. Using CV Data to Determine  B Question: How should we plot the CV data to extract  B ? EvEv EvEv EcEc EfEf EfEf EcEc  Bn qV bi  Bn q(V bi + V) qV

16. Once V bi is known,   can be determined using: Using CV Data to Determine  B V 1/C 2  V bi

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

18. Is the net current zero? Why?

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

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

22. Applications of Schottky Diodes I 0 of a Schottky diode is 10 3 to 10 8 times larger than a PN junction diode, depending on  B. A Schottky diode is the preferred rectifier in low voltage, high current applications. I V PN junction Schottky  B

23. 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?

26. 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