1. Chapter 14. MS Contacts and Practical Contact Considerations
14. MS Contacts and Schottky Diodes
Chapter 14. MS Contacts and
Schottky Diodes
Ideal MS Contacts
Schottky Diode
Practical Contact Considerations

2. Schottky Barriers 14. MS Contacts and Schottky Diodes
Work function of metal : qΦm
Work function of semiconductor : qΦs
When M-S contact, charge transfer until equilibrium
Φm> Φs : Before contact, EFm < EFs
equilibrium contact potential →
potential barrier height
For p-type
For n-type

3. 14. MS Contacts and Schottky Diodes

4. 14. MS Contacts and Schottky Diodes

5. 14. MS Contacts and Schottky Diodes
Rectifying contacts

6. Forward bias : Vo → Vo - V (Fig 5-33(a))
14. MS Contacts and Schottky Diodes
Forward bias : Vo → Vo - V (Fig 5-33(a))
electrons in conduction band → metal : diffusion
Current : metal → semi increase
Reverse bias : Vo → Vo + Vr
   electron : semi → metal : negligible
  electron : metal → semi :        retarded
Diode equation        (Fig 5-33(c))
Reverse saturation current Io determined by      (barrier,indepent of
bias voltage)
→ probability of an electron in the metal surmounting barrier :
Boltzmann factor            

7. 14. MS Contacts and Schottky Diodes
EXAMPLE 5-6
- For heterojunction in the GaAs-AlGaAs system, the direct(Γ) band gap difference ΔEgΓ is accommodated approximately 2/3 in the conduction band and 1/3 in the valance band. For an Al composition of 0.3, the AlGaAs is direct(see Fig. 3-6) with EgΓ=1.85 eV. Sketch the band diagrams for two heterojunction cases : N+-Al0.3Ga0.7As on n-type GaAs, and N+-Al0.3Ga0.7As on p-type GaAs
Solution
- Taking ΔEg = = 0.42 eV, the band offsets are ΔEc = 0.28 eV and ΔEV = 0.14 eV. In each case we draw the equilibrium Fermi level, add the appropriate bands far from the junction, add the band offsets while estimating the relative amounts of band  bending and position of x = 0 for the particular doping on the two sides, and finally sketch the band edges so that Eg is maintained in each separate semiconductor right up to the heterojunction at x = 0.

8. 14. MS Contacts and Schottky Diodes

9. 14. MS Contacts and Schottky Diodes
Potential well

10. Ohmic contact n type Si + metal Φm<Φs : Ohmic contact
14. MS Contacts and Schottky Diodes
Ohmic contact
n type Si + metal
Φm<Φs : Ohmic contact
p type Si + metal
Φm>Φs : Ohmic contact

11. Typical Schottky Barrier
14. MS Contacts and Schottky Diodes
Typical Schottky Barrier
  Schottky barrier → forward current equation

12. Heterojunctions Homojunction Metal-Semiconductor junction
14. MS Contacts and Schottky Diodes
Heterojunctions
Homojunction
Metal-Semiconductor junction
  - two lattice matched semiconductors with different band gaps

13. 14. MS Contacts and Schottky Diodes