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

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

14. MS Contacts and Schottky Diodes

14. MS Contacts and Schottky Diodes

14. MS Contacts and Schottky Diodes Rectifying contacts

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            

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 = 1.85 - 1.43 = 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.

14. MS Contacts and Schottky Diodes

14. MS Contacts and Schottky Diodes Potential well

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

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

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

14. MS Contacts and Schottky Diodes