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Lecture #12

OUTLINE Metal-semiconductor contacts (cont.) I - V characteristics Reading: Finish 14.2. Lecture #12. Current Flow in a Schottky Diode. FORWARD BIAS. Current is determined by majority-carrier flow across the MS junction:

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Lecture #12

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  1. OUTLINE Metal-semiconductor contacts (cont.) I-V characteristics Reading: Finish 14.2 Lecture #12 EE130 Lecture 12, Slide 1

  2. Current Flow in a Schottky Diode FORWARD BIAS • Current is determined by majority-carrier flow across the MS junction: • Under forward bias, majority-carrier diffusion from the semiconductor into the metal dominates • Under reverse bias, majority-carrier diffusion from the metal into the semiconductor dominates REVERSE BIAS EE130 Lecture 12, Slide 2

  3. Voltage Drop across the MS Diode • Under equilibrium conditions (VA = 0), the voltage drop across the semiconductor depletion region is the built-in voltage Vbi. • If VA 0, the voltage drop across the semiconductor depletion region is Vbi - VA. EE130 Lecture 12, Slide 3

  4. Depleted Layer Width, W, for VA 0 Last time, we found that At x = 0, V = - (Vbi- VA) • W increases with increasing –VA • W decreases with increasing ND - (Vbi- VA) EE130 Lecture 12, Slide 4

  5. W for p-type Semiconductor At x = 0, V = Vbi + VA • W increases with increasing VA • W decreases with increasing NA EE130 Lecture 12, Slide 5

  6. Electrons can cross the junction into the metal if Thus the current for electrons at a given velocity is: So, the total current over the barrier is: Thermionic Emission Theory EE130 Lecture 12, Slide 6

  7. For a nondegenerate semiconductor, it can be shown that We can then obtain In the reverse direction, the electrons always see the same barrier FB, so Therefore Schottky Diode I - V EE130 Lecture 12, Slide 7

  8. Applications of Schottky Diodes • IS of a Schottky diode is 103 to 108 times larger than that of a pn junction diode, depending on FB.  Schottky diodes are preferred rectifiers for low-voltage, high-current applications. EE130 Lecture 12, Slide 8

  9. Summary • In a Schottky contact, charge is stored on either side of the MS junction • The applied bias VA modulates this charge and thus the voltage drop across the semiconductor depletion region  Flow of majority carriers into the metal varies exponentially withVA EE130 Lecture 12, Slide 9

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