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

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

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

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

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

Thermionic Emission Theory 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: EE130 Lecture 12, Slide 6

Schottky Diode I - V 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 EE130 Lecture 12, Slide 7

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

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 with VA EE130 Lecture 12, Slide 9