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

7. 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

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 with VA
EE130 Lecture 12, Slide 9