Question E6B08:
Which of the following is a Schottky barrier diode?  

Choices:  
A: Metal-semiconductor junction  
B: Electrolytic rectifier  
C: PIN junction  
D: Thermionic emission diode    

Correct Answer is  A: Metal-semiconductor junction  

Correct answer: A — Metal-semiconductor junction

1) Why A is correct
- A Schottky barrier diode is formed by a metal-to-semiconductor junction (typically metal to n-type silicon), creating a Schottky barrier.
- It is a majority-carrier device (no significant minority-carrier charge storage), giving:
  - Very fast switching and near-zero reverse recovery time ($t_{rr} \approx 0$).
  - Low forward voltage drop: $V_F \approx 0.2$–$0.4$ V (typical).

2) Why the others are incorrect
- B: Electrolytic rectifier
  - Uses electrochemical action in an electrolyte, not a metal-semiconductor barrier. Obsolete and not a Schottky.
- C: PIN junction
  - Has p-type, intrinsic, and n-type regions. Used for RF switches/attenuators and photodiodes; relies on charge storage, not a Schottky barrier.
- D: Thermionic emission diode
  - A vacuum tube diode; current flows across a vacuum from a heated cathode. Not a semiconductor device.

3) Useful formulas and rules of thumb
- Shockley diode equation (applies, with different parameters):
  $$I = I_S\left(e^{\frac{V}{n V_T}} - 1\right), \quad V_T = \frac{kT}{q} \approx 25.9\ \text{mV at 300 K}$$
  - Schottky diodes have larger $I_S$ and $n \gtrsim 1$, yielding lower $V_F$ at a given current.
- Forward-drop quick reference:
  - Schottky: $V_F \approx 0.2$–$0.4$ V
  - Silicon PN: $V_F \approx 0.6$–$0.7$ V
  - Germanium: $V_F \approx 0.2$–$0.3$ V
- Rules of thumb:
  - Choose Schottky for RF detectors/mixers and high-speed rectification.
  - Expect lower reverse breakdown ratings (often < 100 V) compared to PN diodes.