Question E6B06:
Which of the following is a common use of a Schottky diode?  

Choices:  
A: In oscillator circuits as the negative resistance element  
B: As a variable capacitance in an automatic frequency control circuit  
C: In power supplies as a constant voltage reference  
D: As a VHF/UHF mixer or detector    

Correct Answer is  D: As a VHF/UHF mixer or detector  

To use a Schottky diode as a VHF/UHF mixer or detector:
- Schottky diodes are majority-carrier devices with very fast switching (near-zero stored charge) and very low junction capacitance.  
- They have a low forward voltage, typically $V_F \approx 0.2$–$0.3\ \text{V}$, making them sensitive and low-loss for RF detection and mixing at VHF/UHF.

Why D is correct (As a VHF/UHF mixer or detector):
- Fast response (tiny or no reverse recovery time, $t_{rr} \approx 0$) supports high-frequency operation.
- Low $C_j$ (junction capacitance) minimizes RF loading at VHF/UHF.
- Nonlinearity of the diode’s I–V curve enables mixing:  
  $$i \approx I_S\left(e^{v/(nV_T)} - 1\right)$$  
  A nonlinear device driven by RF and LO produces sum/difference frequencies (mixing), ideal for mixers and detectors.

Why the others are incorrect:
- A: Negative resistance elements are tunnel diodes or Gunn diodes, not Schottky diodes. Schottky diodes do not exhibit negative resistance.
- B: Variable capacitance for AFC is done with varactor (varicap) diodes, whose capacitance varies strongly with reverse bias:  
  $$C(V) = \frac{C_0}{(1 + V/\Phi)^m}$$  
  Schottky diodes are not used as the variable-capacitance element in AFC.
- C: Constant voltage references in power supplies use Zener diodes (or bandgap references), not Schottky diodes. Schottkys have a low forward drop but poor temperature stability for precision references.

Useful facts/rules of thumb:
- Schottky forward drop: $V_F \approx 0.2$–$0.3\ \text{V}$ (lower than silicon PN diodes).
- Reverse recovery: $t_{rr} \approx 0$ (excellent for RF switching/mixing).
- Varactor tuning uses the $C(V)$ relation above; Schottky is preferred for RF detection/mixing due to speed and low capacitance, not for variable capacitance or voltage reference.