Special-Purpose Diodes

Semiconductor Devices · Special-Purpose Diodes · Learn

Five common "non-rectifier" diodes — each is still a single PN junction, but exploited in a different operating region or with a structural twist that changes its job in the circuit.

Zener diode

Reverse-biased, intentionally driven into breakdown. Once $V_R$ reaches the Zener voltage $V_Z$ , the device clamps voltage at $V_Z$ regardless of current — a simple voltage reference.

Figure 1. Zener symbol — Z-shaped cathode bar marks the Zener.

Figure 2. Zener I–V curve. Forward like an ordinary diode; reverse breaks at

V_Z

and clamps.

Series resistor sizing — pick $R_S$ so the Zener gets at least its minimum rated current under all load conditions:

R_S = \frac{V_{in,\min} - V_Z}{I_{Z,\min} + I_{L,\max}}

Figure 3. Zener voltage regulator:

V_{in}

→

R_S

→ Zener (reverse-biased, cathode up) parallel to load

R_L

Zener clamps. As long as

V_{in} > V_Z + I_L \cdot R_S

V_{out} = V_Z

. Below that, regulation collapses.

LED (Light-Emitting Diode)

Forward-biased like an ordinary diode, but the bandgap is engineered to release a photon every time an electron-hole pair recombines. $V_F$ depends on the bandgap, hence on colour:

Red ≈ 1.8 – 2.1 V
Yellow / green ≈ 2.0 – 2.4 V
Blue / white ≈ 3.0 – 3.4 V

Figure 4. LED symbol — emitting arrows distinguish it from a plain diode.

Figure 5. LED I–V curve — knee shifted right (≈ 2 V red, ≈ 3 V blue).

R_{series} = \frac{V_{supply} - V_F}{I_F}

Schottky diode

Metal–semiconductor junction (rather than P–N). Two consequences: the forward drop is small ( $V_F \approx 0.2{-}0.4\text{ V}$ ), and there is no minority-carrier storage so it switches in nanoseconds. Used in power-supply rectification, fast logic, and reverse-polarity protection.

Figure 6. Schottky symbol — squared S-shaped cathode bar.

Figure 7. Schottky I–V — knee close to the origin, fast switching.

Schottky = low

V_F

+ fast switching. Trade-off: higher reverse leakage and lower

V_{BR}

than a Si PN diode.

Photodiode

Reverse-biased PN junction whose depletion region is exposed to incident light. Photons knock out electron-hole pairs, the field sweeps them across the junction, and the reverse current scales linearly with light intensity. The dark current is the usual reverse leakage.

Figure 8. Photodiode symbol — incoming light arrows mark photo-generation.

Figure 9. Photodiode I–V — solid: dark. Dashed: illuminated. Light shifts the curve down — extra reverse current.

Varactor (varicap)

Reverse-biased diode used as a variable capacitor. The depletion region is the dielectric; its width grows with $V_R$ , so the junction capacitance $C_j \propto 1/\sqrt{V_R + V_0}$ falls as you increase reverse bias. Used for voltage-controlled oscillators and RF tuning.

Figure 10. Varactor symbol — diode followed by a capacitor plate.

Figure 11. Varactor C vs

V_R

— capacitance falls as reverse bias widens the depletion region.

Each device exploits a different operating region: Zener (reverse breakdown), LED (forward recombination), Schottky (metal junction), photodiode (reverse photo-generation), varactor (reverse depletion-region capacitance).