Special Diodes Summary (with PYQ)

ece-1109 ECE-1109 Introduction to ECE

Special Diodes Summary Table

Diode Type	Construction Structure	Working Principle	Key Characteristic / Application
==1. Tunnel Diode==	Heavily doped P & N (1000x normal); very thin depletion region.	Quantum Tunneling: Electrons pass through the barrier without thermal energy.	Negative Resistance: Current decreases as voltage increases. Used for high-speed switching.
==2. Varactor Diode==	Reverse-biased PN junction acts as a capacitor with depletion layer as dielectric.	Variable Capacitance: Increasing reverse voltage widens depletion layer, decreasing capacitance ($C\propto 1/W$).	Voltage-Controlled Capacitor: Used in tuning circuits (TV/Radio) and frequency modulation.
3. Shockley Diode	4 Layers (PNPN); equivalent to 3 diodes in series.	Breakdown Switching: Stays “OFF” until forward voltage hits breakdown, then drops resistance to turn “ON”.	Latching Switch: Stays ON once triggered. Used in Relaxation Oscillators.
4. LED	Direct band gap materials (GaAs, GaP) encapsulated in resin.	Recombination: Electrons and holes recombine, releasing energy as photons (light).	emits light. Energy efficient, fast switching. Used in displays and indicators.
5. ==Photo-diode==	Reverse-biased PN junction with a glass window.	Light Detection: Incident photons create electron-hole pairs, increasing reverse current ($I_R$).	Dark Current exists without light. Used in alarm circuits, smoke detectors, and sensors.
6. Optoisolator	LED (Emitter) + Photo-detector sealed in an opaque package.	Isolation: Signal transfers via light beam, maintaining electrical isolation between inputs.	Noise Immunity: Protects low-voltage circuits from high voltage and EMI noise.
7. Schottky Diode	Metal-Semiconductor junction (no PN junction).	Majority Carriers: Current flows via majority carriers over the barrier (no minority carrier diffusion).	Fast Switching & Low Drop: Low forward voltage drop (0.2V). Used in RF apps and logic circuits.
8. P-I-N Diode	Heavily doped P & N separated by an Intrinsic (undoped) layer.	Wide Depletion: Intrinsic layer acts as a wide depletion region in reverse bias.	RF Switch: Low capacitance in reverse bias; Variable resistor in forward bias.

Memorization Tip (Analogy)

To memorize these, group them by their “superpower”:

• The Light Couple: LED (makes light) and Photo-diode (eats light). Combine them to get an Optoisolator.
• The Speedsters: Tunnel Diode (Quantum shortcut) and Schottky (Metal shortcut) are built for speed.
• The Shape-Shifters: Varactor (acts like a Capacitor) and P-I-N (acts like a Variable Resistor).
• The Latch: Shockley acts like a heavy door—once you push it hard enough (breakdown), it swings open and stays open.

PYQ

1. Tunnel Diode

PYQs Covered:

• “Write short note on Tunnel diode…” (2015-2018, 2022)
• “What is tunneling effect? Explain the negative resistance region…” (2021, 2024)

Answer:

• Definition & Construction: A Tunnel diode is a PN junction diode heavily doped (1000 times more than conventional diodes) with an extremely thin depletion region. It is typically made of Germanium or Gallium Arsenide.
• The Tunneling Effect: This is a quantum phenomenon where electrons pass through the narrow depletion barrier without requiring sufficient thermal energy to “climb” over it. This occurs because the conduction band of the N-type material aligns with the valence band of the P-type material.
• Negative Resistance Region: The diode exhibits a unique I-V characteristic. As the forward voltage increases from the peak voltage ($V_P$) to the valley voltage ($V_V$), the tunneling effect decreases due to band misalignment. Consequently, the current decreases as voltage increases. This phenomenon is called negative resistance and is used in relaxation oscillators.

2. Varactor Diode

PYQs Covered:

• “What is varactor diode? Describe the operation of a voltage controlled tuning…” (2019, 2020, 2023)
• “Write short note on… Varactor diode.” (2016, 2017, 2022)

Answer:

• Definition: A Varactor diode acts as a variable capacitor when operated under reverse bias.
• Voltage Controlled Tuning Principle:
  1. The depletion region (where no charge carriers exist) acts as the dielectric insulator, while the P and N regions act as the conductive plates.
  2. The capacitance formula is $C=\frac{ϵA}{W}$ (where $W$ is depletion width).
  3. When reverse voltage increases, the depletion width ($W$) increases.
  4. Because $W$ is in the denominator, the total capacitance decreases.
• Application: By changing the voltage, we change the capacitance, allowing the diode to tune frequencies in radio, TV, and FM transmitters.

3. Light Emitting Diode (LED)

PYQs Covered:

• “How can a junction diode be used as a protection element for LED?” (2018, 2023)
• “Write short notes on… LED.” (2015)

Answer:

• Working Principle: LEDs are made of direct band gap materials (like GaAs). When forward biased, electrons and holes recombine at the junction, releasing energy as photons (light).
• Protection Circuits:
  1. Current Protection: To prevent excessive current from burning the LED, a resistor is always connected in series with it.
  2. Reverse Voltage Protection: LEDs have low reverse breakdown ratings (~3V). To protect them, a standard rectifier diode is connected in parallel with the LED (in the opposite direction). If a high reverse voltage is applied, the rectifier diode turns on and shunts the current, saving the LED.

4. Photo-diode

PYQs Covered:

• “What do you mean by dark current and dark resistance?” (2016)
• “Write short notes on… Photo Diode.” (2018)

Answer:

• Operation: A photo-diode is operated in reverse bias. When light strikes the junction, energy transfers to atoms, creating electron-hole pairs that increase the reverse current.
• Dark Current: This is the extremely small reverse current that flows through the device when no light is incident on the PN junction.
• Dark Resistance ($R_{dark}$): This is the resistance of the diode when no light is present, calculated as the ratio of Reverse Voltage to Dark Current ($R=V/I_{dark}$).

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Missing Diodes

• Shockley Diode
• Optoisolator
• Schottky Diode
• P-I-N Diode

Study Tip: If you are prioritizing for an exam based on these sources, focus 80% of your effort on Zener (Regulator Design + Breakdown Theory), Tunnel (Negative Resistance), and Varactor (Tuning/Capacitance).