Electronic Devices and CircuitsUnit 28 min read

PN Junction Diode: IV Curve, Biasing, Applications & Models

Unit 2 of Electronic Devices and Circuits covers the PN junction diode’s construction, IV characteristics, biasing (forward/reverse), equivalent circuit models (ideal/real), and real-world applications in rectifiers, voltage regulation, and signal processing—with visuals of gate symbols, IV curves, and actual diode chi

Padhai · Based on the official syllabus and past papers
Updated 3 Oct 2026

Key Concepts and Theory

1. PN Junction Formation and Depletion Region

A PN junction is formed when a p-type (hole-rich) and n-type (electron-rich) semiconductor are brought into contact. At the junction, electrons from the n-side diffuse to the p-side and recombine with holes, creating a depletion region (a region devoid of free charge carriers). This region establishes an electric field (barrier potential, ~0.7V for Si, ~0.3V for Ge) that opposes further diffusion.

   p-type (holes)       n-type (electrons)
   ----------------------------
   + + + + + +           - - - - - -
   (acceptor ions)       (donor ions)
   ----------------------------
   Depletion Region (E-field)
   ----------------------------

Key Points:

The depletion region width increases with reverse bias and decreases with forward bias.
The built-in potential ( $V_{bi}$ ) is the voltage required to reduce the depletion region to zero under forward bias.

2. IV Characteristics of a PN Junction Diode

The current-voltage (I-V) relationship of a diode is given by the Shockley diode equation: $I = I_{S} (e^{\frac{V _{D}}{n V _{T}}} - 1)$ where:

$I$ = diode current,
$I_{S}$ = reverse saturation current (~pA to nA),
$V_{D}$ = applied voltage,
$n$ = emission coefficient (~1–2 for Si),
$V_{T}$ = thermal voltage (~26 mV at 300 K).

Typical IV Curve of a Silicon PN Junction Diode (Logarithmic Scale for Reverse Bias)

Visual: IV Curve

   I (A)
   |
   |       /-------
   |      /
   |     /
   |____/__________ V (V)
      -Vr   0   Vf

Forward Bias (V > 0): Current increases exponentially after the knee voltage (~0.6–0.7V for Si).
Reverse Bias (V < 0): Current is nearly constant ( $- I_{S}$ ) until breakdown (avalanche or Zener).

Worked Example: A silicon diode has $I_{S} = 1 0^{- 12}$ A and $n = 1.5$ . Calculate $I$ at $V_{D} = 0.7$ V. $I = 1 0^{- 12} (e^{\frac{0.7}{1.5 \times 0.026}} - 1) \approx 1.5 mA$

3. Biasing of PN Junction Diode

Biasing determines the operating point of a diode. There are two primary modes:

Bias Type	Voltage Applied	Depletion Region	Current Flow	Applications
Forward Bias	$V > 0$	Narrows	High ( $I \propto e^{V_{D}}$ )	Rectifiers, Logic Gates, LEDs
Reverse Bias	$V < 0$	Widens	Low ( $- I_{S}$ )	Voltage Regulators, Zener Diodes

Real-World Example:

eSewa App (Nepal): Uses diodes in rectifier circuits to convert AC from the grid to DC for charging mobile wallets during power outages.
Khalti Payments: Diodes in voltage regulators ensure stable power supply to servers handling transactions.

4. Equivalent Circuit Models

Diodes are modeled in two ways for circuit analysis:

A. Ideal Diode Model

Assumes:
- Zero forward voltage drop ( $V_{D} = 0$ ).
- Infinite resistance in reverse bias.
Limitations: Ignores real-world behavior (e.g., knee voltage, leakage).

B. Practical Diode Model

Forward Bias: $V_{D} = V_{o n}$ (e.g., 0.7V for Si).
Reverse Bias: $I = - I_{S}$ (leakage current).
Breakdown: $V = V_{Z}$ (Zener voltage).

Visual: Diode Models

Comparison of Ideal vs. Practical Diode Models (Forward/Reverse Bias)

5. Diode Applications

A. Rectifiers (AC to DC Conversion)

Half-Wave Rectifier:
```
   AC Input
     |
   [Diode] -->|Output| [Load]
     |
   Ground
```
- Output: Pulsating DC (only positive half-cycles).
- Efficiency: ~40.6%.
Full-Wave Rectifier (Center-Tapped or Bridge):
- Output: Both half-cycles used.
- Efficiency: ~81.2%.

Worked Example (Nepal Context): NTC (Nepal Electricity Authority) uses bridge rectifiers in substations to convert 230V AC from transmission lines to DC for electrolysis plants (e.g., chlorine production). If the input is 230V RMS, calculate the peak output voltage (ignoring diode drops). $V_{p e ak} = V_{r m s} \times 2 = 230 \times 1.414 \approx 325 V$

B. Clipping and Clamping Circuits

Clipper: Removes a portion of the input signal.
Clamper: Shifts the DC level of the input signal.

Visual: Clipping Circuit

   Vin
    |
   [Diode] -->|Vout| [Load]
    |       |
   ---      ---
   GND     Vref

C. Voltage Regulation (Zener Diode)

Zener Diode: Operates in reverse breakdown to maintain a constant voltage.
Example: In Ncell’s base stations, Zener diodes regulate voltage for sensitive RF amplifiers.

Worked Example: A Zener diode with $V_{Z} = 5.1$ V and $I_{Z} = 20$ mA is used to regulate voltage. If the load current is 10 mA, calculate the required series resistor for an input of 12 V. $R = \frac{V _{in} - V _{Z}}{I _{Z} + I _{l o a d}} = \frac{12 - 5.1}{20 mA + 10 mA} = 247.5Ω$

6. Temperature Effects

Forward Voltage Drop ( $V_{D}$ ): Decreases by ~2 mV/°C.
Reverse Leakage Current ( $I_{S}$ ): Doubles for every 10°C rise.
Impact: Circuits must include temperature compensation (e.g., using thermistors).

In the Real World

Pathao (Ride-Hailing App):
- Uses bridge rectifiers in its GPS modules to power the device from AC adapters during charging.
- Why? Ensures stable DC power for accurate location tracking.
Daraz (E-Commerce):
- Clamping circuits protect order-processing servers from voltage spikes during lightning strikes.
- Example: A TVS (Transient Voltage Suppressor) diode clamps spikes to safe levels (~5.1V).
Nepal Rastra Bank (NRB) ATMs:
- Zener diodes regulate power supplies for secure transaction processing.
- Scenario: If input voltage spikes to 15V, a 5V Zener ensures the ATM’s logic board sees a stable 5V.

Exam Tip

Memorize Key Values:
- Silicon diode $V_{D} \approx 0.7$ V, Germanium $V_{D} \approx 0.3$ V.
- Zener breakdown voltages (e.g., 3.3V, 5.1V, 12V) are common in exams.
Graphical Analysis:
- Always sketch the IV curve and mark:
  - Knee voltage,
  - Reverse saturation current,
  - Breakdown region.
- For rectifiers, plot input/output waveforms (half-wave vs. full-wave).
Circuit Analysis:
- Assume ideal diodes unless specified otherwise.
- For practical diodes, account for $V_{D}$ drops in forward bias.
- In Zener applications, calculate series resistance using $I_{Z} + I_{l o a d}$ .
Common Pitfalls:
- Forgetting to include $I_{S}$ in reverse bias calculations.
- Misidentifying forward vs. reverse bias in circuit diagrams.
- Ignoring temperature effects in real-world scenarios (e.g., "Why does the diode fail in summer?").
Numerical Problems:
- Rectifiers: Calculate $V_{d c}$ , $V_{r m s}$ , and ripple factor.
- Clippers/Clampers: Determine output voltage levels using diode polarity.
- Zener Regulators: Solve for $R$ or $P_{d i ss i p a t e d}$ in the Zener.

Bridge rectifier circuit diagram Four-diode arrangement for full-wave rectification. (Image: MichaelOReilly at English Wikipedia, Public domain, via Wikimedia Commons)

Based on the PU BE Computer (PU) syllabus for Electronic Devices and Circuits (ELX120), unit 2.

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