PN Junction Diode Explained: Biasing, V-I Characteristics & Rectification Guide

House of Physics October 03, 2025
PN Junction Diode: Complete Guide to Biasing, Characteristics, and Rectification
Mastering Semiconductor Diodes: From Basic Theory to Practical Applications in Electronics
PN Junction Diode Biasing V-I Characteristics Rectification Semiconductor Physics Reading Time: 20 min

📋 Table of Contents

📜 Historical Background

The development of semiconductor diodes marked a revolution in electronics:

  • 1874: Ferdinand Braun discovers the rectifying properties of metal-semiconductor contacts
  • 1906: Greenleaf Pickard patents the silicon crystal detector, the first semiconductor diode
  • 1940s: Development of point-contact diodes during World War II
  • 1950s: Invention of the modern PN junction diode at Bell Labs

These developments paved the way for modern electronics, from radios to computers.

Introduction to Semiconductors

🔬 What are Semiconductors?

Semiconductors are materials with electrical conductivity between conductors (like metals) and insulators (like glass). The most common semiconductor materials are silicon and germanium, which have four valence electrons.

Semiconductors can be classified into two types:

  • Intrinsic semiconductors: Pure semiconductor materials with equal numbers of electrons and holes
  • Extrinsic semiconductors: Doped semiconductors with added impurities to alter their electrical properties

Majority and Minority Carriers

📝 Charge Carriers in Semiconductors

In semiconductors, current flows through two types of charge carriers:

  • Electrons: Negative charge carriers
  • Holes: Positive charge carriers (absence of electrons in the crystal lattice)
Property N-Type Semiconductor P-Type Semiconductor
Doping Element Pentavalent (As, P, Sb) Trivalent (B, Al, Ga)
Majority Carriers Electrons Holes
Minority Carriers Holes Electrons
Doping Effect Increases free electrons Increases holes

💡 Key Insight

In a p-type semiconductor, most current carriers are holes, which are the majority carriers. Although the majority of current carriers are holes, there are also a few free electrons created when electron-hole pairs are thermally generated. Electrons in p-type material are the minority carriers.

PN Junction Formation

🔬 What is a PN Junction?

A PN junction is formed when a p-type semiconductor is joined with an n-type semiconductor. This creates a boundary between the two regions with unique electrical properties.

The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers). The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).

Formation of the Depletion Region

⚙️ Depletion Region Formation

P-Type
Holes (Majority)
| Junction |
Electrons (Majority)
N-Type
Before Diffusion

Process: When a p-type semiconductor is brought close to an n-type to form a PN-junction:

  1. Free electrons near the junction in the n region begin to diffuse across the junction into the p-type region
  2. These electrons combine with holes near the junction
  3. The n region loses free electrons, creating a layer of positive charges (pentavalent ions) near the junction
  4. The p region loses holes as electrons and holes combine, creating a layer of negative charges (trivalent ions) near the junction

These two layers of positive and negative charges form the depletion region.

💡 Depletion Region Characteristics

The term depletion refers to the fact that the region near the PN-junction is depleted of charge carriers (electrons and holes) due to diffusion across the junction. After the initial surge of free electrons across the PN-junction, the depletion region expands to a point where equilibrium is established and there is no further diffusion of electrons across the junction.

In other words, the depletion region acts as a barrier to the further movement of electrons across the junction.

Barrier Potential

⚡ What is Barrier Potential?

In the depletion region there are many positive charges and many negative charges on opposite sides of the PN-junction. The forces between the opposite charges form a "field of forces" called an electric field.

This electric field is a barrier to the free electrons in the n region, and energy must be expended to move an electron through the electric field. That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region.

0.7 V
Typical barrier potential for silicon diodes at 25°C
0.3 V
Typical barrier potential for germanium diodes at 25°C

💡 Barrier Potential Formula

The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts.

\[ V_b = \frac{kT}{q} \ln\left(\frac{N_a N_d}{n_i^2}\right) \]

Where:

  • \( V_b \) = Barrier potential
  • \( k \) = Boltzmann's constant
  • \( T \) = Temperature in Kelvin
  • \( q \) = Electron charge
  • \( N_a \) = Acceptor concentration in p-type
  • \( N_d \) = Donor concentration in n-type
  • \( n_i \) = Intrinsic carrier concentration

Energy Diagrams of PN Junction

📊 Energy Band Theory

In terms of energy levels, free electrons in the n region have more energy than those in the p region. The electric field between the positive and negative charges in the depletion region is a "hill" over which the n region electrons must climb to get to the p region.

Stated another way, the electric field is a potential energy barrier, and external energy must be applied to overcome this barrier.

📈 Energy Band Diagram of PN Junction

[Energy Band Diagram: Showing conduction band, valence band, and Fermi level across the PN junction]

The energy band diagram shows how the conduction and valence bands bend at the junction, creating the potential barrier that prevents further diffusion of carriers.

Diode Biasing

🔌 What is Diode Biasing?

Biasing means applying an external voltage across the PN-junction. There are two types of biasing:

  1. Forward bias: External voltage is applied with the positive terminal connected to the p region and the negative terminal to the n region
  2. Reverse bias: External voltage is applied with the negative terminal connected to the p region and the positive terminal to the n region

Forward Biasing

⚙️ Forward Bias Operation

Forward Bias
+
P-Type
| Junction |
N-Type
-
Current Flows

Process: When forward bias is applied:

  1. The negative terminal of the battery pushes the free electrons in the n region toward the PN-junction
  2. The positive terminal pushes the holes in the p region also toward the PN-junction
  3. When the external voltage exceeds the barrier potential, electrons have enough energy to penetrate the depletion region and cross the junction
  4. As electrons cross the junction, they recombine with holes in the p region
  5. Current flows through the PN-junction

The depletion region narrows under forward bias conditions.

💡 Forward Bias Characteristics

When a diode is forward-biased:

  • The depletion region becomes narrower
  • The barrier potential is reduced
  • Current flows easily once the external voltage exceeds the barrier potential
  • The diode acts like a closed switch

Reverse Biasing

⚙️ Reverse Bias Operation

Reverse Bias
-
P-Type
| Junction |
N-Type
+
Minimal Current

Process: When reverse bias is applied:

  1. The negative terminal of the battery is connected to the p region
  2. The positive terminal is connected to the n region
  3. Free electrons in the n region are attracted toward the positive terminal
  4. Holes in the p region are attracted toward the negative terminal
  5. The depletion region widens
  6. Current flow is extremely small (only due to minority carriers)

The diode acts like an open switch under reverse bias conditions.

💡 Reverse Bias Characteristics

When a diode is reverse-biased:

  • The depletion region becomes wider
  • The barrier potential increases
  • Only a very small reverse current flows (due to minority carriers)
  • The diode acts like an open switch

V-I Characteristics of PN Junction

📈 Voltage-Current Relationship

The relationship between the voltage applied across a PN-junction and the current flowing through it is called the V-I characteristic. This relationship is typically represented graphically.

📊 V-I Characteristic Curve of PN Junction Diode

[Graph: V-I characteristic curve showing forward bias region, reverse bias region, and breakdown region]

The V-I characteristic curve shows the exponential increase in current under forward bias, the small reverse saturation current under reverse bias, and the breakdown region at high reverse voltages.

Forward Bias Characteristics

🧮 Diode Equation

Step 1: Basic Diode Equation

The current through a diode under forward bias is given by the Shockley diode equation:

\[ I = I_s \left( e^{\frac{V}{\eta V_T}} - 1 \right) \]

Step 2: Parameter Definitions

\[ I = \text{Diode current} \]
\[ I_s = \text{Reverse saturation current} \]
\[ V = \text{Applied voltage across diode} \]
\[ \eta = \text{Ideality factor (1 for ideal diode)} \]
\[ V_T = \frac{kT}{q} = \text{Thermal voltage} \]

Step 3: Thermal Voltage Calculation

\[ V_T = \frac{kT}{q} \]
\[ = \frac{(1.38 \times 10^{-23}) \times 300}{1.6 \times 10^{-19}} \]
\[ = 0.0259 \, \text{V at room temperature (300K)} \]
Sample Problem 1: Diode Current Calculation

A silicon diode has a reverse saturation current of 1 nA at room temperature (300K). Calculate the forward current when a voltage of 0.7 V is applied across it. Assume the ideality factor η = 1.

Given:
\[ I_s = 1 \times 10^{-9} \, A \]
\[ V = 0.7 \, V \]
\[ \eta = 1 \]
\[ T = 300 \, K \]
First, calculate thermal voltage:
\[ V_T = \frac{kT}{q} \]
\[ = \frac{(1.38 \times 10^{-23}) \times 300}{1.6 \times 10^{-19}} \]
\[ = 0.025875 \, V \]
Now apply the diode equation:
\[ I = I_s \left( e^{\frac{V}{\eta V_T}} - 1 \right) \]
\[ = 1 \times 10^{-9} \left( e^{\frac{0.7}{1 \times 0.025875}} - 1 \right) \]
\[ = 1 \times 10^{-9} \left( e^{27.05} - 1 \right) \]
\[ = 1 \times 10^{-9} \left( 5.51 \times 10^{11} - 1 \right) \]
\[ \approx 0.551 \, A \]

Reverse Bias Characteristics

🔍 Reverse Saturation Current

Under reverse bias conditions, the diode equation simplifies to:

\[ I \approx -I_s \]

This means the current is approximately equal to the reverse saturation current and flows in the opposite direction to forward current.

💡 Breakdown Region

When the reverse bias voltage exceeds a certain value called the breakdown voltage, the reverse current increases rapidly. This occurs due to two mechanisms:

  1. Avalanche breakdown: High-energy carriers create additional electron-hole pairs through impact ionization
  2. Zener breakdown: High electric field pulls electrons directly from the valence band to the conduction band

Rectification

🔌 What is Rectification?

Rectification is the process of converting alternating current (AC) to direct current (DC). Diodes are used as rectifiers because they allow current to flow in only one direction.

There are two main types of rectifiers:

  1. Half-wave rectifiers
  2. Full-wave rectifiers

Half-Wave Rectification

⚙️ Half-Wave Rectifier Operation

[Circuit Diagram: Half-wave rectifier with AC source, diode, and load resistor]

Operation: In a half-wave rectifier:

  1. During the positive half-cycle of AC input, the diode is forward-biased and conducts
  2. Current flows through the load resistor, creating a positive output voltage
  3. During the negative half-cycle, the diode is reverse-biased and does not conduct
  4. No current flows through the load during the negative half-cycle

The output is a pulsating DC with the same frequency as the input AC.

📊 Input and Output Waveforms of Half-Wave Rectifier

[Graph: AC input waveform and pulsating DC output waveform of half-wave rectifier]

The output waveform shows that only the positive half-cycles of the input AC are passed through to the output.

🧮 Half-Wave Rectifier Calculations

Step 1: Average DC Output Voltage

\[ V_{dc} = \frac{V_m}{\pi} \]
\[ \text{Where } V_m = \text{Peak voltage of AC input} \]

Step 2: RMS Output Voltage

\[ V_{rms} = \frac{V_m}{2} \]

Step 3: Ripple Factor

\[ \gamma = \sqrt{\left( \frac{V_{rms}}{V_{dc}} \right)^2 - 1} \]
\[ = \sqrt{\left( \frac{V_m/2}{V_m/\pi} \right)^2 - 1} \]
\[ = \sqrt{\left( \frac{\pi}{2} \right)^2 - 1} \]
\[ = \sqrt{2.467 - 1} \]
\[ = \sqrt{1.467} \]
\[ = 1.21 \]

Full-Wave Rectification

⚙️ Full-Wave Rectifier Operation

[Circuit Diagram: Full-wave bridge rectifier with four diodes]

Operation: In a full-wave rectifier (bridge configuration):

  1. During the positive half-cycle, diodes D1 and D2 conduct while D3 and D4 are reverse-biased
  2. Current flows through the load in a specific direction
  3. During the negative half-cycle, diodes D3 and D4 conduct while D1 and D2 are reverse-biased
  4. Current still flows through the load in the same direction

The output is a pulsating DC with twice the frequency of the input AC.

📊 Input and Output Waveforms of Full-Wave Rectifier

[Graph: AC input waveform and pulsating DC output waveform of full-wave rectifier]

The output waveform shows that both positive and negative half-cycles of the input AC are converted to positive pulses at the output.

🧮 Full-Wave Rectifier Calculations

Step 1: Average DC Output Voltage

\[ V_{dc} = \frac{2V_m}{\pi} \]
\[ \text{Where } V_m = \text{Peak voltage of AC input} \]

Step 2: RMS Output Voltage

\[ V_{rms} = \frac{V_m}{\sqrt{2}} \]

Step 3: Ripple Factor

\[ \gamma = \sqrt{\left( \frac{V_{rms}}{V_{dc}} \right)^2 - 1} \]
\[ = \sqrt{\left( \frac{V_m/\sqrt{2}}{2V_m/\pi} \right)^2 - 1} \]
\[ = \sqrt{\left( \frac{\pi}{2\sqrt{2}} \right)^2 - 1} \]
\[ = \sqrt{1.2337 - 1} \]
\[ = \sqrt{0.2337} \]
\[ = 0.483 \]
Parameter Half-Wave Rectifier Full-Wave Rectifier
Number of Diodes 1 4 (bridge) or 2 (center-tapped)
Average DC Output \( V_m/\pi \) \( 2V_m/\pi \)
Ripple Frequency Same as input frequency Twice input frequency
Ripple Factor 1.21 0.483
Efficiency 40.6% 81.2%

Practical Diode Parameters

🔧 Important Diode Specifications

When selecting diodes for practical applications, several parameters must be considered:

  • Maximum Forward Current (I_F): The maximum current the diode can handle in forward bias
  • Peak Inverse Voltage (PIV): The maximum reverse voltage the diode can withstand without breakdown
  • Forward Voltage Drop (V_F): The voltage across the diode when conducting (typically 0.7V for silicon)
  • Reverse Recovery Time (t_rr): The time required for the diode to switch from conducting to non-conducting state

🔌 Power Supplies

Diodes are essential components in power supply circuits for converting AC to DC. Bridge rectifiers are commonly used in power adapters and computer power supplies.

📡 Signal Demodulation

In radio receivers, diodes are used as detectors to demodulate AM signals, extracting the audio information from the carrier wave.

⚡ Voltage Protection

Diodes are used in reverse polarity protection circuits to prevent damage to electronic devices if batteries are inserted incorrectly.

💡 LED Lighting

Light Emitting Diodes (LEDs) are specialized diodes that emit light when forward-biased, used in displays, indicators, and lighting applications.

Frequently Asked Questions

❓ Why does a silicon diode have a forward voltage drop of approximately 0.7V?

The 0.7V forward voltage drop in a silicon diode is primarily due to the barrier potential at the PN junction. This barrier potential must be overcome for significant current to flow. The exact value depends on:

  • The semiconductor material (0.7V for silicon, 0.3V for germanium)
  • Temperature (decreases with increasing temperature)
  • Doping concentrations

This voltage drop represents the energy required for electrons to cross from the n-type to the p-type region and recombine with holes.

❓ What happens if a diode is connected in reverse bias with voltage exceeding the PIV rating?

If the reverse bias voltage exceeds the Peak Inverse Voltage (PIV) rating:

  • The diode enters the breakdown region
  • Current increases rapidly, potentially damaging the diode
  • In Zener diodes, this is a controlled process used for voltage regulation
  • In regular diodes, this usually causes permanent damage due to excessive heat generation

To prevent this, diodes should always be operated within their specified voltage ratings, and protection circuits may be needed for applications with voltage spikes.

❓ Why is the full-wave rectifier more efficient than the half-wave rectifier?

Full-wave rectifiers are more efficient than half-wave rectifiers for several reasons:

  • Higher average output voltage: \( 2V_m/\pi \) vs \( V_m/\pi \)
  • Better ripple factor: 0.483 vs 1.21, meaning smoother DC output
  • Higher ripple frequency: Twice the input frequency, making filtering easier
  • Better transformer utilization: Both halves of the AC cycle are used

The main disadvantage is that full-wave rectifiers require more diodes (four in bridge configuration or two with a center-tapped transformer).

📚 Master Semiconductor Electronics

Understanding PN junction diodes is fundamental to electronics. These components form the basis of more complex semiconductor devices like transistors, integrated circuits, and microprocessors. Continue your journey into the fascinating world of semiconductor physics and applications.

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© Electronics Education Initiative | PN Junction Diodes: Complete Guide

Based on standard electronics curriculum with additional insights from professional engineering resources

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