“Types of Semiconductors and P-N Junction Explained with Energy Band Diagrams | Intrinsic, Extrinsic, n-Type & p-Type”

Author:

Prof. Kali Chandrakant

M.Sc., M.Ed., D.C.S.

50+ Years of experience in Physics teaching

 1.Introduction:

  Have you ever wondered what makes your smart-phone, laptop, or even your car’s smart features possible? The answer lies in semiconductors. These materials are the backbone of modern electronics, sitting perfectly between conductors (like copper) and insulators (like glass).

     In this guide, we’ll break down the different types of semiconductors, how “doping” changes their behavior, and the role of the Fermi level in making technology work.

   Also, this document provides a comprehensive overview of the P-N junction, a fundamental building block of modern semiconductor devices. It covers the formation of the junction, its working principle under different biasing conditions, the resulting voltage-current (V-I) characteristics, and a range of important applications. Understanding the P-N junction is crucial for anyone studying or working with electronics and semiconductor technology.

 2.What Are Semiconductors?

      A semiconductor is a material whose electrical conductivity lies between that of a conductor (like copper) and an insulator (like glass).

The most common semiconductor materials are:

• Silicon (Si) – Energy gap: 1.1 eV
• Germanium (Ge) – Energy gap: 0.72 eV

Semiconductors are classified into two main types:

1. Intrinsic Semiconductors
2. Extrinsic Semiconductors

 3. Intrinsic Semiconductors: The Pure Form:

    An intrinsic semiconductor is a substance in its chemically pure state, without any intentional impurities.

• Structure: They are tetravalent, meaning they have four valence electrons that form covalent bonds with neighboring atoms. (Fig. A)

Fig.A

• At Absolute Zero (0 K): These materials act as perfect insulators because all electrons are locked in covalent bonds.
• At Room Temperature: Thermal energy causes a few electrons to break free, leaving behind a “hole” (a positive charge carrier). In intrinsic semiconductors, the number of free electrons (n) is always equal to the number of holes (p).(Fig. A)
• Energy Gaps:
• Germanium: 0.72 eV 
• Silicon: 1.1 eV

 4. Extrinsic Semiconductors: Power Through Impurity:

    Pure semiconductors aren’t very efficient at conducting electricity. To fix this, scientists use a process called doping—the intentional addition of specific impurities to increase the number of charge carriers.

Extrinsic Semiconductor = Intrinsic Semiconductor + Impurities

There are two primary types of extrinsic semiconductors: n-type and p-type.

 4.1. n-type (Negative Type Carriers):

        An n-type semiconductor is created by adding penta-valent impurities (atoms with 5 valence electrons) like Phosphorus (P), Arsenic (As), or Antimony (Sb) to a pure crystal.

     How it works: A penta-valent atom forms four covalent bonds with surrounding silicon atoms. (Fig. B)

• The fifth electron is loosely bound.
• Even a small amount of thermal energy frees this electron.

Fig.B

• Majority Carrier: Electrons (negative charge).
• Minority Carrier:
• Charge: Despite having “extra” electrons, the material remains electrically neutral because the total number of protons in the nuclei equals the total number of electrons.

 4.2. p-type (Positive Type Carriers):

        A p-type semiconductor is created by adding trivalent impurities (atoms with 3 valence electrons) like Boron (B), Aluminum (Al), or Gallium (Ga).

How it works:

• A trivalent atom forms three covalent bonds.
• One bond remains incomplete. (Fig. C)

• This creates a hole.

The impurity atom accepts an electron from a neighboring atom, creating hole conduction.

Fig.C

• Majority Carrier: Holes (positive charge).
• Minority Carrier:
• Charge: Like n-type, p-type materials are electrically neutral.

 5. Understanding the Fermi Level (E_F):

    “The Fermi Level is a theoretical energy level that represents the highest energy state an electron can occupy at absolute zero.”

Think of it as the “water level” of electrons in a material.

 5.1. Where does the Fermi Level sit?

Fig.D

 5.1.1. Fermi Level in Intrinsic Semiconductor:

• Number of electrons = Number of holes
• Equal probability of occupation in both bands
• Fermi level lies exactly in the middle of the band gap. (Fig. D)

 5.1.2. Fermi Level in n-Type Semiconductor:

• More electrons in conduction band
• Higher probability of electron occupation
• Fermi level shifts closer to the conduction band. (Fig. D)
• Lies above the donor level

 5.1.3. Fermi Level in p-Type Semiconductor:

• More holes in valence band
• Higher probability of hole occupation
• Fermi level shifts closer to the valence band. (Fig. D)

 

Semiconductor Type	Fermi Level Position	Reason
Intrinsic	Exactly in the middle of the band gap	Equal number of electrons and holes.
n-type	Close to the Conduction Band	High concentration of electrons (donors).
p-type	Close to the Valence Band	High concentration of holes (acceptors).

 6. Formation of a P-N Junction:

    “A P-N junction is formed when a p-type semiconductor material is joined with an n-type semiconductor material, creating an interface between them.”

 Fig.E

      When a p-type and an n-type semiconductor are brought together, the concentration gradient of charge carriers causes diffusion. Electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side. (Fig. E)

 6.1. Working Principle:

   The diffusion of charge carriers across the junction leads to the following key phenomena:

 6.1.1. Depletion Region:

      As electrons diffuse from the n-side to the p-side, they recombine with holes near the junction. Similarly, holes diffusing from the p-side to the n-side recombine with electrons. This recombination depletes the region near the junction of free charge carriers (electrons and holes), creating a region called the depletion region or space charge region. The depletion region is essentially devoid of mobile charge carriers and acts as an insulator.

 6.1.2. Barrier Potential (Built-in Potential):

       The diffusion of charge carriers and the formation of the depletion region create an electric field across the junction. This electric field opposes further diffusion of charge carriers. The potential difference associated with this electric field is called the barrier potential or built-in potential (V_bi)The barrier potential depends on the doping concentrations of the p-type and n-type materials and the temperature.

  6.2. Biasing the P-N Junction:

    The behavior of the P-N junction is significantly affected by the external voltage applied across it. There are two primary biasing conditions:

 6.2.1. Forward Bias:

Fig.F

In forward bias, the positive terminal of the voltage source is connected to the p-side, and the negative terminal is connected to the n-side. This applied voltage opposes the barrier potential, effectively reducing the width of the depletion region. As the forward voltage increases, the barrier potential decreases, allowing more majority carriers to cross the junction. When the forward voltage exceeds the barrier potential, a large current flows through the junction.(Fig. F)

 6.2.2. Reverse Bias:

Fig.G

In reverse bias, the positive terminal of the voltage source is connected to the n-side, and the negative terminal is connected to the p-side. This applied voltage reinforces the barrier potential, widening the depletion region. The increased depletion region further reduces the number of majority carriers that can cross the junction. Consequently, only a small reverse saturation current (I_s) flows due to the minority carriers. (Fig. G)

 6.3. V-I Characteristics:

   The voltage-current (V-I) characteristics of a P-N junction diode describe the relationship between the voltage applied across the diode and the resulting current flowing through it. (Fig. H)

Fig.H

6.3.1. Forward Bias Region:

    In the forward bias region, the current increases exponentially with the applied voltage. The current is described by the diode equation:

 6.3.2. Reverse Bias Region:

    In the reverse bias region, small reverse saturation current (Is) flows, which is relatively independent of the applied voltage. However, if the reverse voltage exceeds a certain breakdown voltage (V_br), a large reverse current flows, potentially damaging the diode.

6.3.3. Breakdown Region:

    When the reverse voltage applied to the diode exceeds the breakdown voltage, a large current flows in the reverse direction. This can occur due to two mechanisms:

 6.3.4. Avalanche Breakdown:

 High electric field accelerates minority carriers, which collide with other atoms, generating more electron-hole pairs. This process repeats, leading to a large current.

 6.3.5. Zener Breakdown:

     Occurs in heavily doped diodes. The high electric field directly breaks covalent bonds, generating a large number of carriers.

 6.4. Applications of P-N Junctions:

   P-N junctions are the fundamental building blocks of many semiconductor devices, including:

• Diodes: Diodes are used for rectification (converting AC to DC), signal detection, switching, and voltage regulation.

• Transistors: Bipolar junction transistors (BJTs) and field-effect transistors (FETs) utilize P-N junctions to amplify or switch electronic signals.

• Solar Cells: Solar cells convert light energy into electrical energy using the photovoltaic effect, which relies on the properties of P-N junctions.

• Light-Emitting Diodes (LEDs): LEDs emit light when electrons and holes recombine in the P-N junction under forward bias.

• Photodiodes: Photodiodes detect light by generating a current when photons strike the P-N junction.

• Varactor Diodes: Varactor diodes (also known as varicaps) are used as voltage-controlled capacitors, where the capacitance of the P-N junction varies with the applied reverse voltage.

• Integrated Circuits (ICs): P-N junctions are essential components in the fabrication of integrated circuits, enabling the creation of complex electronic systems on a single chip.

 7. Conclusion:

Quick Comparison Table

Feature	Intrinsic	n-Type	p-Type
Purity	Pure	Doped	Doped
Majority Carrier	Equal electrons & holes	Electrons	Holes
Minority Carrier	Equal	Holes	Electrons
Conductivity	Low	High	High
Fermi Level	Middle of band gap	Near conduction band	Near valence band

• Doping is the secret sauce that makes semiconductors useful.
• n-type uses donor impurities to provide extra electrons.
• p-type uses acceptor impurities to create holes.
• The Fermi Level shifts depending on the type of doping, dictating how the material will behave in a circuit.

   The P-N junction is a crucial component in modern electronics. Its unique properties, stemming from the formation of the depletion region and the behavior under different biasing conditions, make it indispensable for a wide range of applications. Understanding the principles of P-N junction operation is essential for anyone working in the field of electronics and semiconductor technology.

    Understanding these fundamentals is the first step toward mastering electronics and solid-state physics.