Semiconductors 03 : Intrinsic Semiconductors - Concept Of electron + Hole Conduction

Name: Semiconductors 03 : Intrinsic Semiconductors - Concept Of electron + Hole Conduction
Uploaded: 2020-04-07T00:30:59.000Z
Duration: 2 h 18 s

Introduction to Intrinsic Semiconductors

Overview of Lecture

The lecture introduces intrinsic semiconductors, focusing on electron and hole conduction.

It distinguishes between intrinsic and extrinsic semiconductors, with extrinsic types (n-type and p-type) to be covered in the next lecture.

Emphasizes that current can flow due to both electrons and holes, a new concept for students.

Key Concepts of Intrinsic Semiconductors

Defines intrinsic semiconductors as pure materials without external impurities; examples include pure silicon and germanium.

Explains that "intrinsic" means natural or fundamental, highlighting the absence of impurities in these semiconductors.

Properties of Silicon and Germanium

Atomic Structure

Identifies pure silicon (atomic number 14) and germanium (atomic number 32) as key examples of intrinsic semiconductors.

Discusses electronic configuration: Silicon has 4 valence electrons from its outer shell configuration.

Valence Electrons

Both silicon and germanium have 4 valence electrons, which are crucial for their bonding properties.

Highlights that these valence electrons form covalent bonds with neighboring atoms, resulting in no free electrons available for conduction.

Covalent Bonding in Crystals

Formation of Covalent Bonds

Describes how all four valence electrons participate in forming covalent bonds within silicon or germanium crystals.

Illustrates the structure where each atom forms four covalent bonds with adjacent atoms, creating a stable crystal lattice.

Conductivity at Low Temperatures

At absolute zero temperature, there are no free electrons since all valence electrons are involved in covalent bonding.

Concludes that at absolute zero Kelvin, intrinsic semiconductors behave like insulators due to the lack of free charge carriers.

Understanding Semiconductor Behavior at Zero Kelvin

Semiconductor as Insulator

At zero Kelvin, semiconductors behave like insulators due to the absence of free electrons. All electrons are confined within the valence band, preventing electrical conductivity.

Energy Bands in Semiconductors

The structure of semiconductors includes two primary bands: the valence band (VB) and conduction band (CB), separated by a forbidden energy gap where no electron states exist. Electrons cannot transition between these bands without sufficient energy.

Characteristics of Valence and Conduction Bands

At absolute zero, all electrons occupy the valence band; thus, there are no free electrons available for conduction in the conduction band. This lack of free electrons results in zero current flow under these conditions.

Energy Gap Values

The forbidden energy gap is approximately 1.1 eV for silicon and 0.72 eV for germanium, with germanium having a smaller gap due to its position lower on the periodic table, indicating weaker bonding of valence electrons to the nucleus.

Role of Electric Field

Applying an electric field can influence electron movement; however, if insufficient energy is provided, it won't enable electron transitions from VB to CB or lead to breakdown unless a strong field is applied. Thus, at zero Kelvin, semiconductors act as insulators with no conductivity.

Increasing Temperature Effects on Conductivity

Thermal Energy Contribution

Raising temperature increases thermal energy available to electrons, allowing some covalent bonds to break and generating free electrons that contribute to electrical conductivity in semiconductors.

Temperature Threshold for Conductivity

As temperature rises from absolute zero towards room temperature (300 K), conductivity gradually improves; however, significant conductivity only emerges after reaching certain thresholds (e.g., around 300 K). A minimal number of bonds break even at elevated temperatures like 800 K (27°C).

Bond Breaking Dynamics

At room temperature, one covalent bond breaks per approximately 10^29 atoms in a semiconductor crystal structure—indicating that while some conductivity occurs at this temperature, it remains relatively low compared to metals or other conductors due to limited free electron generation.

Implications for Extrinsic Semiconductors

Given that intrinsic semiconductor conductivity is not very high even at room temperature due to limited free electron availability from bond breaking, this leads into discussions about extrinsic semiconductors which enhance conductivity through doping processes or other methods aimed at increasing charge carrier density.

Understanding Semiconductor Conductivity and Electron-Hole Concept

Introduction to Semiconductor Behavior

The speaker initiates a discussion on the behavior of semiconductors at room temperature, emphasizing the need for silicon to enhance conductivity.

A foundational concept is introduced: the electron-hole pair, which is crucial for understanding semiconductor functionality.

Covalent Bonds and Thermal Effects

The formation of covalent bonds among four electrons is explained, highlighting how increasing temperature can break these bonds.

When covalent bonds break due to heat, vacancies (holes) are created where electrons were previously bound. This process generates thermally induced free electrons.

Energy Bands in Semiconductors

The distinction between valence and conduction bands is made clear; electrons transition from the valence band to the conduction band when energy levels increase.

As electrons gain energy from heat, they jump into the conduction band, creating vacancies in the valence band that are referred to as holes.

Current Flow and Electric Fields

The movement of free electrons under an external electric field leads to current flow; this motion is described as opposite to that of conventional current direction.

An explanation follows about how many electrons move downwards in response to an applied electric field, contributing to current generation.

Understanding Holes in Semiconductors

A detailed exploration of what constitutes a hole: it represents a vacancy left by a thermally generated electron.

Holes are characterized as positively charged entities resulting from missing negatively charged electrons within the crystal structure.

Charge Neutrality and Conservation Principles

The speaker discusses charge neutrality within crystals before and after electron movement; while individual charges may separate, overall neutrality remains intact.

It’s emphasized that for every negative charge (electron), there exists a corresponding positive charge (hole), maintaining balance within the crystal structure.

This structured overview captures key concepts related to semiconductor physics discussed in the transcript while providing timestamps for easy reference.

Understanding Electron-Hole Recombination in Semiconductors

Mechanism of Electron and Hole Generation

The generation of electrons and holes occurs similarly to their recombination, influenced by external electric fields. Electrons move opposite to the field direction, potentially leading to recombination as they return.

Concept of Recombination

Recombination can be illustrated with analogies like a ball falling into a pit, representing how electrons can fall back into holes.

Temperature Effects on Recombination

At equilibrium, temperature affects bond stability; higher temperatures reduce recombination rates due to increased energy breaking bonds. However, some electrons will still recombine despite high temperatures.

Recombination occurs at all temperatures but is less likely at higher temperatures where bond energies are diminished. Both generation and recombination processes happen simultaneously.

Relationship Between Electrons and Holes

The number of free electrons (N_e) equals the number of holes (N_h), indicating a balance in semiconductor materials.

This relationship holds true for intrinsic semiconductors where thermally generated free electron numbers match hole numbers per unit volume.

Intrinsic vs Extrinsic Semiconductors

In extrinsic semiconductors, N-type has more electrons while P-type has more holes. Understanding this helps clarify charge carrier dynamics within these materials.

Carrier Density and Temperature Dependence

Charge carriers (electrons and holes) have densities that depend on temperature; as temperature increases, more bonds break leading to higher concentrations of both carriers.

At room temperature (~300K), intrinsic carrier concentration values are approximately 2.4 x 10^19 m³ for germanium and 1.5 x 10^16 m³ for silicon, illustrating significant differences in electron availability based on material properties.

Current Generation in Semiconductors

The discussion transitions towards understanding how current is generated in semiconductors through the movement of both electrons and holes, emphasizing their roles as charge carriers.

Understanding Charge Carriers in Semiconductors

Energy and Tension in Crystals

The concept of energy and tension is introduced, emphasizing the clarity of holes within a crystal structure. Heating can break bonds, leading to discussions on bond-breaking mechanisms.

When an electron leaves a "hole," it creates a positive charge where the electron was previously located, maintaining overall neutrality in the crystal.

Intrinsic vs. Extrinsic Charge Carrier Density

The discussion shifts to intrinsic charge carrier density, highlighting that both free electrons and holes are essential for current flow in semiconductors.

Focus is placed on thermally generated free electrons as a fundamental aspect of semiconductor behavior.

Current Flow Mechanisms

An electric field's role is explained; without it, there would be no current. The relationship between electric fields and electron movement is clarified.

The direction of current (IE) is defined as opposite to the motion of negative charges (electrons), establishing foundational concepts for understanding current flow.

Understanding Holes and Their Movement

A transition to discussing holes includes creating diagrams to visualize their behavior alongside electrons under an applied electric field.

It’s noted that while holes appear to move in the direction of the electric field, this movement does not imply actual physical movement but rather represents a conceptual model.

Current Generation from Charge Carriers

The creation of current due to moving charge carriers—both positive (holes) and negative (electrons)—is emphasized. This principle applies universally regardless of charge type.

Diagrams are used again to illustrate how crystals behave when subjected to external forces or conditions affecting their internal structure.

Bond Breaking and Electron Movement

A narrative about heating crystals explains how bond breaking leads to hole formation alongside thermally generated free electrons.

The impact of applying an external electric field on bonded electrons within valence bands is discussed, likening them to children wanting freedom from constraints.

This structured overview captures key insights into semiconductor physics as presented in the transcript while providing timestamps for easy reference.

Understanding Electron Behavior in Electric Fields

The Concept of Recombination

The speaker emphasizes that the behavior of electrons in an electric field is not a recombination process; rather, it involves new interactions.

A distinction is made between falling electrons and recombination, highlighting that recombination occurs when thermally generated electrons fall into specific energy states.

Movement of Electrons and Holes

The discussion explains how electrons lack sufficient energy to reach the conduction band under certain conditions, remaining in the valence band instead.

The interaction between holes (positive charge carriers) and electrons is described, illustrating how an electron can break its bond and create a hole.

Misconceptions About Charge Movement

It is clarified that while holes appear to move with the electric field, it is actually the movement of bonded electrons that creates this effect.

The apparent movement of holes in response to an electric field is explained as a result of bonded electron movements rather than actual hole displacement.

Current Generation from Electron Movement

External forces influence electron behavior; thus, while it seems like holes are moving, it's primarily the bonded electrons creating what’s termed as hole current (IH).

An explanation follows on how thermal generation leads to electron movement within energy bands when an external electric field is applied.

Energy Band Dynamics

As temperature increases, some electrons transition from the valence band to the conduction band due to thermal excitation.

A visual representation of energy bands illustrates how at low temperatures all electrons are confined within their respective bands until heat causes some to rise into the conduction band.

This structured summary captures key concepts regarding electron dynamics in electric fields as discussed in the transcript. Each point links back to specific timestamps for easy reference.

Understanding Electron Movement in Electric Fields

The Role of Electric Fields and Electrons

Discussion begins on the behavior of thermally generated electrons in an electric field, questioning if positive charge is necessary for functionality.

When an electric field is applied, electrons move opposite to the direction of the field, indicating that current flows in the direction opposite to electron motion.

Clarification on how bonded electrons cannot be moved within the valence band due to insufficient energy; they remain stationary despite the electric field.

Understanding Holes and Current Flow

The concept of holes is introduced; as electrons from the valence band are displaced, it creates a perception that holes are moving downwards.

It’s explained that while holes appear to move downwards, it's actually the movement of valence band electrons upwards due to external electric fields that creates this effect.

Current Contributions in Semiconductors

The net current in a semiconductor is described as being composed of contributions from both free electrons (IE) and holes (IH), which can be added like scalar quantities.

A distinction is made between actual electron movement and apparent hole movement; while both contribute to current, their behaviors differ under an electric field.

Intrinsic Semiconductors Dynamics

In intrinsic semiconductors, it’s noted that free electron density (nE), hole density (nH), and intrinsic carrier concentration (ni) are equal. However, currents may not be equal due to differences in drift velocities.

The discussion emphasizes that even with equal numbers of charge carriers, variations in mobility lead to different current values; thus understanding these dynamics is crucial for semiconductor behavior.

Understanding Mobility in Intrinsic Semiconductors

The Concept of Mobility

Reduced mobility in holes leads to a lower hole current in intrinsic semiconductors. Mobility refers to the ability of charge carriers (holes and electrons) to move freely.

The current due to electrons in the conduction band (IE) is greater than that due to holes in the valence band (IH), indicating that electron mobility is higher than hole mobility.

Electron and Hole Conduction

It is emphasized that while holes are less mobile compared to free electrons, they still contribute to conduction. Understanding this distinction is crucial for grasping semiconductor behavior.

Increasing temperature enhances both free electrons and holes, leading to an overall increase in current within intrinsic semiconductors. This highlights the relationship between temperature and conductivity.

Temperature Effects on Conductivity

The equation NI ∝ E^(-EG/2kT) illustrates how increasing charge carrier numbers boosts conductivity. A graph depicting conductivity versus temperature would show an upward trend for semiconductors, contrasting with conductors where increased temperature typically reduces conductivity.

The discussion transitions towards extrinsic semiconductors, suggesting further exploration of their properties will follow.