1. Introduction
Devices that control the flow of electrons are fundamental Basic Building Blocks to electronic circuits.
Historical Context
Before the transistor's invention in 1948, vacuum tubes (or valves) were the primary devices used, including:
- Vacuum diode (2 electrodes: anode and cathode)
- Triode (3 electrodes: cathode, plate, and grid)
- Tetrode (4 electrodes)
- Pentode (5 electrodes)
- Electrons are emitted by a heated cathode.
- The flow of electrons is controlled by varying the voltage between electrodes.
- Vacuum is necessary to prevent energy loss from collisions with air molecules.
- Electrons flow only from cathode to anode, leading to the term "valves" for these devices.
- Bulky and high power consumption.
- Operate at high voltages (~100 V).
- Limited lifespan and low reliability.
Emergence of Semiconductor Electronics:
- Development began in the 1930s with the realization that solid-state semiconductors could control charge carrier flow.
- Simple excitations (light, heat, voltage) can alter the number of mobile charges in semiconductors.
- A naturally occurring crystal of galena (Lead sulfide, PbS) was used as a radio wave detector before the full understanding of semiconductor devices.
Comparison with Vacuum Tubes:
- Semiconductor devices do not require external heating or a vacuum.
- They are smaller, consume less power, operate at lower voltages, and have longer lifespans and higher reliability.
Replacement of CRTs: Cathode Ray Tubes (CRTs) in TVs and monitors are being replaced by Liquid Crystal Display (LCD) monitors, which utilize solid-state electronics.
The Chapter will cover basic concepts of semiconductor physics and discuss specific devices like junction diodes and bipolar junction transistors, along with their applications in circuits.
2. Classification of Metals, Conductors, and Semiconductors
On the basis of conductivity
On the basis of energy bands
Classification of Solids Based on Conductivity
Solids can be classified based on their electrical conductivity (σ) or resistivity (ρ = 1/σ). The classification is broadly divided into three categories: metals, semiconductors, and insulators.
1. Metals
Conductivity: Very high
- Resistivity (ρ): 10⁻² to 10⁻⁸ Ω·m
- Conductivity (σ): 10² to 10⁸ S/m
Characteristics:
- Excellent conductors of electricity.
- Low resistivity allows for efficient current flow.
2. Semiconductors
Conductivity: Intermediate between metals and insulators
- Resistivity (ρ): 10⁻⁵ to 10⁶ Ω·m
- Conductivity (σ): 10⁵ to 10⁻⁶ S/m
Characteristics:
- Conductivity can be modified by temperature and impurities (doping).
- Used in electronic devices due to their controllable conductivity.
3. Insulators
Conductivity: Very low
- Resistivity (ρ): 10¹¹ to 10¹⁹ Ω·m
- Conductivity (σ): 10⁻¹¹ to 10⁻¹⁹ S/m
Characteristics:
- Poor conductors of electricity.
- High resistivity prevents current flow.
Classification of Solids Based on Energy Bands
Materials are distinguished into three main categories: conductors, semiconductors, and insulators, based on their energy band structure.
1. Conductors
- Definition: Materials that allow the flow of electric current with minimal resistance.
- Energy Band Structure: In conductors, the valence band and conduction band overlap, allowing electrons to move freely.
- Examples: Metals like copper, silver, and gold.
2. Semiconductors
- Definition: Materials that have electrical conductivity between that of conductors and insulators.
- Energy Band Structure: Semiconductors have a small energy gap (band gap between the valence band and conduction band, typically ranging from 0.1 to 3 eV. At room temperature, some electrons can gain enough energy to jump from the valence band to the conduction band.
- Examples: Silicon, germanium, and gallium arsenide.
3. Insulators
- Definition: Materials that do not conduct electricity under normal conditions.
- Energy Band Structure: Insulators have a large band gap (greater than 3 eV) between the valence band and conduction band, preventing electrons from moving freely.
- Examples: Rubber, glass, and most ceramics.
3. Intrinsic Semiconductors
Intrinsic semiconductors are pure semiconductor materials without any significant dopant atoms present. The most common examples are Germanium (Ge) and Silicon (Si), both of which have a diamond-like crystal structure.
Crystal Structure
- Lattice Structure: Each atom in Ge and Si is surrounded by four nearest neighbors, forming a tetrahedral arrangement.
- Valence Electrons: Both Si and Ge have four valence electrons. In their crystalline structure, each atom shares one of its valence electrons with each of its four nearest neighbors, forming covalent bonds.
Covalent Bonding
- Bond Formation: The shared electron pairs create strong covalent bonds, holding the atoms together.
- Ideal Conditions: At low temperatures, all bonds are intact, and no electrons are free.
Thermal Excitation
- Temperature Effects: As temperature increases, thermal energy allows some electrons to break free from their covalent bonds, contributing to electrical conduction.
- Ionization: The breaking of bonds creates free electrons and vacancies (holes) in the lattice:
- Free Electron: Carries a negative charge (–q).
- Hole: The vacancy left behind behaves as a positive charge (+q).
Carrier Concentration
- In intrinsic semiconductors, the number of free electrons (ne) is equal to the number of holes (nh):
- Equation: ne = nh = ni
- Intrinsic Carrier Concentration: ni represents the concentration of charge carriers in the material.
Movement of Charge Carriers
- Electron Movement: Free electrons move independently, contributing to the electron current (Ie) under an electric field.
- Hole Movement: Holes can also be visualized as moving:
- An electron from a neighboring bond can jump into a hole, effectively moving the hole to a new position.
- Total Current: The total current (I) in the semiconductor is the sum of the electron current and the hole current:
- Equation: I = Ie + Ih
Recombination Process
- Generation and Recombination: Electrons can recombine with holes, which occurs when an electron collides with a hole.
- Equilibrium: At equilibrium, the rate of generation of charge carriers equals the rate of recombination.
Temperature Dependence
- Behavior at Absolute Zero (0 K): An intrinsic semiconductor behaves like an insulator, as no electrons can move to the conduction band.
- Behavior at Higher Temperatures (T > 0 K): Thermal energy excites some electrons from the valence band to the conduction band, partially filling the conduction band and creating holes in the valence band.
Energy Band Diagram
- At higher temperatures, the energy band diagram of an intrinsic semiconductor shows:
- Electrons in the conduction band that have been thermally excited from the valence band.
- An equal number of holes remaining in the valence band.
Summary
Intrinsic semiconductors like Si and Ge are characterized by their diamond-like crystal structure and covalent bonding. Their electrical properties are significantly influenced by temperature, which affects the generation of free electrons and holes. Understanding these concepts is essential for applications in electronics and semiconductor technology.