Metallic Bonds, Semiconductors, and Electronic Components

Posted on Mar 18, 2025 in Technology

Metallic Bond and Band Theory

Metallic bond describes the bond strength between atoms in a metal. Two main theories explain this: electron sea theory and band theory.

Band Theory

When atoms combine, they form molecular orbitals (MOs) that belong to the entire molecule. The linear combination of two atomic orbitals (AOs) results in two MOs, corresponding to two energy levels. One MO has higher energy than the original AOs (antibonding MO), and the other has lower energy (bonding MO). When s orbitals combine, they form sigma (σ) MOs. When px and py orbitals combine, they form pi (π) MOs.

  • Valence Band: This occurs when a metal has two electrons in its s orbitals, and the energy band is full.
  • Conduction Band: This is formed by combining the empty p orbitals.

Semiconductors

Semiconductors are materials that are normally insulators. However, a small increase in temperature provides enough energy for electrons to jump into the conduction band, leaving behind holes.

  • Intrinsic Conductivity: This occurs when electrons and holes can move due to external electric fields.
  • Doping of Semiconductors: This is a method to enhance electrical conductivity. For example, in Germanium (Ge) and Silicon (Si), elements from groups 3 and 5 are used as impurities.
  • Extrinsic Conductivity: This occurs when the fifth electron of a group 5 element is weakly bound to the nucleus and becomes free with an energy of approximately 0.01 eV.

PN Junction: LED

When two semiconductors (N-type and P-type) are joined, a diffusion of electrons and holes occurs between them. Near the junction, a series of electrical charges is created: negative (-) in the P-type region and positive (+) in the N-type region. This forms the basis of a Light Emitting Diode (LED).

Application of External Voltage to a PN Junction

  • Direct Polarization: The negative pole (-) is connected to the N-type semiconductor, and the positive pole (+) is connected to the P-type.
  • Reverse Polarization: The connections are reversed.

Diodes

A diode is an electronic component that allows current to flow primarily in one direction.

Zener Diode

A Zener diode utilizes the reverse-bias characteristics of a PN junction. Under reverse voltage, conduction occurs via an avalanche effect, resulting in a high current that is limited only by the resistance (R) of the circuit.

Transistors

A transistor consists of three extrinsic semiconductor regions, alternating between N-type and P-type (e.g., NPN or PNP).

The transistor operates with the base-emitter junction forward-biased and the collector-base junction reverse-biased. Electrons injected from the emitter into the base (which is very thin) mostly reach the collector. The collector current (IC) has two components:

  1. Carriers originating from the emitter, dependent on the size, shape, and doping of the emitter.
  2. Collector leakage current due to the reverse bias between the collector and the base.

The collector current of a transistor is proportional to the base current.

Transistor Operating Regions

  • Cut-off Region: The transistor behaves like an open switch (IB = 0).
  • Saturation Region: The transistor behaves like a closed switch.
  • Active Region: The middle region, where the I-V relationship can be considered approximately linear.

Transistor Gain

When a transistor is used as an amplifier, it is typically operated in the active region. When used in switching applications, it operates in the cut-off and saturation regions.

Field-Effect Transistors (FETs)

FETs are divided into two main groups: JFETs and MOSFETs.

JFET (Junction Field-Effect Transistor)

JFETs consist of a thin layer of N-type material with two contacts: the source (S) and the drain (D). Two P-type regions are joined to form the third terminal, the gate (G).

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

MOSFETs consist of a P-type substrate into which two N-type regions are diffused, forming the source and drain. The area between them is covered with a layer of SiO2, and a metal layer is deposited on top to form the gate contact. MOSFETs come in two types: enhancement-mode and depletion-mode.

Thyristors

Thyristors are used for control functions such as switching, rectification, and frequency conversion. They can be unidirectional or bidirectional.

SCR (Silicon-Controlled Rectifier)

An SCR is a unidirectional thyristor. It consists of a silicon crystal with four alternating P-N-P-N layers.

  • Negative Blocking Region: When the SCR is reverse-biased, only a small leakage current flows. If the voltage becomes sufficiently high, avalanche conduction occurs.
  • Positive Blocking Region: The SCR transitions from a blocking state to a conducting state when the breakdown voltage is reached.