Semiconductor Electronics Overview

Semiconductor Electronics: Materials, Devices, and Simple Circuits

• Devices control the flow of electrons, forming electronic circuits.
• Prior to transistors, vacuum tubes (valves) were common, but these were bulky, consumed high power, operated at high voltages, and had limited life.
• Modern solid-state semiconductor electronics uses semiconductors and their junctions to control charge flow.
• These semiconductors, like silicon (Si) and germanium (Ge), enable efficient and reliable devices requiring low power and low voltage.

Classification of Metals, Conductors, and Insulators

• Metals: Possess low resistivity and high conductivity.
• Semiconductors: Have intermediate resistivity and conductivity values.
• Insulators: Possess high resistivity and low conductivity.
• Resistivity values are indicative, not the sole criteria.

Elemental and Compound Semiconductors

• Elemental: Silicon (Si) and Germanium (Ge) are common examples.
• Compound: Inorganic (CdS, GaAs, CdSe, InP) and organic forms (anthracene, doped pthalocyanines, polypyrrole, polyaniline, polythiophene) are other possibilities.
• Current semiconductor devices primarily utilize elemental Si and Ge and inorganic compound semiconductors.

Band Theory of Solids

• An isolated atom's electron energy depends on the orbit.
• When atoms form a solid, their outer electron orbits overlap or interact, creating energy bands.
• The valence band contains valence electrons, while the conduction band lies above the valence band.
• In a conductor, the valence band overlaps the conduction band.
• In insulators, there's a significant gap between the valence and conduction bands.
• Some electrons from the valence band can gain energy to cross the gap, making the material a semiconductor.

Intrinsic Semiconductors

• In intrinsic semiconductors (Si or Ge), both electrons and holes contribute to conductivity.
• At absolute zero(0K), the valence band is completely filled, and the conduction band is empty.
• As temperature increases (T > 0K), some valence electrons gain enough energy to jump to the conduction band.
• These electron transitions create holes in the valence band. The electron-hole pairs contribute to the current.
• The number of electrons equals the number of holes in equilibrium.

Extrinsic Semiconductors

• Intrinsic semiconductors have low conductivity at room temperature.
• Impurity atoms (dopants) can be added to increase their conductivity and are called extrinsic semiconductors.
• Pentavalent dopants: Increase the number of free electrons (n-type).
• Trivalent dopants: Increase the number of holes (p-type).
• Doping level and temperature affect free charge carrier numbers.

P-N Junction

• A p-n junction is formed by joining p-type and n-type semiconductors.
• The p-side and n-side acquire positive and negative space charges, respectively.
• This space-charge region between p and n is called the depletion region.
• A barrier potential develops across the junction, opposing current flow.
• With forward bias, the barrier reduces, leading to current flow.
• In reverse bias, the barrier increases, reducing current to a small reverse saturation value.

Semiconductor Diode

• A p-n junction diode allows current flow in one direction (forward bias) and restricts current flow in the opposite direction (reverse bias).
• The diode's V-I characteristics show non-linear behavior in the forward and reverse bias regions.

Diode as a Rectifier

• A diode allows current to flow in one direction, converting alternating current (AC) to pulsating direct current (dc).
• Half-wave rectifiers use one diode to rectify only half the input cycle.
• Full-wave rectifiers use two diodes to rectify both halves of the input cycle, producing a smoother dc output signal.
• A capacitor can smooth the rectified voltage, closer to a true dc output.

Zener Diode

• A Zener diode is a heavily doped p-n junction diode that operates in reverse breakdown.
• It exhibits a nearly constant voltage across it when the reverse current increases exceeding its critical threshold voltage - called the Zener voltage.
• A series resistor in the circuit controls current, allowing voltage regulation.

Optoelectronic Devices

• Photodiodes: Detect light by generating electron-hole pairs. They operate under reverse bias for enhanced sensitivity.
• Light Emitting Diodes (LEDs): Generate light when a forward current is applied. LEDs use semiconductors with controlled band gaps for specific colors.
• Photovoltaic devices (solar cells): They produce electricity when exposed to light by generating electron-hole pairs. This is due to the photovoltaic effect.

Transistor

• Transistors are three-terminal semiconductor devices (p-n-p or n-p-n junctions)
• In their active region, they amplify a signal by controlling the current flow between the collector and emitter.
• Basic biasing configurations like common emitter, common collector, and common base have different roles.
• Transistors can act as switches (cutoff or saturation) or in amplification modes.
• Input and output characteristics are shown graphically and relate changes/currents to voltage levels.

Transistor as Amplifier

• Transistors, biased in a particular range or region, amplify input signals, often shown as voltage or current changes, resulting in amplified output signals.
• The input and output characteristics can be determined from a test circuit for identifying critical parameters.

Digital Electronics and Logic Gates

• Digital signals use discrete voltage levels (0 or 1) to represent information.
• Logic gates (NOT, AND, OR, NAND, NOR) are basic elements in digital circuits, performing logical operations with inputs.
• These gates, represented symbolically and by truth tables, show the relationship between inputs and outputs.

Explore the fundamentals of semiconductor electronics including materials, devices, and circuits. This quiz covers the properties of metals, semiconductors, and insulators, emphasizing their roles in electronic devices. Understand the distinction between elemental and compound semiconductors.