Semiconductor Devices and p-n Junctions

p-n Junction Formation

• A depletion region forms at the boundary between p-type and n-type materials as free electrons fill available holes, permitting current flow in one direction.
• p-n junctions are the foundation for devices such as solar cells and LEDs.
• p-n junctions can be created via three methods:
  • Grown junction method
  • Alloying method
  • Diffusion method
• In a p-n junction diode, the n region contains majority carriers (electrons) and the p region contains majority carriers (holes).

Depletion Layer Creation

• Depletion layer is emptied of mobile charge carriers due to electron-hole recombination at the junction.
• The n region becomes positively charged as electrons leave, while the p region becomes negatively charged as it accepts electrons.
• The width of the depletion region is dependent on doping levels:
  • Heavily doped materials lead to a thin depletion layer.
  • Lightly doped materials lead to a thicker depletion layer.

Biasing of Diodes

• Biasing refers to applying external voltage to a p-n junction diode.
• Forward biasing allows current to flow; the p-type connects to the positive terminal and n-type to the negative terminal of a battery.
• Reverse biasing blocks current flow; the p-type connects to the negative terminal and n-type to the positive terminal.

VI Characteristics of PN Junction

• The VI characteristics curve illustrates the relationship between voltage and current through the diode.
• In forward bias, current begins to flow once the barrier voltage (0.3 V for Ge) is overcome, increasing with higher voltage.
• In reverse bias, the potential barrier increases, leading to negligible current until a breakdown voltage is reached.
• Exceeding 25 V in reverse bias can cause permanent damage to the junction.

Applications of Junction Diodes

• Used as rectifiers to convert AC to DC.
• Function as switches in computer circuits.
• Act as detectors for audio signals in radios.
• Emitted light in LEDs of various colors.

Zener Diodes Overview

• Zener diodes allow current to flow in both directions but primarily operate in reverse bias.
• In forward bias, Zener diodes work like normal diodes; in reverse bias, they exhibit a small leakage current until breakdown voltage (Vz) is reached.
• After reaching Vz, current increases sharply until it stabilizes.

V-I Characteristics of Zener Diodes

• Forward characteristics mirror normal diode behavior.
• In reverse conditions, a small current occurs until breakdown, marked as Zener Voltage (Vz).

Breakdown Mechanisms in Diodes

• Two types of breakdown:
  • Avalanche Breakdown: Occurs at high reverse voltages, leading to an uncontrolled increase in current and potential damage.
  • Zener Breakdown: Occurs in heavily doped diodes at lower reverse voltages (2-8 V), characterized by a rapid current increase at breakdown voltage.

Rectification Methods

• Half Wave Rectifier: Removes one-half of the AC input signal to produce a DC level, using a single diode.
  • Conducts during the positive half-cycle and blocks during the negative half-cycle.
• Full Wave Rectifier: Allows current flow during both half cycles using a bridge circuit, resulting in a pulsating DC output.
  • The output frequency is double the input frequency.

Transistors Introduction

• Transistors are three-layer semiconductor devices capable of functioning as insulators or conductors.
• Transistors, especially bipolar junction transistors (BJTs), serve as switches and amplifiers in electronic circuits.
• Components of a transistor:
  • Base: Activates the transistor.
  • Collector: Positive lead.
  • Emitter: Negative lead.

This quiz focuses on the fundamental concepts of p-n junctions, including their creation and characteristics in semiconductor devices like solar cells and LEDs. Explore different methods used to form p-n junctions and understand how these structures enable electric current to flow in one direction.