Basic Electronics - PN Junction (Lecture Notes)
Topics Covered
• Introduction
• Diode Representations
• The P-N Junction Under No Bias, Reverse Bias, and Forward Bias
• V-I Characteristics of Ideal & Practical Diode
• Si Diode vs Ge Diode
• Effect of Temperature
• Examples
• Junction Capacitances (Transition & Diffusion)
• References
Introduction
• A P-N junction diode is a two-terminal semiconductor device.
• It allows current in forward bias and blocks it in reverse bias.
• Formed by doping a single crystal with p-type and n-type impurities.
• Manufacturing methods: Ion implantation, diffusion, epitaxy.
PN Junction Behavior
• No Bias: Built-in potential (barrier potential) forms, diffusion current = drift current.
• Reverse Bias: Depletion region widens, reverse saturation current flows (minority carriers).
• Forward Bias: Depletion region narrows, diffusion current increases, conduction occurs.
V-I Characteristics
• Ideal diode: Piece-wise linear, conducts perfectly in forward bias, zero in reverse.
• Practical diode: Knee voltage ≈ 0.7V (Si), ≈ 0.3V (Ge). Breakdown occurs in reverse bias.
• Diode Equation: ID = IS (e^(VD/ηVT) - 1), where VT ≈ 26mV at room temp.
Silicon vs Germanium Diodes
• Si knee voltage ≈ 0.7V, Ge ≈ 0.3V.
• Si operates at higher temperature, Ge is more sensitive.
• Si is cheaper and more widely used than Ge.
Effect of Temperature
• Reverse saturation current doubles for every 10°C rise.
• Barrier potential decreases by ~2mV/°C.
• At higher temperature, conduction increases due to increased carrier generation.
Examples
1. Find forward voltage if ID = 1mA, IS = 10^-10 A, T = 25°C, η = 1.5. 2. Threshold voltage of Si diode at 100°C =
0.55V (from 0.7V at 25°C). 3. Reverse saturation current increases with temperature using IS = IS0 * 2^(∆T/10).
Junction Capacitance
• Transition Capacitance (CT): Dominant in reverse bias, decreases with voltage. Used in varactor diodes.
• Diffusion Capacitance (CD): Dominant in forward bias, proportional to current. Larger at high current.
References
1. Boylestad & Nashelsky, *Electronic Devices and Circuit Theory*, PHI, 7e (2001). 2. D.C. Kulshreshtha,
*Electronic Devices and Circuits*, New Age, 2e (2006). 3. Sedra & Smith, *Microelectronic Circuits*, Oxford Univ.
Press, 7th Edition (2012).
