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chap3 - Diode Introduction Carrier Diffusion A diode is...

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Diode Introduction A diode is formed by interfacing an n -type semiconductor with a p -type semiconductor. A pn junction is the interface between n and p regions. Diode symbol Carrier Diffusion Carriers diffuse from high concentration to low concentration. Drift Currents Diffusion currents lead to localized charge density ( ρ ) variations near the pn junction. Gauss’ law predicts an electric field due to the charge distribution: Assuming constant permittivity ( ε S ), Resulting electric field gives rise to a drift current. With no external circuit connections, drift and diffusion currents cancel. There is no actual current, since this would imply power dissipation, rather the electric field cancels the diffusion current ‘tendency.’ ∇⋅ E = ρ c ε s E ( x ) = 1 ε s ρ ( x ) dx (Change in electric field is proportional to net charge per unit volume.) Space Charge Region Formation at the pn Junction Potential across the Junction Charge Density Electric Field (define E =0 in neutral region) Potential (define φ =0 at junction) φ j = − E ( x ) dx = V T ln N A N D n i 2 , V T = kT q Width of Depletion Region w d 0 = ( x n + x p ) = 2 ε s q 1 N A + 1 N D φ j Combining the previous expressions, we can form an expression for the width of the space-charge region, or depletion region. It is called the depletion region since the excess holes and electrons are depleted from the dopant atoms on either side of the junction. The 0 subscript indicates that no voltage is applied to diode terminals.
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