Diode Introduction•A diode is formed by interfacing an n-type semiconductor with a p-type semiconductor.•A pnjunction is the interface between nand pregions.Diode symbolCarrier DiffusionCarriers diffuse from highconcentration to lowconcentration.Drift Currents•Diffusion currents lead to localized charge density (ρ) variations near the pnjunction.•Gauss’ law predicts an electric field due to the charge distribution:•Assuming constant permittivity (εS),•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=ρcεsE(x)=1εsρ(x)dx∫(Change in electric field is proportional to net charge per unit volume.)Space Charge Region Formation at the pnJunctionPotential across the JunctionCharge DensityElectric Field(define E=0 in neutral region)Potential(define φ=0 at junction)φj= −E(x)dx∫=VTlnNANDni2⎛ ⎝ ⎜ ⎞ ⎠ ⎟ , VT=kTqWidth of Depletion Regionwd0=(xn+xp)=2εsq1NA+1ND⎛ ⎝ ⎜ ⎞ ⎠ ⎟ φjCombining 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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