PN Junction Diodes

A Brief Description Definitions The Depletion Region Varying VA Diode Currents
The Equation Derivations from the Ideal Let's Draw! Related Topics

Definitions:

Bias:
The fixed voltage applied to an electrode.
Built-in potential:
Also known as Vbi , it is the change in potential across the depletion region of a pn junction under equilibrium conditions.
Current:
A flow of electric charge.  The amount of electric charge flowing past a specified circuit point per unit time.
Density:
The amount of something per unit measure.
Deplete:
To reduce or lessen in quantity, value, or effectiveness; exhaust.  To empty.
Depletion region:
The transition region from p-type semiconductor to n-type semiconductor in a pn junction.  This region is characterized by a large electric field, huge changes in hole and electron concentrations, large amounts of fixed charge (can't move), and a varying potential.
Depletion approximation:
An idealization of the actual charge distribution in the depletion region that originates from the fact that the majority carriers have been removed.  We say this region is "depleted" of majority carriers.  It facilitates the use of Poisson's equation because we can obtain a closed-form solution.  When using depletion approximation, we are assuming that the carrier concentration (n and p) is negligible compared to the net doping concentration (NA and ND) in the region straddling the metallurgical junction, otherwise known as the depletion region.  Outside this region, it is assumed that the net charge density is zero.
Poisson's Equation:
Charge Density:
Simplified to one dimension by
the depletion approximation:
Depletion width:
The width of the region straddling the metallurgical junction of a pn diode where the diffusion of carriers has left a charge, otherwise known as the width of the depletion region, where W = xn + xp .
Diode:
An electronic device that restricts current flow chiefly to one direction.
xn:
The width of the depletion region on the n-side.
xp:
The width of the depletion region on the p-side.
Equilibrium:
A condition in which all acting influences are canceled by others, resulting in a stable, balanced, or unchanging system. The condition of a system in which the resultant of all acting forces is zero and the sum of all torques about any axis is zero. The state of a reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants does not change with time.
Forward bias:
A voltage VA > 0 is applied to the diode terminals.  The potential on the n-side of the junction is lowered relative to the p-side of the pn junction. VA must be =< Vbi since the voltage drop across the depletion region becomes Vbi - VA.
Ideal:
A conception of something in its absolute perfection.
Linearly graded junction:
A more realistic approximation than the step junction to model junctions formed by deep diffusions into moderate to heavily doped wafers.  The linearly graded profile uses a grading constant in the formula ND - NA = ax, where a's units are cm-4.
pn-junction:
The region that is formed by adjoining a p-type semiconductor to an n-type semiconductor.  The line dividing the two regions, where ND - NA= 0,  is the metallurgical junction.
Region:
A specified district or territory.
Reverse bias:
A voltage VA < 0 is applied to the diode terminals.  This lowers the potential on the p-side of the junction relative to the n-side of the pn junction. VA must be =< Vbi since the voltage drop across the depletion region becomes Vbi - VA.
Space charge region:
Antoher name for depletion region.
Step junction:
The doping profile we use to approximate the  pn junction electrostatics of a lightly doped starting wafer.  It means that is n-type with NA = 0 next to a region that is p-type with ND = 0.  The doping profile looks like the figure below.

 

 
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