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

Varying VA:

Vbi is the built in voltage in a pn-junction diode in equilibrium.  VA is the voltage that is applied to the diode terminals.

Applying VA > 0, the diode is forward biased.  Reverse bias is when VA < 0.  The diode is in equilibrium when VA = 0. VA is considered positive when the higher potential is applied to the p-side of the diode.

The question is, how does varying VA affect the diode?  The simple answer is that xp and xn  change as VA changes, therefore changing the width of the depletion region.  If you take a look at the equations for charge density, electric field and potential on pages 210-213, they all include the bounderies of the depletion region, xp and xnso they all change as well.  The width increases with VA < 0 and decreases with VA > 0.

Let's take as an example a pn diode.  When VA = 0, the Fermi level is constant and we can determine that the diode is in equilibrium.  If we reverse bias the diode, apply VA < 0, we increase the potential hill by lowering the n-side of the diode with respect to the p-side, increasing the barrier to diffusion.  If we forward bias the diode, apply VA > 0, we decrease the potential hill by lowering the p-side of the diode with respect to the n-side, making it almost flat.  This can be seen with band diagrams.

Electrostatic variables, such as potential, electric field, and charge density, all change with changing the applied bias in a similar manner since they are all a function of x. The easiest way to determine the electrostatic potential, V,  is to follow the band diagram, since it can be drawn as an "upside down" conduction band, so as the band diagram changes, the potential changes by the same degree, take a look.

The electric field, E, is also affected by varying the depletion region.  There's a larger electric field when the diode is reverse biased since there is a wider depletion region. A smaller one with forward biasing corresponding to a narrower depletion region.  Take a look at how the electric field is affected in this example.

Charge density, r, only depends on the doping and charge.  What makes the graph vary is that it is graphed only between xp and xn, so as the depletion width changes, the graph for charge density changes, although the height doesn't.  To see how charge density is affected by increasing the width of  the depletion region of the pn diode with forward biasing and decreasing it with reverse biasing, see this example.

Now, putting them all together for each VA:

You can also take a look at the demo for pn junction statics.  You will need shockwave plug-in for your browser.  It should run properly on the Vectras using Internet Explorer, but it will not run on the Suns.  Use the "full screen" option for better viewing, and you will need to use the browser's "back" button to return to this page.

Next Concept: 
Diode Currents
Previous Concept: 
The Depletion Region
Table of Contents