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Applet Worksheet
(Open-Circuit PN Junction Virtual Lab)

Objectives: After this Lab, students should understand the definitions of built-in voltage, depletion width, and the space-charge of the depletion region.  Students should be able to calculate the built-in voltage, depletion width, the space charge, and the distance depletion extends into the n-side and p-side.  These calculated parameters are to be visually understood from the applet.

PreLab:
Useful formulas for this PreLab:
Built-in voltage V0 = VT ln(NdNa/ni2);
Thermal voltage VT = kT/q = 25.9 meV at 300 K;
Depletion width Wdep = [(2es/q)(1/Na + 1/Nd) V0]1/2, where
Dielectric permittivity of Si es = 11.7 e0 = 1.04x10-12 F/cm;
Assume ni = 1010/cm3 at 300 K for Si;
xn = Wdep*Na/(Na+Nd);
xp = Wdep*Nd/(Nd + Na); and
Space charge neutrality: Ndxn=Naxp.

1) Review (optional): If you are not familiar with the Energy Band concepts, study the “Fermi Level and Carrier Concentration Applet” and its worksheet before continuing with this virtual laboratory.  The applet is at http://jas2.eng.buffalo.edu/applets/education/semicon/fermi/bandAndLevel/fermi.html.  You need to know at least the Energy Band concepts for bulk semiconductors, the significance of Fermi level, carrier concentrations and their relationship to the Fermi level, and the formulas to calculate the carrier concentrations.

2) Calculate the PN Junction parameters: For a Si PN junction diode with Na=1017/cm3, Nd=1016/cm3 and T=300 K,

a) Find built-in voltage, Vo
b) The width of the depletion region., Wdep
c) The distance depletion extends on the n and p side , xn and xp.

Procedure: Open-Circuit PN Junction Virtual Lab
1) Visit the applet entitled “Formation of PN Junction Energy Band Diagram” at http://jas2.eng.buffalo.edu/applets/education/pn/pnformation/pnformation.html.  This applet lets you visualize the Energy Band Diagram of a PN Junction under thermal equilibrium (i.e., under the Open Circuit condition).

2) Select Si, Na=1E15, Nd=1E15, click “SetNewValue” button, and click “FormJunction” button.  Repeat clicking the “FormJunction/Separate” button until you are able to visualize the final Energy Band Diagram of the PN junction.  Now, repeat the procedure with Na=1E17 and Nd=1E15.  Pay attention to the condition when the animation of Energy Band Diagram finally comes to completion.  [Hint: watch the green horizontal line in the band.  What is this green line ?]

3) Click “Separate” button.  Select Si, Na=1E15, Nd=1E15, click the “SetNewValue” button, click the “ShowParameters” button, and click the “FormJunction” button.  From the applet, find the built-in voltage Vo, the distance depletion extends on the n-side xn and p-side xp, and find the total depletion width Wdep:

From applet: Vo = (        ) volts,   xn = (        )mm,   xp = (     ) mm, and Wdep = (        ) mm.

Use the formulas given at the beginning of PreLab and calculate the above parameters below:
Calculation of Vo, xn, xp, and Wdep:

From calculation in PreLab: Vo = (        ) volts,   xn = (        )mm,   xp = (     ) mm, and Wdep = (        ) mm.

4) Click “Separate” button.  Select Si, Na=1E17, Nd=1E16, click the “SetNewValue” button, click the “ShowParameters” button, and click the “FormJunction” button.  From the applet find the following parameters:

From applet: Vo = (        ) volts,   xn = (        )mm,   xp = (     ) mm, and Wdep = (        ) mm.

Compare this with your calculation in the PreLab:

From calculation in PreLab: Vo = (        ) volts,   xn = (        )mm,   xp = (     ) mm, and Wdep = (        ) mm.

5) Self-graded Quiz: For a Si PN junction with Na=1E15, Nd=1E16 at room temperature (300K), calculate the Fermi level Ef deep inside the p-side and n-side bulks, relative to the intrinsic Fermi level Ei:

Calculation
n-side: Ef-Ei = (    ) eV
p-side: Ef-Ei = (    ) eV.
[Hint:   “Fermi Level and Carrier Concentration Applet”]
Calculate other PN junction parameters:  Vo, Wdep, xn, and xp.
Calculation
Vo = (        ) volts,   xn = (        )mm,   xp = (     ) mm, and Wdep = (        ) mm.

Sketch the Energy Band Diagram below without looking at the applet.  In your diagram you must have all the important parameters indicated clearly: Ec, Ev, Ef, Ei, Vo, Wdep, xn, and xp..
Energy Band Diagram Sketch: