QuickNote | Introduction | Mathematical Analysis | Applet Tutorial | Applet Worksheet | Quiz | SPICE/CAD | References | Feedback
Applet Worksheet
(Drift, Diffusion, and Recombination Virtual Lab)

Objectives:
Upon completion of this laboratory students should be able to understand the processes of diffusion, drift,
recombination and generation, for carriers in semiconductors.

Pre-lab:
1) For a piece of silicon doped with 1016 Arsenic Atoms.
a. Find the electron and hole concentrations at equilibrium.
b. Use the plot in the book to find the mobility (if data are not available, then use the mobility in Intrinsic Si) and then calculate the resistively of this material.
c. Are holes or electrons the "minority carriers"?
Procedure:

Diffusion, Drift and Generation and Recombination

1) Visit the virtual lab entitled "Diffusion, Drift and Recombination Virtual Experiment" at http://jas2.eng.buffalo.edu/applets/education/diffusion/diffusion.html. This applet
lets you visualize the following processes of excess minority carriers in a semiconductor:
· diffusion, due to the concentration gradient of carriers,
· drift, by the applied bias (and therefore electric filed)
· recombination or loss of a carrier.
· generation of carriers
2) Check the checkboxes of width vs. t, displacement. vs. t, and N vs. t. This shows the three
graphs at the bottom which are the width of the distribution of the minority carriers, the
location of the peak of the distribution of the minority carriers, and the total number of
minority carriers in the entire bar.

Diffusion: First you will examine diffusion.

3) In the applet, set: the lifetime to infinity, bias to 0 V and material to p-type, length to .4
cm, temp to 300K, and mobility to 1900 cm2 /Vs.

4) Start the applet, and observe the distribution of the minority carriers. Draw below what the
distribution of minority carriers looks like over time, and describe it in words.

5) Notice that location of the peak doesn't move, Why?

6) Why does the total number of minority carriers stay the same?

7) Increase the mobility to 3900 cm2 /VS, and re-start the applet. What effect does the mobility
have on the distribution of minority carriers?

8) What is the direction(s) of the diffusion current? (a drawing would be good here!)

Drift:  Second you will study drift.
9) Examine drift by setting the lifetime to infinity, bias to +2V, material to p-type, length to
.4 cm, temp to 300K, and mobility back to 1900 cm2 /Vs.

10) Start the applet, and observe the distribution of the minority carriers. Draw below what the
distribution of minority carriers looks like over time, and describe it in words.

11) Which direction does the peak move and why?? (remember the plots are for the minority
carriers!)

12) Change the material to n-type, and re-start the applet. Does the peak move in the direction
you expect?

13) Change the material back to p-type, and compare running the applet at +2V bias and +4V
bias. What can you say about the dependence of the voltage on the velocity of the carriers?
(Some ## would be good here!)

14) Calculate the drift current in above situation for both the +2 and +4 V case.  For the calculation, assume a uniform concentration of minority carrier electrons at 1014/cm3.

Jdrift (@ 2V) = ___________________ Jdrift(@ 4V) = ____________________

Recombination: Last we examine the recombination.
15) Examine recombination by setting the lifetime to .01 ms, bias to 0 V, material to p-type,
length to .4 cm, temp to 300K, and mobility to 1900 cm2 /Vs.

16) Run the applet, and notice the third plot (total number of carriers). If the total number of
electron carriers is decreasing, where are they going?

17) How many ms does it take for 2/3 of the electrons to disappear?

18) Run the applet with the lifetime set to 0.1 ms and .001 ms as well. What does the time-dependence
of the Number (of minority carriers)-vs.-t data appear to have ?

19) What does the "lifetime" represent then? (think RC circuit)

Generation & general questions:
20) How are the electrons in the p-type Ge generated in this applet? (can you say anything about the maximum wavelength the laser must have for Ge) [If this subject is not covered in class, then you may skip this question.]

21) How many electron-hole pairs does the laser create in this applet? [If this subject is not covered in class, then you may skip this question.]

22) Why aren't the majority carriers distributions shown in the plots??

(This worksheet was originally contributed by Prof. J. Ross of the Univ. of Pacific, 2/17/1999)