See below:
Grace Kim wrote:
Dr.
Topp,
Could you check this over and see if my explanation is more accurate?
I think my misconception stemmed from a misunderstanding of how pumping
occurs in a diode laser.
*P and N-doping & a natural voltage/ "population inversion" in a
p-n junction*
I did not realize that the p-n junction of a diode already has an
artificially created population inversion because of the structure of a
n-doped and p-doped semiconductor. The n-doped semiconductor is
made by doping with atoms with an extra electron, which must occupy the
conduction band because the valence band is fully occupied. In
the p-doped semiconductor, there are "holes" or vacancies in the
valence band due to doping with electron deficient atoms.
I
would say that the condition for a population
inversion is created by bringing together n- and p-type doped
materials.
*Pumping the diode with an electrical current*
As the conduction band electrons relax and occupy the "holes," the
population inversion can only be maintained by applying a current that
removes electrons from the p-type semiconductor and replenishes the
electrons in the n-type semiconductor (i.e. applying a positive current
to the p-type semiconductor and a negative current to the n-type
semiconductor). An electrical current is only used to pump the diode
laser to maintain the original pre-existing population inversion and
allow for continuous lasing action. Without the current, the
extra electrons from the n-conductor would relax into the holes of the
p-conductor and produce an electrically neutral region (effectively
quenching the ability to produce light).
There is a single definable current
although there are two types of charge carrier. What you apply is
a VOLTAGE, or a POTENTIAL DIFFERENCE and not a current.
Essentially OK, with a significant caveat.
What you
actually describe is an LED and not yet a diode laser.
In the LED, you simply need to make electrons fall into holes to
generate light. It so happens that the population is naturally
inverted, but this asset is not used in the LED.
In the
laser, you need to have the population inversion as part of the
amplification process. For the laser, of course, you further need
to form the device so that the junction becomes part of an optical
resonator.
*An analogy, kind of?*
Rightly or wrongly, I kind of think of the application of electrical
current to a diode as similar (not equivalent!) to the use of a salt
bridge in a voltaic cell--it keeps the charges moving so that things
don't fizzle out and die.
I wouldn't go there. The salt bridge
is passive like a wire, except that there is more than one type of
charge carrier. The battery applied to an LED is a power source,
where a chemical reaction generates a suitable potential difference
needed to drive a current through a circuit.
--
Michael Topp Professor,
Department of Chemistry, University of Pennsylvania
231, South 34 St., Philadelphia, PA 19104-6323
Tel: (215) 898-4859 Fax: (215) 573-2112
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