Hello All
By the way
Anyone knows why in some devices, using epitaxial layer is good? Good
in what parameters?
Kindly advise
Thank you
Jason
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In the diffused process that John described, say you have a silicon
N-type wafer and wanted to make a PN junction. A P dopant is
deposited in a fairly high concentration of the top of an N type
silicon wafer, then the temperature is raised so that the P dopant
atoms diffuse into the body of the N silicon. At the end of the
diffusion step, you would find that the concentration of the P dopant
atoms decrease from the surface toward to bottom of the wafer, but not
in a linear way-it follows an error function distribution(erf). The P
dopant concentration at the surface of the wafer is usually greater
than the wafer background N atoms, so that the surface of the wafer
has been converted to P type silicon. As you go deeper and deeper
into the Si, the concentration of P dopant atoms gradually decreases
until you reach the point at which P=N. This is the PN junction
depth. Below this point N>P and you are into N type silicon on the
other side of the junction. This makes a diffused diode.
With the epitaxial process, the N wafer is heated while a mixture
of a silane gas and P dopant gas is passed over the N wafer. Silicon
doped with P dopant is deposited on the wafer, producing a P type
silicon layer on top of the N wafer. In contrast to the diffused
process, the concentration of dopant in the top layer is very nearly
constant throughout its depth. Ideally, it produces a "step" junction
with no N type carriers. In some cases, the dopant can be left out
altogether to produce a carrier free(ideally) top layer, which is
called "intrinsic" silicon.
This distribution of carriers around a PN junction will have a
pronounced effect on the electrical characteristics of a device.
Leakage current of a diode, for example. Or junction
capacitance-avoided for high frequency devices, exploited for
varicaps.
"Sic hoc legere scis nimium eruditionis habes."
(If you can read this, you're overeducated.)