| Sahib wrote on Thu, 06 December 2007 17:11 |
| Quote: |
.... Microscopy at a moderate x200 magnification reveals the structure of the conductor. Bar-refined copper shows a highly crystalline makeup, of some 150 000 crystalls per metre. An analysis of the structure indicates that the crystals have a pure interior, while the impurities congregate at the crystal boundaries. The oxygen content is present in the reduced form of Cu2O, a semiconductor. Considering te conductive path between crystals, the boundary has the properties of a junction diode, a capacitor and a low shunt resistance, the latter being the dominant feature.
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Cemal
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There will indeed be some oxide in copper, and on the copper surface. Copper readily oxidizes at normal pO2! Its also very easy to measure the relative crystallographic orientations of the rolled copper, by making something called a "pole figure" on an X-ray diffractometer. None of this is news.
To consider a smattering of anything in a matrix of conductive copper at audio frequencies is laughable. This is easily shown by doing impedance spectroscopy using a potientiostat, an extremely common and useful technique in the world of electroceramics. This is not tremendously different than the use of a network analyzer in the ee world.
In recent years people have attempted to extend impedance spectroscopy to the world of metal alloys, with very unimpressive results. One the professors trying to pioneer that technique is in my department here at Georgia Tech. Eventually, if you go high enough in frequency, you can measure effects that are not purely resistive, but linking them to characteristics of the microstructure is nearly impossible.
That is not to say that certain materials defects don't play a tremendous role in the electrical performance of certain systems...Just not at audio frequencies!
When I was fabbing microchips there were all kinds of materials limiting factors to electrical behavior. The speed of waves in the metal back end layers is tremendously influenced by the dielectric constant of the surround substrate. Notice I say "wave" because we are at transmission line frequencies here.
Another high frequency effect that is common in microchips is the residual parasitic capacitance in two specific places. The first is at the materials boundary between the gate oxide and the source drain channel. The apparent capacitance here is tremendously influenced by crystal defects that can trap electrons (or holes). The triumph of silicon as a materials system for CMOS is in large part a function of silicon's ability to grow a very thin, stable, epitaxial, oxide layer. Silicon is not a direct bandgap material, nor does its oxide have a particularly high dielectric constant. Regardless, it has dominated CMOS processing due to silicon dioxide's stable, low defect thermal growth.
The second parasitic capacitance of note is in material forming the gate conductor (above the gate oxide dielectric). The gate conductor is capacitively coupled to the source drain channel through the gate oxide. The gate conductor has historically been polycrystalline silicon (polysilicon), which is then heavily doped via ion implantation to make it more conductive. The material has substantial residual capacitances, in part due to trapping of implanted material at the grain boundaries between the crystals of silicon.
Intel just turned the CMOS world on its head very recently, by introducing the Penryn chip. Penryn returns to a (shock!
) metallic gate conductor a titanium/nitraded titanium combination. The gate dielectric is now halfnium oxide. This is a really big deal in the CMOS world, since even though the "M" in "CMOS" stands for "Metal," polysilicon has ruled the day/gate for many years.
The gate oxides in the computer you are reading this post on are at most 4 atomic planes thick. This is the size/frequency scale of materials science where these sorts of behaviors matter, and are clearly observable!
Not in the audio band, period, regardless of some physicist's misinformation.
[/Materials Engineering Hat]
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Topic: Microdiodes in copper conductor (Read 25129 times)