ZaKi AbaaS
مؤسس المنتدى
عدد الرسائل : 91
العمر : 54
البلد : ismailia
العمل/الترفيه : التربية والتعليم
المزاج : رايق
تاريخ التسجيل : 18/11/2007
 |  موضوع: Chapter 15  السبت يناير 12, 2008 2:48 am | |
|
|
1- A metallic crystal consists of positive ions and a cloud of free electrons roaming around the crystal in random motion.
There is a force of attraction between the ions and the electron cloud. But the resultant of all forces of attraction on a single free electrons is zero. If an electron tries to escape from the metal, a net force of attraction due to the atom layer at the surface pulls it in.
2- A pure silicon (semiconductor) crystal consists of atoms covalently bonded. At low temperatures, there are no free electrons. If temperature increases, some bonds are broken, electrons become free, leaving behind holes move randomly.
3- The number of broken bonds increases with temperature. It may increase also by an external stimulus, such as light, provided that the photon energy is sufficient to break the bond.
4- The number of free electrons and holes increases by adding impurities (doping). Thus, the material becomes n-type or p-type.
5- The conductivity of a semiconductor depends on the conduction of free electrons and holes. Thus a semiconductor has two current carriers: electrons and holes, while in a metal there is only one current carrier (the electron). Electron concentration in a metal is constant and does not depend on temperature.
6- Semiconductors are environment - sensitive. They can be used as sensors to light, heat, pressure, humidity, chemical pollution, radiation…etc.
7- A diode (pn junction) consists of a p-type region and an n-type region. If the p-side is connected to the positive terminal of the battery and the n-side to the negative terminal (forward connection or forward bias) current flows. If the battery is reversed no current flows. This is why a diode is used in rectification.
8- A transistor may be pnp or npn, and can be used as an amplifier, since the ratio of the collector current to the base current be is large. Therefore, any small change in the base current leads to an amplified change in the collector current.
9- A transistor may also be used as a switch. It is used in logic gates, such as an inverter (NOT, AND, OR) gates.
10- Digital electronics is superceding analog electronics for its ability to overcome electrical noise. Its basic concept is to code. Information in binary form (0, 1).
11- ICs have the advantage of small size and weight, increased speeds and capacity, and yet low cost. This is the reason for the proliferation of PCs. |
|
| |
|
