About Bryce Randolph Expertise Analog and transistor circuit design, inventions involving sensors and electronics, the physics of inventions, the practicality of inventions, and how to document inventions.
Experience
Past/Present clients Texas Instruments
North American Rockwell
EXAR
Government Systems Division
Quantor
Kaiser Aerospace
Expert: Bryce Randolph Date: 9/4/2004 Subject: "Fabrication of transistors""urgent"
Question Hi,
I have to present the topic "Fabrication of transistors". I am not able to find much information on this topic. Can you please suggest some web sites which cover this topic or some books on this topic? I am doin my BTech ( Electrical), 3rd semester. What are the detailed steps of fabrication? Are there many fabrication methods?
Thank You.
Answer I searched under Google as follows:
Search Terms: transistor fabrication structure diagrams
Must Contain: "Theory"
This search seems to work.
The site jas.eng.buffalo.edu sems to be a good one for your question. See
Minu-
I will give you a brief sketch of it here and then see if i can find some detailed information which I will send in a follow-up answer.
CRYSTALS
All electronic devices depend on the existence of crystals of elements which do not conduct electricity. Silicon is the universal element used for this today. Silicon is found in sand all over the earth, as silicon dioxide.
Since silicon will not conduct electricity, it has to be made to conduct. This is done by raising the silicon to high temperatures on the order of 1000 degrees and then allowing a gas of some other conductive element to make contact with the hot silicon. Some of the hot gas molecules will then diffuse into the hot silicon just like sprinkling salt into a glass of water. Boron is an example of one of the 'dopants' used to make the silicon crystal conduct electricity. Boron makes what is called 'P' type silicon.
The dopant is put into certain areas and not others by a process of masking using silicon dioxide. Silicon dioxide is grown on top of the silicon by allowing some oxygen into the hot chamber. Next a very powerful acid called hydrofluoric acid is used to open windows into the silicon. This is just like opening the windows of your house and letting fresh air in. The longer the window is open, the more outside air will get in.
After the dopant has diffused into the silicon a certain distance, the oven is turned off and the cystal lattice again 'freezes' and all of tha toms present stop moving.
For a bipolar transistor, there is needed three regions of consuction; two P and one N or two N and one P, making either a PNP transistor or an NPN transistor.
For example, if we allowed Boron into the silicon making a P region, we will next allow some N atoms such as phosphorous into the same area only not quite as deep as the P. We finish with a third diffusion of more P material only not as deep as the N. This makes a transistor with P on the bottom, then an N region, and finally a P region on top, making a PNP transistor. The very bottom region is called the collector, the middle one the base, and the top one the emitter.
Wires are connected to these three regions and the whole thing is covered with a final layer of glass (silicon dioxide) for protection from the air. At room temperature the atoms will remain in place for millions of years, so our PNP transistor will work for quite a while.
The transistor has current gain so a small voltage on the base will control a large current in the collector. For example a small voltage from a tape head will eventually drive a large loudspeaker all because of transistors and current-gain.
