How are transistors designed

amal banerjee wrote in news:d083fd7c-93b6-4f6d- snipped-for-privacy@googlegroups.com:

Read up on the structure of a modern power transistor...

Read up on the history of the fin fet...

The start of transistor technology was a surface-contact experiment that had gain. William Shockley wrote a theoretical description of a possible mechanism (not a very good fit to the experiment, but very productive) around 1948.

Practical transistors initially were three-layer Germanium devices created by alloying a slab, and diffusion of the alloying impurities determined the dopant distribution.

Improvements over the years were important; Silicon can be passivated by oxidation (foolproof compared with Germanium passivation schemes), and has higher service temperatures. Ion bombardment can implant impurities at depth, with diffusion as a finishing step. Masking operations can print shapes for different parasitic characteristics. Chemical vapor deposition can add a layer of pure material overtop a partial construction.

Finally, there are packaging considerations (you need to connect wires somehow) with some importance in final-construction circuit parameters. For silicon, aluminum is the universally approved way to make an ohmic contact, to either P-type or N-type silicon.

So, to design a BJT, you might want to start with a design that nearly fills your needs, and change its scale, or its chemistry, or its manufacture process. Then, it's just a matter of knowing the three-dimensional composition result and applying physical conditions to the model (biasing the transistor) and predicting the results. There's a lot of physics, chemistry, and lore involved.

There are always some surprises (fringe fields, emitter push, and cosmic ray sensitivity), so a final assessment is done experimentally, after cooking up a few batches.

How likely is it that there isn't a device already being made that will do the job?

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  Rick C. 

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Right. Or at least one that was made in the past.

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 Thanks, 
    - Win

Winfield Hill wrote in news:qvdapr023l6 @drn.newsguy.com:

Zetex makes some pretty amazing small signal stuff.

Old Plessey stuff, they have fabs in Oldham.

Depressingly likely, if you need a fast PNP. :)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

The HV BJT has been forgotten, outshined by easy to use Gets

Look at the saturation voltage for HV switching circuits below 200W, BJTs outperforms FET at same price. A little slower, but many applications do not need blazing fast operation

Cheers

Klaus

Whats his face, Shockley, was beat by over 10 years WRT invention of the transistor.

Julius Edgar Lilienfeld' first patent "Method and Apparatus for Controlling Electric Currents" was filed Oct 8, 1926 and published Jan

28, 1930 #1745175. And it was no fluke; he clearly knew what was happening and his second patent 1677140 published Sept 13, 1932 teaches about a junction transistor. His THIRD patent 1,900018 was published March 7, 1933.

Clearly way before whats his name.

Wasn't that a FET ?

Point-contact transistors were still made until about 1970, iirc. They weren't very reliable or repeatable, but were much much faster than the old alloy-junction devices.

Yup. SiO2 is a very useful material, whereas GeO2 is water soluble. (Diamond also doesn't have a good oxide.) ;)

You don't need to do diffusion if you have ion implantation--that's more or less the point of the exercise. You do need to do a rapid thermal anneal (RTA) to get rid of the lattice damage caused by the implantation. (It needs to be rapid precisely to _avoid_ significant dopant diffusion.)

In 1955, maybe. There are two basic ways of making an ohmic contact to silicon: you either dope the contact region so heavily that the bandgap goes away (i.e. it effectively becomes metallic) or else you pick a metal that makes a Schottky barrier with negative barrier height. (These exist, but I forget which ones they are.)

Nah, physical simulations are both easy and accurate today, and there are lots of well-understood knobs to twist. The cleanliness of modern semiconductor processes is amazing, so there's not much mystery about what's happening inside the device.

Did you read the patent? He has no very deep concept of the device physics. From P. 2, starting at line 76:

"The basis of the invention resides appar ently in the fact that the conducting layer at the particular point selected introduces a resistance varying with the electric field at this point; and in this connection it may be assumed that the atoms (or molecules) of a conductor are of the nature of bipoles. In order for an electron, therefore, to travel in the electric field, the bipoles are obliged to become organized in this field substantially with their axes parallel or lying in the field of flow. Any disturbance in this organization, as by heat movement, magnetic field, electro static cross-field, etc., will serve to increase the resistance of the conductor; and in the instant case, the conductivity of the layer is influenced by the electric field. Owing to the fact that this layer is extremely thin the field is permitted to penetrate the entire volume thereof and thus will change the conductivity throughout the entire cross-section of this conducting portion."

"...the bipoles are obliged to become organized...?"

Pretty tough to find anything about doping, band structure, conduction vs. valence electrons, or even any real electrostatics. Also his copper-oxide semiconductor is polycrystalline or amorphous.

Should be 1877140. What he describes isn't anything like a BJT--it's a metal-semiconductor-metal structure, and he apparently had no idea how it worked, if it ever did. It may have been a reach-through device, with back-to-back Schottky barriers, but with the vacuum technology available in 1930, who knows? Certainly it isn't a BJT.

That seems to be a MESFET-sort of structure, controlled by a back gate. It's far from clear how he achieves channel modulation, because there's no front gate at all.

Nah, no band structure, no P- and N- doping, no semiconductor-semiconductor junction, nada. Shockley & Co. are safely dead, but their fame remains secure. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

There is nothing new here. My Masters's thesis included sections on how C-V measurements of sub-micron MOSFETs may be automated. I am fully aware of the structures of common semiconductor devices.

amal banerjee wrote in news: snipped-for-privacy@googlegroups.com:

"touchy" Hahahah! At the molecular level!

So your claim is that you know how they are constructed, and yet your original post asks how they are designed...

I would have said thet they get designed in colleges like MIT and Purdue, but You apparently did not even need to ask the question.

Did you not word your query well enough?

Brattain and Shockley had stumbled upon the FET, but did not know it, till Dr. John Bardeen joined them. Within a few weeks, after careful analysis of the experimental data and modifications of data measurement and analysis scheme, the BJT was discovered.

My suspicions are confirmed, given that SPICE device model parameters are obtained with curve fitting. Physics has always been fun, and all engineering is just applied physics. Electronic/electrtcal engineering is all about controlling current flow. Mechanical, civil engineering is distributing stresses|loads.

Still?

Gem Mill (old Ferranti discretes fab, taken over by Plessey) is now a housing estate. Plessey Hollinwood is now a supermarket. Worked at both for a short period in the 80's.

--
Ian 

"Tamahome!!!" - "Miaka!!!"

Any good book recommendation on how the BJT was discovered?

Many excellent examples in Youtube, BSTJ, and on the web.

YOUTUBE

Transistor Full Documentary

Plus others.

BSTJ

The Bell System Technical Journal (1922-1983)

Example

BSTJ 28: 3. July 1949: The Theory of p-n Junctions in Semiconductors and p-n Junction Transistors. (Shockley, W.)

It's a little amusing to see it discussed in this form -- it sounds like the incremental theory of fields, i.e. infinitesimal dipoles, lines of force, that sort of thing. Traditional introductory fields stuff. In other words, just applying contemporary frameworks to a new and poorly understood phenomenon, and not really getting it right as a result, it was just the closest thing they had at the time.

As it happens, it's right in a manner of sorts, but only qualitatively so, not in a functional and explanatory way. For exactly the reasons you give (lacks semiconductor theory, especially the notorious problems that held FETs back i.e. surface states).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

To design an npn transistor the emitter is heavily doped. The base is medium impurity level. The collector is lightly doped.

The base-emitter diode conducts more electrons than holes. That means the holes from the base are few, so low base current. That means high beta.

beta is about Ie / Ib

emitter current over base current is about beta.