DIY transistor fabrication: Are you up to the challenge?

By Aalyia Shaukat, contributing writer

Do-it-yourself transistors have been generated from a few hobbyists online including Neil Steiner and some guy named Jim back in 2009. The most notable homebrew transistors, however, come from Jeri Ellsworth , a tinkerer with a knack for device physics. She built a home-based fabrication facility (also known as “fab”) in 2010 and managed to generate some simple circuits with her homemade chips. Interestingly enough, no other well-documented DIY transistors have since been published. This is likely due to the fact that this type of manufacture necessarily leads to the handling of dangerous chemicals and can take years to perfect (it took Jeri two years). Moreover, because transistors are the building block for electronics, they can be purchased cheaply to create much more complex ICs that perform something that can be more directly functional. With that being said, it is a pretty substantial achievement for a hobbyist to build a transistor from scratch with a home laboratory, so one would imagine there is a personal sense of accomplishment that comes with making one.

Nowadays, most fabless firms outsource the actual production overseas because the overhead cost of employing the required number of engineers and technicians to operate the plant at a profit is too high following the $10 billion invest that it typically costs to construct a state-of-the-art fabless plant. Most companies cannot sustain the level of sales needed to justify that kind of financial investment.

Being that there’s no other option to generate transistors cheaply, it makes sense why these foundries are making a killing with a new record of $400 billion in sales. In essence, it might actually make sense to explore more accessible modular transistor fabrication, particularly for prototyping, despite the major cost and technological hurdles because the gigantic facilities do have some drawbacks .

Jeri Ellsworth and Neil Steiner have both created transistors from prefabricated germanium diodes and cadmium sulfide photocells, respectively. This is not the same as building a transistor from just bare doped wafer because the diode and photocell are prefabricated.

Building a transistor from scratch would require some tools that are not commonly found in a home-based lab. This video from San Zeloof is a pretty thorough tour of his home chip fab and is reminiscent of Jeri’slab . Some more uncommon tools include the following:

• Nitrogen tank
• Kiln
• Hydrofluoric acid (HF)
• Phosphosilicate film
• Prime-grade silicon wafer
• Vinyl sticker in place of photoresist for masks
• Color chart for identifying thickness of oxide layer

Some of these materials are more harmful than others. Anyone handling HF must do so very carefully because this chemical can penetrate tissue, causing some pretty gnarly burns. A nitrogen tank is not completely necessary, but it helps in controlling the atmosphere of the kiln to more predictably grow the oxide layer on silicon, and when you’re growing layers that are only several hundred angstroms (Å) in thickness, it will probably save a lot of time. Also worth noting, doping pure silicon is another project in and of itself, so purchasing predoped prime-grade silicon online might be more feasible. Some more mundane hobbyist equipment includes a power supply, oscilloscope, tweezers, CPU fan, conductive epoxy, and solder.

How are transistors fabricated?
A transistor is fabricated through the process of photolithography , which, in essence, patterns the surface of a substrate to form various transistor topologies. One such topology is that of a metal-oxide field-effect transistor (MOSFET), as shown below.

Image source: Shutterstock.

Once again, the bulk p-type substrate can be readily bought online.

Growing the initial oxide layer is accomplished through time in the kiln. Jeri, for instance, states that it takes her six hours to grow a 500- to 600-Å-thick layer of oxide with the addition of steam pumped into the furnace. There is no need to pull out a super tiny caliper and microscope to measure this thickness as it is apparent by the color. A 600-Å-thick layer corresponds to a green color.

Etching, or removing the oxide to accomplish a particular pattern, is accomplished through the use of HF. For many etch steps, part of the wafer is protected from the etchant by a “masking” material, which resists etching. The vinyl sticker mask is precut to resist the etchant in certain areas.

The source and drain region is created by spinning the phosphosilicate film onto the wafer so that there is a thin layer of the liquid on the wafer piece; this is normally accomplished with a CPU fan.

Once again, the device is placed in the kiln at 1,000^o C for a certain amount of time to deposit high concentrations of phosphorus onto the surface of the substrate and oxide layers — creating two doped n-type regions and, ultimately, a channel for electrons to flow.

Finally, vinyl masks are used to etch away the gate region and place it back into the kiln to grow a gate oxide layer. Jeri specifies a thickness of 800 to 1,000 Å, which is a pink to dark red color.

Contacts to the gate, source, drain, and substrate can be made with conductive epoxy. This is accomplished with another vinyl mask and additional etching to make access points down to the source and drain to place bits of epoxy that can be soldered to with leads.

More details are listed on a number of different websites with just a little bit of research. It is apparent that this process gets exponentially more complicated when designing masks and planning on the steps for a topology with multiple transistors. The size and scope of the task can get unwieldy; thus, the design of huge fabrication facilities in which transistor gate widths are down to nanometers and CAD programs for the layout of millions of transistors.