💾 Archived View for tanelorn.city › ~vidak › old-blog › diy-alloy-junction-transistors.gemini captured on 2020-11-07 at 01:36:51. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2020-09-24)
-=-=-=-=-=-=-
---
generator: pandoc
title: DIY Alloy Junction Transistors
viewport: 'width=device-width, initial-scale=1.0, user-scalable=yes'
---
2018-09-03T03:05:18+10:00
Part 0 - The Rationale for Early 1950s Transistors
==================================================
Before you ask, "Why make your own transistors at home??" -- read my
[Manfiesto for
Why](https://bootlicker.party/posts/manifesto-for-communist-technology-use/).
Not Just MOSFETs!
-----------------
Most of the homebrew community has been focused on fabricating Field
Effect Transistors (FETs) at home. Sam Zeloof and Jeri Ellsworth are
probably wiser to try and 'etch FETs' because they are much better
suited to fabricating Integrated Circuits (ICs). ICs were a real
revolution in electronics because they miniaturised sometimes enormous
circuits into small, convenient packages. Discrete circuits also do not
last as long as integrated circuits, because it is expensive to render
them mechanically inert. ICs can be completely encased in plastic,
shielding their components from dust, and heat, and other kinds of
physical mechanical interference.
on their references, it seems that manufacturing the kind of transistors
they have, in the ways they have, may still be too expensive and
difficult for hackers. So in this article I am going to argue that *one
option* for hackers is to fabricate not *silicon, planar process FETs*,
but *germanium, alloy-junction, Bipolar Junction Transistors*.
Alloy-junction BJTs are a much older and more primitive type of
transistor to fabricate than the planar process transistors that
Ellsworth and Zeloof talk about, but I will argue that alloy-junction
transistors present themselves as an attractive option for hackers who
cannot afford expensive equipment and materials, and who have to push
most of the cost of hacking onto using their own labour in order to get
things done.
Why Choose Alloy-Junction Transistors
-------------------------------------
Alloy-junction transistors are not *the earliest* and most primitive
kinds of transistors, but they are one of the earliest and most
primitive. These kinds of transistors are **necessarily discrete**
transistors. I argue they present themselves as an attractive kind of
transistor to fabricate at home in a DIY, homebrew setting because:
- Their die-size is much larger than your average planar transistor,
which means they are far better suited to making in small batches,
by hand, one-at-a-time. [This Wikipedia
Reference](http://www.thevalvepage.com/trans/manufac/manufac1.htm)
explains that the 600mW Mullard OC81 transistor wafer size is 2.4mm
x 2.44mm. OC44 and OC45 transistors have a circular wafer size of
1.45mm diameter. The actual transistor die is created by melting
pellets of indium or antimony into the very thin germanium wafer.
The pellets are relatively simple to make, conceptually. The pellets
are also quite large -- they are visible individually to the naked
eye. See the following image below:
- The temperatures of furnaces required for fabrication are much, much
lower, in the order of hundreds of degrees Celsius, and not
thousands.
- The techniques of transistor fabrication are much more primitive,
and are therefore much more suited to beginners, and require the
knowledge of far less complex chemistry.
- Most of their materials seem inexpensive to gather (germanium,
indium, antimony).
- Most of the materials for the fabrication of these transistors seem
relatively safe to be exposed to in reasonable amounts, with some
notable exceptions, like indium. But even indium is a lot safer than
the etching fluid Sam Zeloof recommends-- Hydrofluoric acid, HF.
- The process of their fabrication does not require photo-lithography.
So, making these transistors does not require expensive projection
equipment to be fabricated at ''small'' sizes, and does not require
**complex proprietary materials** in order to etch.
Part 1 - The Actual Manufacture of Alloy-Junction Transistors
=============================================================
Now, I will describe the rough process of how to fabricate this
primitive type of transistor.
The Creation of Monocrystalline Wafers
--------------------------------------
First, a wafer of germanium make of a single crystal is formed. This can
be done yourself with great amounts of heat, or more conveniently, and
be obtained online. This is not the most important process to consider
when making these transistors.
Dicing
------
Dice your germanium wafers into wafers required to manufacture the base
of the transistors.
The wafer of germanium forms the alloy-junction in this way. It is
sandwiched between two melted pellets of semiconductor alloy:
The [Wikipedia
reference](http://www.thevalvepage.com/trans/manufac/manufac1.htm)
mentioned above suggests that an ''ultrasonic drill'' can satisfactorily
dice germanium wafers into the right size.
Etching
-------
The wafers of germanium then need to be ''etched'' into the correct
thickness in order to satisfy the operating conditions of the
transistor. This is not photo-lithographic ''etching'', it is the
chemical erosion of the diced germanium wafer into a much smaller
thickness than previous.
Pellets
-------
Pellets of indium or antinomy need to be fabricated in order to dope the
''etched'' germanium wafer which is intended to be the base of the
transistor. These pellets of semiconductive precious metals form the
collector and emitters of the transistor.
The [Wikipedia
reference](http://www.thevalvepage.com/trans/manufac/manufac1.htm) above
describes the process of forming the small pellets of metal:
Indium wire or strip is cut into portions containing the amount of
material required for the pellets. The pellet which forms the
Collector is three to five times the size of the one used for the
emitter, according to the type of transistor.
The process for shaping or 'balling up' the pellets bears some
resemblance to that used for making lead shot. The pieces of indium
are dropped down a glass tube about three feet high and filled with
liquid. At the top the liquid is sufficiently hot to melt the pieces
of indium into droplets. Further down the liquid is cooler and the
drops of indium solidify into spherical pellets.
Alloying
--------
Using a jig, you then alloy to the wafer of germanium first the emitter
of the transistor, and then the collector. This is the part of the
fabrication that requires a furnace that can reach a high temperature.
The temperature of the furnace is much lower than that required for the
fabrication of planar process FETs, and the actual stages of heating are
also much simpler. The *chemistry* of the alloying process is also less
mission-critical than the chemical process of FET construction.
The furnace temperature is *below* the melting point of germanium, but
degrees Celsius.
Part 2 - Conclusion
===================
I will not discuss the soldering of lead connections to the finished
germanium transistor, because they're not different from any other
transistor fabrication process -- either Zeloof's or Ellsworth's. *But*,
these transistors are much larger and easier to solder than the ICs of
Zeloof etc.
Anyway that's it!