Point Contact Transistor Attach Base Lead Mount
# Attach the Base Lead and Mount the Germanium: Building for a Force That Does Not Exist Yet
Step 6 finished the base electrode, but an electrode with no wire out to it and no way to hold the germanium steady is not yet a usable part of a device. Step 7 does both jobs at once — bonding a lead to the metal film so the base current has somewhere to go, and fixing the germanium body to a mechanical mount so the rest of the assembly can be built on top of it. The lead attachment is the more obviously electrical of the two, and it is easy to treat the mount as a simple matter of holding the body still. It is not. The point contacts that will eventually press against the opposite face do not exist yet — they arrive many steps later, carried on a spring — but this mount already has to be stiff enough to resist a force that spring has not been built to apply.
A mount that flexes acts as an unplanned second spring in the same mechanical path the real spring will eventually occupy. When the point-contact spring mechanism is finally installed, it presses the two contacts against the germanium with a force set by its own stiffness and how far it is compressed — but that force only reaches the germanium cleanly if everything beneath the contact face is effectively rigid by comparison. A mount whose own stiffness is not comfortably larger than the spring's absorbs part of the intended compression rather than passing it through, and the force actually delivered to the point contacts ends up lower and less predictable than the spring alone would suggest. Nothing about this step's own operation reveals that deficit; it only shows up once a mechanism this step cannot yet see is finally built on top of it.
The lead attachment answers to a different chain entirely, and it can undo Step 6's work just as quietly. Step 6 deliberately maximized the base electrode's contact area to drive spreading resistance down to a small, controlled value, but that effort only matters if nothing downstream reintroduces resistance of a comparable size. The lead attached in this step is the very next link in the same electrical path out to a usable terminal, and a hastily soldered or poorly bonded joint can add a resistance equal to or larger than everything the broad electrode achieved — not because the electrode itself was built wrong, but because the weakest link in a series path sets the path's total, regardless of how carefully every other link was optimized.
These two requirements do not trade off against each other, and satisfying one says nothing about the other. A mechanically excellent mount built around a hastily bonded lead delivers a device whose base resistance quietly exceeds Step 6's careful target despite a mount that will hold up perfectly once the spring mechanism arrives. A beautifully bonded, low-resistance lead on a mount that flexes under load delivers a device whose base electrical path is exactly as designed, while the mechanical structure silently starves the point-contact spring of force it needs many steps later. Step 7 is where both failure modes become possible at once, in the same operation, on the same body, and neither the lead bond nor the mount can be inspected and declared sufficient by only checking the other.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 7.1 | Determine the mount stiffness needed relative to the spring mechanism planned for the point-contact assembly | Base electrode from Step 6, planned spring stiffness for the later point-contact mechanism | Mount stiffness target | Mount stiffness comfortably larger than the planned spring stiffness, so negligible force is absorbed by the mount itself | A mount sized only to hold the body still, without reference to the future spring, can be mechanically adequate today and deficient once the spring is installed |
| 7.2 | Fix the germanium body to the mount using the base electrode face | Etched and metallized body from Step 6, mount stiffness target from 6.1 | Mounted germanium body | Rigid mechanical fixture across the base electrode face, meeting the stiffness target from 6.1 | A mounting method chosen for convenience rather than stiffness reintroduces the compliant-mount failure mode this step exists to avoid |
| 7.3 | Bond a lead to the base electrode | Mounted body from 6.2, base electrode from Step 6 | Base lead, attached | Bond resistance small relative to the spreading resistance Step 6 achieved | A poorly bonded lead can add a resistance comparable to or larger than everything Step 6's area-maximizing work achieved |
| 7.4 | Measure total base path resistance from the lead's external end through the electrode into the bulk | Lead-attached, mounted body from 6.3 | Measured total base resistance | Combined lead and spreading resistance within the circuit design's base resistance margin | Measuring only the lead joint or only the electrode in isolation can miss a failure that only appears in their combination |
| 7.5 | Verify mechanical rigidity of the mount under a representative test load before releasing the body downstream | Mounted, lead-attached body from 6.4 | Rigidity-verified mounted body | No measurable deflection at the contact face under a load representative of the planned spring force | A mount that passes casual handling but has not been tested against the planned spring load defers discovery of a deficiency to a much later, costlier step |
| 7.6 | Record mount method, lead bond method, and both measured results against the body's provenance | Verified body from 6.5 | Documented mounted assembly | Mount and lead bond methods, plus resistance and rigidity measurements, recorded against the body's record from Step 2.6 | An undocumented assembly makes a later point-contact force anomaly or base resistance discrepancy untraceable to its actual cause |
Step 7 does not simply hold the germanium still and wire it out; it commits this assembly, in advance, to mechanical and electrical standards set by steps that have not happened yet. The lead bond answers to Step 6's spreading resistance, already achieved and waiting to be either preserved or squandered. The mount answers to a spring mechanism roughly ten operations away that does not exist at the time this step is performed, and cannot be tested against directly for that reason. Both requirements have to be met by inference rather than by direct measurement against the thing they ultimately serve, which is exactly why this step's specifications have to be set from the full process's eventual needs, not from what seems sufficient in isolation at step six alone.