Point Contact Transistor Cut Small Block or Disc
# Cut a Small Block or Disc: The First Operation That Can Destroy What Step 1 Selected For
Step 1 accepted a germanium sample on the strength of a property that lives in the bulk — a minority carrier lifetime long enough that injected holes survive the trip between two point contacts. Step 2 takes a saw to it. That is not a trivial sequencing detail: cutting a crystal creates new free surfaces, and a free germanium surface is an extremely efficient recombination site, far more efficient than the bulk the lifetime measurement characterized. The cut therefore does two things at once. It produces the semiconductor body the device needs, and it introduces the one mechanism most capable of invalidating the measurement that qualified the material in the first place.
The block's as-cut size is set by two bounds that push in opposite directions. The lower bound comes from surface recombination: the free side faces of a finished body are recombination sinks, so they must sit far enough from the future emitter–collector pair that they do not compete with the collector for injected holes. Shrink the body too far and the nearest side face becomes a better destination than the collector point. The upper bound comes from everything else — germanium was scarce and expensive, a larger body spreads base current over a longer path and raises series resistance between the base contact and the region under the points, and a larger body is harder to mount and to keep thermally stable. "Small" in the process description is the result of that squeeze, not a free parameter.
The thickness dimension is the one the device's circuit topology actually constrains. The two point contacts go on one face; the large-area base contact goes on the opposite face. The germanium between them carries the base return current, so the block's thickness sets a resistance in series with the base, in the same body the emitter is injecting into. Thinner means less series resistance but less mechanical margin for the grinding in Step 3 and a greater risk of fracturing the body during mounting in Step 6. There is no thickness that optimizes both, which is why this dimension is chosen with the mounting operation already in mind rather than purely electrically.
Cutting also severs the sample from the measurement that qualified it. Step 1 recorded resistivity, a rectification check, and a lifetime estimate against a sample. Minority carrier lifetime varies along a germanium ingot and between regions of a single piece, so when one qualified sample is cut into several blocks, the lifetime figure belongs to the parent, not to any particular child. Either each block inherits the parent's provenance record explicitly and is treated as provisionally qualified, or the qualification has to be redone per block. The failure mode this prevents is specific and nasty: a device that will not show transistor action because its particular block came from a poor region of a sample that measured well on average, with nothing in the records to distinguish it from its siblings.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 2.1 | Determine the finished body dimensions before any cutting | Accepted sample and planned point spacing from Step 1 | Target finished dimensions | Lateral clearance from the intended point pair to any free side face large compared with the planned point spacing; thickness chosen with the Step 6 mounting method in mind | Dimensioning after the cut is too late — the allowance in 2.2 depends on knowing the finished size first |
| 2.2 | Add a machining allowance to every face that will be cut | Target finished dimensions from 2.1 | As-cut dimensions | Allowance on each sawn face at least the expected depth of saw-induced damage, so Steps 3 and 4 can remove all of it | Cut to the finished dimension and the damaged layer becomes part of the active device, overriding the bulk lifetime Step 1 verified |
| 2.3 | Choose block or disc form, and the cutting method | Sample, as-cut dimensions from 2.2 | Cutting plan | Form and method selected for handling and yield; cutting method consistent with the allowance budgeted in 2.2 | A more aggressive cut leaves deeper damage and silently consumes the allowance 2.2 budgeted for it |
| 2.4 | Plan the cut layout across the sample to maximize usable blocks | Sample, cutting plan from 2.3 | Cut layout with each block's position recorded | Each planned block identified by its position within the parent sample | An unrecorded layout makes the per-block provenance in 2.6 impossible to reconstruct afterward |
| 2.5 | Cut the block or disc from the sample | Sample, cut layout from 2.4 | As-cut germanium body, oversize | Dimensions within the as-cut targets of 2.2, with no chipping that reaches inside the intended finished outline | Chipping that crosses into the finished outline cannot be ground out without violating the finished dimensions |
| 2.6 | Propagate the parent sample's provenance to each cut block | As-cut bodies from 2.5, Step 1 records | Per-block provenance record | Each block carries its parent's resistivity, rectification, and lifetime figures, plus its own position within the parent | Treating a block as fully qualified because its parent measured well hides within-sample variation in lifetime |
| 2.7 | Set aside blocks with visible cracks or inclusions before further work | As-cut bodies from 2.5 | Blocks released to Step 3 | Blocks with cracks reaching the intended finished outline, or visible inclusions, withdrawn before grinding | Grinding and etching a cracked block spends effort on a body that will fracture during mounting or behave unpredictably |
Step 2 does not shape the device; it decides how much of the germanium the next two steps are allowed to throw away. The operation reads like simple stock preparation, and in a rectifier program it largely would be — a rectifier needs one clean junction on one face and is indifferent to what the bulk a few thousandths of an inch away is doing. A point-contact transistor is not indifferent, because its mechanism is a carrier crossing that bulk. Cutting is the moment the project stops working with a measured crystal and starts working with a manufactured object whose surfaces it created itself, and the only defense against those surfaces is to have left enough material in hand to remove them.