Point Contact 1947 Cut a Small Block or Disc

# Cut a Small Block or Disc: Sizing a Body Around a Resistance Problem, Not a Junction Depth

Neither a grown-junction nor an alloy-junction wafer's thickness problem applies here at all — this device has no junction depth to leave room for, because current doesn't cross a planar junction, it spreads outward from a tiny point contact into a block of bulk germanium on its way to a distant base electrode. Cutting this block to size is really a decision about how much resistive bulk material sits between the active point contacts and the base connection, and getting that choice wrong doesn't threaten a dimension like $W_B$ from the other series — it directly adds unwanted series resistance to every signal this device will ever amplify, a resistance that would later become one of the point-contact transistor's most famous practical headaches.

## 1. Current Spreads From a Point Contact According to a Classic Resistance Law

Current injected through a small, roughly circular contact of radius $r$ into a semi-infinite bulk of resistivity $\rho$ encounters a real, calculable spreading resistance as it fans out hemispherically before reaching a distant electrode — a classical electrostatics result, not a hand-wave. Because step one's material was selected for a high-resistivity, high-reverse-voltage rating rather than for low resistivity, this spreading resistance is not negligible here the way it might be in a more heavily doped device: the same resistivity that gives this germanium its useful rectifying reverse-voltage behavior also means current leaving a tiny point contact has real, measurable resistance to fight on its way to the base.

## 2. Real Diagram: A Point Contact's Current Fans Out Before It Ever Reaches the Base

## 3. The Block Has to Be Much Larger Than the Contact It Will Carry

A point contact in this device is physically tiny — the wedge and wire contacts described later in this sequence measure in mils, thousandths of an inch — while the block this step cuts has to measure many times that in every dimension, because the spreading-resistance picture above only holds while the hemispherical current pattern has genuinely unobstructed bulk material to expand into. A block cut too close to the contact's own scale truncates that spreading geometry against a nearby edge, which doesn't reduce resistance, it distorts the current path in ways the simple formula above no longer predicts and typically makes the practical resistance worse, not better. This is why "small block" in this step's own name is relative only to a wafer-scale sensibility borrowed from later devices — relative to the point contacts it will host, this block is enormous.

## Cut a Small Block or Disc's Place in the Process Lineage

Cutting a small block or disc is step two of the twenty-five-step sequence documenting the original 1947/48 Bell Labs point-contact transistor — immediately after suitable N-type germanium was selected, and before either face of this piece is ground, etched, or metallized. It is the step that establishes a geometry governed entirely by spreading-resistance physics rather than any junction-depth concern, sizing the block large enough relative to the microscopic point contacts it will eventually host that step one's deliberately high-resistivity material doesn't translate into an unmanageable series resistance before the device even has its first contact. Step three, grinding the upper and lower faces flat, is what actually prepares those two faces for the contacts this step's sizing decision was made to accommodate.