Surface Barrier 1953 Attach the Emitter Lead

# Attach the Emitter Lead: Bonding a Wire Without Unmaking the Contact Step Eighteen Just Formed

Attaching this lead cannot simply apply whatever heat and force reliably bonds a wire to metal in general, because the metal this wire is bonding to is not an ordinary pad — it is one face of the rectifying barrier step eighteen's electroplating just created, sitting on a membrane step sixteen already identified as unable to absorb rough mechanical treatment. This step has to satisfy both constraints at once: enough bonding energy to make a reliable mechanical and electrical connection, and little enough that the barrier's own interface and the membrane's own structure survive the attachment undisturbed.

## 1. Excess Bonding Heat Threatens the Barrier Itself, Not Just the Membrane

$$\phi_B(T) = \phi_B(T_0) - \kappa\,\Delta T$$

Step seventeen's barrier height $\phi_B$ was set by a clean metal-germanium interface formed at low temperature, and that interface is not thermally inert — excessive heat during lead attachment can drive interdiffusion at the contact, degrading $\phi_B$ by an amount $\kappa\,\Delta T$ that depends on how far bonding temperature rises above the temperature the contact actually formed at. This is a second, independent reason beyond step sixteen's fracture-stress limit to keep this step's thermal input low: too much heat does not just risk cracking the membrane, it risks quietly converting the clean rectifying contact this series took seventeen and eighteen steps to establish into something electrically different.

## 2. Real Diagram: Two Independent Failure Budgets, Both Capped by the Same Bonding Step

This Bond Has Two Separate Ways to Fail, Not One a budget from step sixteen and a new one from step seventeen, enforced together here mechanical failure, step 16's budget bond force > σ_fracture membrane cracks electrical failure, this step's own budget bond heat too high barrier height φ_B degrades a bonding process has to clear both ceilings, not just the one any single step established alone

## 3. Pulse Bonding Trades Peak Force and Duration to Satisfy Both Budgets at Once

$$E_{\text{bond}} = F_{\text{bond}}\cdot\Delta t_{\text{pulse}}, \qquad F_{\text{bond}} < \sigma_{\text{fracture}}A_{\text{membrane}},\ \ \Delta T(\Delta t_{\text{pulse}}) < \frac{\phi_{B,\text{tol}}}{\kappa}$$

A short, controlled pulse of bonding energy $E_{\text{bond}}$ — brief force applied for a short duration $\Delta t_{\text{pulse}}$, rather than sustained pressure and heat — lets this step satisfy step sixteen's force ceiling and Section 1's thermal ceiling simultaneously, because a short pulse delivers enough localized energy to form a reliable bond without sustaining either the force or the temperature long enough to approach either failure threshold. This is the same principle this series has already used twice before, in steps thirteen and fifteen, to control a process variable precisely in time rather than relying on sustained conditions — pulse bonding is this constraint's answer at the lead-attachment stage.

A Short Pulse Satisfies Both Ceilings at Once sustained force and heat would fail; a brief pulse of each does not time → force / temperature fracture / interdiffusion ceiling brief pulse, stays under both ceilings sustained condition, same peak, risks both failures peak amplitude is not the whole story — duration decides whether either ceiling is actually crossed

## Attach the Emitter Lead's Place in the Process Lineage

Attaching the emitter lead is step nineteen of Philco's surface-barrier process, immediately after the emitter contact has been electroplated and before the collector lead is attached on the opposite face. It is the step that must simultaneously respect step sixteen's mechanical fracture-stress budget and a thermal budget protecting the rectifying barrier step seventeen first established, and it resolves that dual constraint the same way this series has resolved every other tight timing requirement since step thirteen: by controlling duration precisely rather than relying on sustained conditions. Step twenty, attaching the collector lead, faces the identical dual constraint on the opposite, larger contact.

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