Surface Barrier 1953 Electroplate Emitter Top Pit

# Electroplate the Emitter Into the Top Pit: Why the Second Contact Cannot Simply Repeat the First

This step forms the same kind of metal-semiconductor barrier step seventeen just formed on the opposite face, using the identical Faraday's-law deposition relationship — but repeating step seventeen's procedure exactly would leave this device without the one geometric asymmetry it actually needs to work as an amplifier. A transistor needs an emitter that injects carriers and a collector that gathers them, and gathering is inherently harder than injecting: carriers leaving the emitter spread out as they cross the base, so a collector sized to match the emitter exactly would miss some of that spread, while an emitter that matches the collector exactly wastes none of its own footprint on margin it does not need.

## 1. Collection Efficiency Depends on Relative, Not Absolute, Contact Area

$$\alpha \approx 1 - \left(\frac{A_{\text{collector}}}{A_{\text{collector}}+A_{\text{loss}}(W_B)}\right)^{-1}$$

Current gain in this device depends on collection efficiency $\alpha$, which falls short of unity by an amount set by how much emitter-injected current spreads laterally beyond the collector's own footprint before it can be gathered — a spreading loss $A_{\text{loss}}(W_B)$ that grows with base width $W_B$. Because step fifteen already fixed $W_B$ at a specific target, this step and step seventeen jointly control the one remaining lever available to minimize that loss: making the collector's contact area larger than the emitter's, so the collector's footprint overlaps more of the spreading carrier stream than a matched pair of equal-sized contacts ever could.

## 2. Real Diagram: Matched Contacts Waste Collection; Asymmetric Contacts Recover It

Why This Pit Is Not Simply a Mirror of Step Seventeen's carriers spread crossing the base; the two contacts' sizes should not match matched contact sizes emitter collector, same size spread current misses collector edges asymmetric contact sizes emitter, small collector, larger spread current stays within collector footprint this is why step seventeen and this step cannot simply use identical procedures

## 3. Smaller Contact, Same Electroplating Law, Different Target

$$t_{\text{plate,E}} = \frac{I\,t\,M}{nF\rho_{\text{metal}}A_{\text{emitter}}}, \qquad A_{\text{emitter}} < A_{\text{collector}}$$

This step deposits metal using the identical Faraday's-law relationship step seventeen established, holding thickness to the same target — but over a deliberately smaller area $A_{\text{emitter}}$, which for the same deposited volume means this step must apply correspondingly less total charge than step seventeen did, not merely repeat step seventeen's current and time settings verbatim. Getting this step's area wrong in either direction costs the device something specific: an emitter plated too large approaches the collector's own footprint and erodes the asymmetry Section 1 showed the device needs, while an emitter plated too small reduces the total injected current the whole device has to work with in the first place.

The Same Law, a Smaller Target Area, a Different Charge Budget step seventeen's settings are a starting point, not a template to copy exactly step 17: collector, larger A more charge for same thickness step 18: emitter, smaller A less charge for the same thickness t_plate same target, A different, so Q must differ too copying step seventeen's charge setting would overplate this smaller pit this step's own calculation, not step seventeen's record, sets the right current-time product

## Electroplate the Emitter Into the Top Pit's Place in the Process Lineage

Electroplating the emitter into the top pit is step eighteen of Philco's surface-barrier process, immediately after the collector has been plated into the bottom pit and before the device's two leads are attached. It completes this device's active structure — two metal-semiconductor contacts, formed exactly as step seventeen's analysis explained, now deliberately sized unequally so the collector can gather the carrier spread the base's own width guarantees. Step nineteen, attaching the emitter lead, and step twenty, attaching the collector lead, give each of these two freshly formed contacts the external connection the rest of the finished device will depend on.

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