Surface Barrier 1953 Electroplate Collector Bottom Pit

# Electroplate the Collector Into the Bottom Pit: Where This Device's Name Actually Comes From

Every junction this project has documented before this step — point-contact's formed-point barrier, grown-junction's regrown crystal interface, alloy-junction's fired regrowth front, diffused-base's dopant-driven profile — is a junction between two differently-doped regions of the same semiconductor. This step forms something categorically different: a metal electroplated directly onto bare N-type germanium, at the floor of the pit step fifteen's etching left behind, creates a rectifying contact between a metal and a semiconductor, not between two semiconductor regions. This metal-semiconductor interface is the surface barrier this entire process is named for — formed here, for the first time in this series, rather than inherited from any earlier step.

## 1. A Metal-Semiconductor Contact Rectifies for a Different Reason Than a p-n Junction Does

$$\phi_B = \phi_M - \chi_s$$

Where every earlier device in this project's histories built its rectifying junction from a doping discontinuity inside one semiconductor crystal, this contact's barrier height $\phi_B$ comes from the difference between the plated metal's work function $\phi_M$ and the germanium's electron affinity $\chi_s$ — a property of which two materials touch, not of how either one is doped. This is why electroplating, not alloying or diffusion, is the correct tool for this step: the junction this process needs is not grown or fired into existence, it is simply formed the instant the right metal contacts bare, clean germanium, which is exactly what step sixteen's rinse existed to make possible.

## 2. Real Diagram: A Junction Formed by Contact, Not by Regrowth

Every Prior Junction Was p-n; This One Is Metal-Semiconductor this is the step that makes "surface barrier" a literal description, not a brand name every earlier device in this project N-type P-type junction = doping discontinuity this step N-type Ge plated metal junction = contact itself this is the device's one genuinely novel electrical structure, not just a novel way to reach a familiar one

## 3. Plating Thickness Follows the Same Law as Etching, Running in Reverse

$$t_{\text{plate}} = \frac{I\,t\,M}{nF\rho_{\text{metal}}A}$$

Depositing metal into the pit obeys the same Faraday's-law relationship step eleven's etching current already established, with the sign of material flow reversed: plating current $I$ applied for time $t$ deposits a thickness $t_{\text{plate}}$ set by the metal's molar mass $M$ and density $\rho_{\text{metal}}$ over the plated area $A$. This step's target is not merely "fill the pit" — overplating risks the metal spreading past the pit's edge toward the fragile membrane region step sixteen's rinse was built to protect, while underplating leaves an incomplete contact that cannot form a reliable barrier. The same electrical control this process used to stop etching at the right depth now controls stopping plating at the right thickness.

Plating Needs Its Own Endpoint, for a Different Failure Mode too little metal fails the contact; too much metal threatens the membrane target plated volume underplated — weak contact overplated — risks membrane t_plate = I·t·M / (nF·ρ_metal·A) — the same law, deposition instead of dissolution

## Electroplate the Collector Into the Bottom Pit's Place in the Process Lineage

Electroplating the collector into the bottom pit is step seventeen of Philco's surface-barrier process, immediately after the etched wafer has been rinsed and before the emitter is formed on the opposite face. It is the step that actually creates this device's active electrical structure — a metal-semiconductor rectifying contact formed by direct electroplating, not a p-n junction formed by any of the thermal or diffusion techniques every earlier device in this project's process histories relied on. Step eighteen, electroplating the emitter into the top pit, repeats this same contact-forming operation on the opposite face, completing the second half of the device's active structure.

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