Enclose the Transistor

# Enclose the Transistor: Why Plastic Doesn't Actually Close the Loop Step Twenty-Eight Opened

Step twenty-eight's rinse removed mobile ionic contamination before it could drift under the junction's own field, framed as a race against a hard deadline — this enclosure step. But not every enclosure actually ends that race the same way. Plastic encapsulation, used on early devices, protects the bar mechanically and keeps out gross contamination, but polymer is not a true moisture barrier: water vapor and the ions it carries diffuse through plastic slowly over years, which means a plastic-encased device's surface is never truly sealed from the environment the way step twenty-eight's rinse assumed the final enclosure would guarantee. A hermetically sealed metal can, by contrast, is an actual barrier — nothing diffuses through solid metal and a proper glass-to-metal seal, which is why later production moved to cans specifically for the reliability step twenty-eight's rinse was trying to protect.

## 1. Moisture Ingress Through Plastic Operates on a Timescale Nothing Else in This Process Has Used

$$x_{\text{moisture}}(t) = \sqrt{D_{\text{polymer}} \, t}$$

Every diffusion-length calculation this process has used before — zone refining, solder thermal budget, pulse bonding — operated on timescales of seconds to minutes during manufacturing. This equation is the same mathematical form, but $t$ here is the device's operating lifetime in the field, years rather than seconds, and $D_{\text{polymer}}$ describes how readily water vapor moves through the specific plastic chosen rather than how readily a dopant moves through germanium. Over a long enough service life, moisture ingress through a plastic encapsulant can reach the same surface step twenty-eight worked to clean, reintroducing the ionic drift mechanism that step's rinse was specifically designed to prevent — not from process residue this time, but from the operating environment itself.

## 2. Real Diagram: Two Enclosures, Two Very Different Guarantees

Plastic Protects Mechanically; Only the Can Seals Hermetically step twenty-eight's clean surface stays clean only behind a true barrier plastic encapsulation bar moisture, over years hermetic can bar nothing diffuses through metal both enclosures stop gross contamination; only one stops the slow kind

## 3. This Choice Determines Which Reliability Curve From Step Twenty-Eight Actually Applies

$$\text{plastic:} \;\; I_{\text{leak}}(t) \nearrow \text{ over years}; \qquad \text{hermetic can:} \;\; I_{\text{leak}}(t) \approx \text{constant}$$

Step twenty-eight's leakage-drift diagram compared a rinsed-clean device against a residue-contaminated one, both held at a fixed enclosure condition. This step adds a second variable to that same picture: even a perfectly rinsed device can still drift over time if its enclosure doesn't actually keep the environment out, which means the enclosure choice made here determines whether step twenty-eight's careful cleaning translates into a genuinely stable device or only a temporarily stable one. A hermetic can is what makes step twenty-eight's investment pay off for the device's entire service life rather than just its first several years.

Leakage Over Service Life, Same Rinsed-Clean Device step twenty-eight's clean start, two different enclosure outcomes service life, years → leakage current hermetic can — stays clean indefinitely plastic — moisture eventually reaches the surface both devices passed every test through step thirty-three identically

## Enclose the Transistor's Place in the Process Lineage

Enclosing the transistor is step thirty-four of the 1951 grown-junction transistor's full manufacturing sequence — immediately after the completed assembly has been inspected, and before the device is aged or stabilized. It is the step that finally determines whether step twenty-eight's mobile-ion cleaning holds for the life of the device or only until environmental moisture eventually reaches the same surface through an encapsulant that was never actually a true barrier.

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