Diffused Base 1954 Inspect Completed Assembly

# Inspect the Completed Assembly: The Last Chance to Catch What the Mesa Sidewall Was Warned Could Go Wrong

This step checks terminal separation, electrical continuity, and junction isolation across the completed device, and it exists as the last non-destructive opportunity to confirm two specific risks this sequence has already flagged: that Step 22's lead attachment never actually bridged the emitter and base contacts on their shared mesa top, and that Step 21's exposed mesa sidewall is not already leaking more than this process can tolerate. Nothing before this step could check either risk directly — Step 22 could confirm its own leads were placed correctly without ever confirming they stayed electrically separate under the mechanical stress of bonding, and Step 21 could etch a clean sidewall without any guarantee that the surface it exposed is actually behaving the way an undisturbed planar junction would.

## 1. A Short Between Emitter and Base Reads as a Specific, Recognizable Failure Signature

$$R_{EB} \to 0 \quad \text{vs.} \quad R_{EB} \approx R_{\text{expected}}$$

A measured emitter-to-base resistance $R_{EB}$ collapsing toward zero is this step's direct signature of the proximity risk Step 22 carried — a lead bridge, a stray conductive path, or simple mechanical contact between two leads this process placed deliberately close together. This measurement either confirms Step 22's attachment held the separation it was supposed to, or it reveals, immediately and unambiguously, that it did not, which is exactly the kind of binary result a destructive technique like Step 16's groove-and-stain could never provide this quickly or this early.

## 2. Real Diagram: Two Specific Risks, Two Specific Measurements, One Inspection Step

This Step Closes Two Risks Earlier Steps Could Only Flag, Not Confirm Step 22's proximity risk and Step 21's sidewall risk both get their first real test here Step 22's risk emitter-base lead proximity tested: R_EB measurement Step 21's risk exposed mesa sidewall leakage tested: reverse leakage current this is the first and only inspection in the sequence positioned to confirm both of these specific, already-named risks

## 3. Sidewall Leakage Has a Specific Signature Too, Distinct From a Healthy Junction's Reverse Current

$$I_{\text{reverse}} = I_{\text{bulk}} + I_{\text{sidewall}}(\text{etch quality})$$

Measured reverse leakage current $I_{\text{reverse}}$ includes a bulk-junction component this process expects and a sidewall component $I_{\text{sidewall}}$ that depends entirely on how cleanly Step 21's etch actually left that exposed edge — a device whose reverse leakage runs well above what a healthy bulk junction alone would produce is showing exactly the sidewall degradation Step 21 warned this new surface could introduce. This step's acceptance threshold for reverse leakage therefore has to be set with that sidewall contribution already budgeted in, rather than borrowed from a planar-junction leakage standard that never had an exposed etched edge to account for.

Reverse Leakage Now Carries a Term No Earlier Series Had to Budget this step's threshold has to include the sidewall term, not just the bulk one bulk leakage expected, planar junction + sidewall leakage new, from Step 21's etched edge a threshold copied from an earlier series' planar-junction standard would silently pass a device failing only on this new term

## Inspect the Completed Assembly's Place in the Process Lineage

Inspecting the completed assembly is step twenty-three of Bell Labs' twenty-six-step diffused-base manufacturing sequence — immediately after the three leads were attached, and before this device is ever sealed inside an encapsulant. It is the only inspection positioned to directly confirm both the lead-proximity risk Step 22 carried and the sidewall-leakage risk Step 21 introduced, using two distinct measurements rather than one generic pass-fail check. Step twenty-four, encapsulating the device, proceeds only with assemblies this step has actually confirmed clear both risks.

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