Alloy Junction 1952 Place the Collector Disk

# Place the Collector Disk: Why It Goes On Larger, and Why It Goes On First

RCA's documented example placed a collector disk measurably larger than the emitter disk that would later go on the opposite face — not an arbitrary asymmetry, but a direct consequence of what the collector junction is actually required to do. A collector has to gather substantially all of the minority carriers the emitter injects into the base, and those carriers don't travel in a straight line across the base thickness alone — they also spread laterally as they diffuse, so a collector junction sized to exactly match the emitter's footprint would miss every carrier that wandered sideways during transit. Placing this disk first, before the emitter is ever positioned, also does something else: it sets the only coordinate reference this featureless wafer face has, which the opposing disk's later placement will have to align against rather than measure independently.

## 1. Collection Efficiency Requires a Radius Margin, Not Just a Larger Area

$$r_C \ge r_E + L_p^{\text{lateral}}$$

For the collector to intercept substantially all of the carriers the emitter will inject, its radius $r_C$ has to exceed the emitter's radius $r_E$ by at least the lateral spread $L_p^{\text{lateral}}$ a minority carrier accumulates while diffusing across the base — a sideways application of the same diffusion-length physics step one introduced for longitudinal transit survival, now governing how far a carrier wanders perpendicular to its average direction of travel rather than how far it gets before recombining. A collector only as large as the emitter's own footprint systematically loses every carrier that diffused outward past the emitter's edge before reaching the base-collector junction, which shows up as a current gain lower than the device's bulk lifetime alone would predict.

## 2. Real Diagram: The Margin Carriers Actually Need

Carriers Spread Sideways While Crossing the Base a collector sized to the emitter's own footprint misses the carriers that wandered collector disk, r_C emitter footprint, r_E lateral wander during base transit L_p^lateral this margin, not a larger area for its own sake, is what the collector's size has to buy

## 3. The First Disk Placed Becomes the Reference the Second Must Align Against

$$\vec{r}_C = \vec{r}_0 + \vec{\epsilon}_{\text{placement}}$$

This wafer face carries no built-in alignment mark before this step — the collector disk's own placement vector, $\vec{r}_C$, relative to an intended center $\vec{r}_0$ and a real placement error $\vec{\epsilon}_{\text{placement}}$, is the first physical coordinate this assembly actually has. Because the collector goes on first, its placement error isn't something a later step corrects; it's the baseline the emitter disk's eventual alignment has to measure against on the opposite face, through a wafer that is, from the emitter side, completely opaque to where the collector actually landed. Getting this disk centered well doesn't just serve this junction alone — it determines how much alignment budget is left for the one placed later.

Placement Error Here Becomes the Reference, Not a Correction Target nothing downstream re-measures where the wafer's true center actually is wafer, intended center r_0 collector, placed at r_0 + ε ε_placement step seventeen's job: align the emitter to this actual point the emitter disk aligns to where the collector landed, not to the wafer's nominal center

## Place the Collector Disk's Place in the Process Lineage

Placing the collector disk is step thirteen of RCA's forty-two-step alloy-junction manufacturing sequence — the first disk-placement step of Phase Two, immediately after the positioning fixture was loaded, and before this disk is fired into the wafer at all. It is the step that fixes two things simultaneously: the collector's physical size, chosen to exceed the eventual emitter footprint by enough margin to collect carriers that diffuse laterally during base transit, and the only spatial reference this wafer face has, which step seventeen's alignment of the opposing disks will have to work against rather than independently establish. Step fourteen, the first firing, is what actually bonds this disk in place.

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