Alloy Junction 1952 Position the Base Tab

# Position the Base Tab: The One Contact That Isn't Supposed to Form a Junction

Every metal this process has placed on the wafer so far was chosen specifically to dissolve germanium and leave behind a P-type region when it regrows — the base tab is the first contact in this entire process whose entire job is to do the opposite. Indium alloys deliberately; nickel, the material RCA's documented example used for this tab, has to make electrical contact to the remaining N-type germanium without forming a rectifying junction there at all. Getting this wrong doesn't threaten a dimension or a budget the way earlier steps' failures did — it threatens the base connection's basic ability to conduct current in both directions, which a rectifying contact simply can't do.

## 1. An Ohmic Contact Needs a Barrier Low Enough to Not Rectify

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

The Schottky-Mott relation sets the barrier height $\phi_B$ a metal-semiconductor contact presents as roughly the difference between the metal's work function $\phi_M$ and the semiconductor's electron affinity $\chi_s$ — and for this contact to behave ohmically rather than like a second, unwanted diode, that barrier has to stay low enough that current flows freely in either direction rather than being blocked on one side the way the emitter and collector junctions are deliberately designed to block current on one side of their own barriers. This is the same underlying metal-semiconductor physics every contact in this device depends on, but aimed at the opposite outcome from every other contact this process has made.

## 2. Real Diagram: Two Contacts, Two Opposite Jobs, Same Wafer

A Rectifying Contact Beside an Ohmic One both are metal touching germanium — only the barrier height differs emitter / collector, indium high barrier — rectifies by design P-type region forms underneath base tab, nickel low barrier — conducts both ways N-type germanium, unchanged underneath same furnace, same wafer — the metal's own work function decides which outcome results

## 3. The Tab's Footprint Has to Stay Entirely Outside Both Junction Areas

$$A_{\text{tab}} \cap \left(A_{\text{collector}} \cup A_{\text{emitter}} \cup A_{\text{margin}}\right) = \varnothing$$

Beyond getting the contact physics right, this step has a purely geometric requirement step seventeen's overlap measurement made concrete: the tab's footprint has to avoid both junction footprints entirely, with a safety margin $A_{\text{margin}}$ added around each, because landing even partially on exposed emitter or collector material would short the base connection directly across a junction the rest of this process spent seventeen steps building correctly. This is the opposite instinct from step seventeen's alignment problem — there, the goal was maximizing intentional overlap between two regions; here, the goal is guaranteeing zero overlap between the base contact and either of them.

Wafer Face, Viewed From Above the base tab has to land entirely in the open N-type area left over wafer face junction + margin, keep-out zone base tab the tab sits in open N-type germanium, fully clear of the dashed exclusion boundary

## Position the Base Tab's Place in the Process Lineage

Positioning the base tab is step eighteen of RCA's forty-two-step alloy-junction manufacturing sequence — immediately after the two disks were aligned, and before the furnace atmosphere is established for the main firing. It is the first step in this process to require a contact that explicitly does not form a junction, relying on a metal whose work function keeps the barrier low enough to conduct in both directions, and it is the step that converts step seventeen's overlap measurement into a hard exclusion zone this contact's own footprint has to clear entirely. Step nineteen, establishing the furnace atmosphere, is the first step of the main firing sequence this tab now has to survive electrically intact.

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