Diffused Base 1954 Attach Leads
# Attach the Emitter, Base, and Collector Leads: Three Connections, Two of Them Dangerously Close Together
This step attaches the device's three external leads — emitter and base both landing on the same small mesa top surface Step 21 just defined, and collector landing on the wafer's opposite face, where the bulk material this process never diffused into still does the collector's job. The emitter and base contacts sit only as far apart as the mesa's own small top surface allows, which is close enough that this step's real risk is no longer just making a good connection to each one — it is making both connections without the two leads, or the bonding tools placing them, ever touching each other.
## 1. Lead Separation on the Mesa Top Inherits a Proximity Risk This Process Has Not Faced Before
The separation $d_{\text{lead gap}}$ between the emitter and base leads on this mesa is set by how small this process made the device in the first place, a direct consequence of diffusing both junctions from one compact surface rather than aligning two separately placed alloy disks the way an alloy-junction device does. This step therefore works at a precision scale closer to what this project's earlier point-contact series already had to manage between two independently placed spring-loaded contacts than to what any junction-formed device in this project's history has faced before.
## 2. Real Diagram: Two Leads, One Small Mesa, Very Little Room for Error
## 3. The Collector Contact Inherits the Opposite Lesson — Area Solves Its Problem Entirely
The collector contact on the wafer's opposite face answers to a completely different design logic than the emitter and base contacts do — its spreading resistance falls as its own effective contact radius grows, which means this step can simply make the collector contact as large as the wafer's back face allows, with none of the proximity risk the top-side leads carry. This step therefore has to hold two genuinely different disciplines in the same operation: micrometer-scale placement precision on one face, and simple area maximization on the other, with no procedure that treats both sides identically actually serving either one well.
## Attach the Emitter, Base, and Collector Leads's Place in the Process Lineage
Attaching the emitter, base, and collector leads is step twenty-two of Bell Labs' twenty-six-step diffused-base manufacturing sequence — immediately after individual devices were isolated by mesa etching, and before any inspection of the completed assembly. It is the step where this process's own miniaturization, the direct consequence of diffusing both junctions from one compact surface instead of aligning separate alloy disks, becomes a genuine attachment-precision risk on the mesa top, answered by a completely different, far more forgiving discipline on the collector face beneath it. Step twenty-three, inspecting the completed assembly, is the first chance to confirm this step actually kept the two leads on the mesa top from ever touching.