Double Diffused Silicon Mesa 1956 Classify Accept Release
# Classify, Accept, Downgrade, or Reject: Where Nineteen Steps of Process Become a Decision About One Part
## 1. Why This Step Exists to Sort, Not to Fix
This final step takes the $f_T$ measurement from Step 19, along with the other electrical parameters gathered alongside it — current gain $h_{FE}$, breakdown voltage, leakage current — and sorts every finished device into a bin: accept for its intended high-frequency application, downgrade to a lower-speed grade where the drift advantage matters less, or reject outright. Nothing about the device changes at this step; it only decides, based on everything the prior nineteen steps actually produced, what that device is now allowed to be sold or used as. A device whose $f_T$ falls short of the target for a drift transistor but still clears ordinary diffused-transistor specifications is not scrapped — it is downgraded into service as an ordinary device, recovering value from a wafer that did not fully deliver on this process's specific goal:
where $f_{T,\text{baseline}}$ is roughly the cutoff frequency an ordinary diffusion-only device of similar geometry would achieve — the same comparison Step 19's second diagram drew, now turned into an actual sorting threshold rather than a diagnostic observation.
## 2. Real Diagram: A Project's Entire Lineage, Closing on the Measurement This Step Finally Acts On
This is the last step of the last device series this project has documented across nine years of transistor history. The diagram below closes the loop this project opened with the point-contact transistor in 1947, ending exactly where that lineage's defining question — can a transistor be made faster and more reliable through better control of its base region — reaches its most deliberate answer yet: a field engineered on purpose, not discovered by accident.
## 3. Why This Closing Step Carries More Finality Than the 1954 Diffused-Base Process's Equivalent Step
The 1954 diffused-base germanium process this project has already documented also ended with a classify-and-release step sorting devices by measured performance, so the mechanism of this step is not new. What is different is what the sorting threshold represents: in 1954, the dividing line was simply "does this device meet diffused-transistor spec," a question about manufacturing consistency. Here, the dividing line additionally asks whether this specific wafer successfully executed the one design intent — a deliberately graded, field-producing base — that distinguishes this entire twenty-step sequence from every simpler process this project has documented before it. A device landing in the downgrade bin here has not merely fallen short of a quality target; it has reverted, electrically, to the kind of transistor the 1954 process was already capable of making, two years and six steps' worth of added process discipline earlier.
Step 20 does not change anything about the physical device in front of it; it is the step where everything the prior nineteen steps built finally gets to matter, or not, depending on a number this step did not create but is the first to act on.