Bicmos Logic 1980s Watch Pure CMOS Scaling Close the Drive Current Gap

# BiCMOS Logic (Late 1980s): Step 19 — Watch Pure CMOS Scaling Close the Drive-Current Gap on Its Own

## 1. The Premise Step 1 Opened Starts to Erode

Step 1 opened this series with pure CMOS's drive-current weakness at the output pin. Through the early 1990s, continued geometry scaling — shorter channel lengths, thinner gate oxides, higher transconductance per unit width — steadily increased how much drive current a given-size pure CMOS output transistor could supply, independent of anything BiCMOS-specific, simply as a consequence of the same scaling trend that had already driven CMOS's density and power advantages throughout the `cmos_logic_1980s` series.

## 2. The Crossover Point From Step 10 Moves, It Does Not Vanish Overnight

This scaling did not make BiCMOS's drive-current advantage disappear instantly; it moved the crossover point from Step 10 — the load capacitance above which BiCMOS wins — further out, to progressively larger loads, as each new CMOS generation closed part of the gap pure CMOS had with bipolar. For a growing fraction of the nodes BiCMOS had been used for in Steps 12 through 14 — ordinary bus drivers, moderate fan-out nets — a newer-generation pure CMOS output stage eventually became fast enough on its own that paying BiCMOS's Step 16 process premium for those specific nodes no longer made economic sense, even though the hardest cases — the very largest loads, the tightest timing budgets — continued to favor bipolar output stages for longer.

## 3. Companion Diagram One: Quantitative View

The Crossover Point Migrates as CMOS Scaling Continues crossover load capacitance against process generation, successive CMOS scaling steps crossover point, late-1980s process crossover point, early-1990s process, several generations later the gap that scaling steadily closes the load above which BiCMOS won in Step 10 same ranking rule, now requiring a much larger load each new CMOS generation needs a larger load before BiCMOS pays off the physics of Steps 6 through 10 is unchanged — only where the line falls has moved

## 4. Companion Diagram Two: Structural View

A Shrinking Region Where BiCMOS Still Wins nodes that justified bipolar output stages, across successive scaling generations LATE 1980s wide region above the crossover — many nodes qualify EARLY 1990s crossover has moved right — fewer nodes qualify HARDEST CASES ONLY largest loads, tightest timing — still favor bipolar the region BiCMOS was useful for narrows gradually, it does not close all at once

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