MOSFET 1962 Step 15 Die Yield Economics Bipolar Comparison

# Measure the Yield Economics: Why a Simpler Device Did Not Mean a Cheaper One Yet

## 1. Fewer Process Steps, Not Yet Fewer Defects

A MOS transistor needs fewer photomasking and diffusion steps than a bipolar transistor of comparable function, which should mean higher yield and lower cost -- and in 1962-63 it did not, because the oxide-charge and mobile-ion sensitivities this series' Steps 7 and 14 describe made MOS yield highly dependent on contamination control bipolar lines had already spent a decade perfecting. Early commercial MOS yields ran well below contemporary bipolar yields on comparable die sizes, which meant the simpler device was, for its first several years on the market, often the more expensive one per working unit.

Fewer Masking Steps, Lower Working-Die Yield (1962-63) process simplicity did not yet translate to lower cost per unit 75 Bipolar mask steps 40 MOS mask steps 70 Bipolar yield 35 MOS yield MOS needed fewer steps but lost more die to contamination-driven threshold variation. The cost advantage from fewer steps did not survive this yield gap.

## 2. The Economics That Eventually Flipped

The yield gap closed over the following several years as oxide-cleanliness practices (the same practices named in Step 13's account of Fairchild's process-control program) matured industry-wide, and MOS's fewer-step cost advantage began to show up in actual unit economics by the mid-to-late 1960s -- the same period this project's siliconGate_1967 series covers for a different, specific process improvement. This step closes on the honest 1962-63 picture only: a technically simpler device that was not yet the cheaper one.

The Cost Argument, Honestly Timed simplicity promised savings; yield had to catch up first 1962-63 fewer steps lower early yield higher cost per working die MID-1960s cleanliness practices mature yield gap narrows cost advantage starts to show 1967 onward siliconGate process arrives named, not retold, here The cost story this step tells stops at the honest 1962-63 picture. Whether and when the fewer-step advantage became a real cost win is this project's later material.

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