Micrologic 1960 Step 12 Bipolar over Mos Logic Choice

# Choose Bipolar Transistors Over Any Field Effect Alternative: What Practical MOS Logic Could Not Yet Deliver

## 1. Why the Device This Series Already Trusted Was the Only Real Candidate

This step explains a choice that earlier steps made silently: every transistor in Micrologic is a bipolar junction device, diffused by the same process this project has refined since 1954, and not a field-effect device switched by a gate oxide, because a field-effect transistor built with 1960's oxide and surface-cleanliness control could not yet deliver the threshold stability a production logic family needed. A field-effect channel's threshold voltage depends on surface-state charge trapped at the oxide-silicon interface, and in 1960 that trapped charge drifts with time, temperature, and bias history in ways the planar process's own oxide, proven for isolation and masking rather than for carrying a gate signal, had never been characterized to control. A bipolar junction transistor has no such interface to drift; its switching threshold is set by a diffused junction's built-in potential, a quantity this series has measured reliably since Step 2.

$$V_{T,\text{FET}} = f(Q_{\text{surface}}, t, T), \qquad V_{BE,\text{on}} \approx 0.6\text{ to } 0.7\ \text{V, stable}$$

where $V_{T,\text{FET}}$ denotes a field-effect threshold as an unresolved function of trapped surface charge, time, and temperature, and $V_{BE,\text{on}}$ denotes a bipolar turn-on voltage already pinned to a narrow, repeatable range by the diffusion recipes this series inherited — a production logic family cannot ship a part whose switching point is the left-hand side's open question.

A Stable Threshold Versus an Unresolved One bipolar turn-on voltage against field-effect threshold drift, in 1960 time and temperature since manufacture → switching threshold bipolar V_BE, on diffusion 1960 field-effect threshold, drifting a production logic family needs the flat line, not the drifting one

## 2. Real Diagram: The Interface That Bipolar Devices Simply Do Not Have

A bipolar transistor's active region is buried under the surface, inside the diffusion itself. A field-effect transistor's channel runs along the surface, directly beneath the gate oxide, exactly where 1960's oxide process leaves the most uncontrolled trapped charge — the device types differ in nothing more consequential than where their active region sits relative to that one unreliable interface.

Where the Active Region Sits buried junction versus surface channel, relative to the oxide interface BIPOLAR, THIS SERIES surface oxide, not in the signal path buried junction, active region FIELD EFFECT, NOT YET VIABLE gate oxide directly over the channel channel, right at the interface the active region far from the interface is the one this series can actually ship

## 3. Why This Choice Was Not Conservatism but a Correct Read of What 1960 Could Deliver

Nothing in this series' prior nine steps forced bipolar devices as a rule; Step 11's universal NOR gate could, in principle, have been built from a different kind of switch. The reason it was not is that a field-effect logic family in 1960 would have inherited an unsolved surface-physics problem into every single gate this series needed to qualify under Step 9's military specification, and Step 7's whole-chip yield penalty would have multiplied that instability across every transistor on a die before any customer ever saw a part. Bipolar devices let this series ship on the schedule Step 10's commercial classification actually met.

This is not a verdict against field-effect logic as an idea — only a statement that the interface it depends on was not yet a controlled one in 1960, and that a commercial logic family could not afford to be the project that controlled it first.

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