Lilienfeld 1925 Prepare the Insulating Support
# Prepare the Insulating Support: Why the Base Material Has to Stay Electrically Invisible
Every later step in this 1925 field-effect concept depends on one current path existing between the two main terminals and nowhere else — a thin copper-sulfur film, deliberately conductive, sitting on top of an otherwise ordinary glass block. Before anything is deposited, split, or sulfurized, the choice of that glass block already has to satisfy a requirement nothing downstream can fix: its own bulk and surface conduction has to stay so far below the intended film's conductivity that the glass is, electrically, invisible. If the support itself carries any appreciable current between the terminals, the control electrode's field — the entire mechanism this concept depends on — has no way to act on a path it was never meant to control.
## 1. The Support Has to Lose the Conduction Contest by a Wide Margin
For the copper-sulfur film to be the only current path worth measuring, its resistance between the two terminals has to sit far below the glass support's own resistance over that same geometry. Expressed as a ratio of resistivities, scaled by whatever geometric factor relates the two parallel paths' cross-sections and lengths:
Ordinary glass's resistivity is routinely many orders of magnitude higher than a thin, deliberately conductive metal-sulfide film's — which is precisely why glass, rather than any arbitrary insulator, is a reasonable starting choice here. But the inequality is not automatically satisfied just because glass is "an insulator" in the everyday sense; it has to actually hold at the specific geometry Step 5 through Step 9 will later establish, which is the real reason this choice gets made deliberately, as its own first step, rather than assumed.
## 2. Real Diagram: One Intended Path, One Path That Must Stay Negligible
## 3. The Support Also Has to Survive Whatever Bias the Device Will Carry
Beyond keeping its own conduction negligible, the support has to physically withstand the electric field the finished device will actually impose on it once Steps 11 and 12 apply the main and control biases. A simple dielectric-withstand criterion sets a lower bound on how much voltage a given glass thickness can tolerate before breaking down:
where $E_{\text{bd,glass}}$ is glass's own breakdown field strength and $t_{\text{glass}}$ is the thickness of material actually separating the biased elements once the block is split and reassembled in Step 2 through Step 4. This is the same kind of air-gap and solid-dielectric withstand reasoning that shows up anywhere a thin insulator separates biased conductors, applied here at the very first step of a completely different device concept — before any film, terminal, or control electrode exists, the raw support material already has to be chosen with this bound in mind, since nothing later in the sequence can compensate for a support that was simply too thin or too conductive from the start.
## Real Diagram: The Resistivity Gap That Has to Hold
## Prepare the Insulating Support's Place in the Process Lineage
Prepare the Insulating Support is the first step of this 1925 field-effect concept, and the foundation every later step builds directly on top of; it precedes Step 2, Split the Support Transversely, which will cut directly into the very block this step selects and verifies as adequate. There is no step before it to reference, only the requirement every step from Step 2 onward assumes has already been satisfied: a support material whose own conduction stays far below the intended copper-sulfur film's, and whose dielectric strength comfortably exceeds whatever bias the finished device is meant to carry.