Lilienfeld 1926 Attach Terminal Leads Film
# Attach Current-Carrying Terminals to the Film: Two Contacts Define the Measurement Axis
The copper-sulfide film deposited in Step 4 has no way to carry a measurable current until two separate ohmic contacts are attached to it. These two terminals — not the buried control electrode — are the current-carrying path that the control electrode modulates. Their placement is not a minor mechanical detail: where they land relative to the buried electrode determines how much of the film's area actually participates in field-effect modulation, and how much carries current that the control electrode cannot influence at all.
Contact method matters as much as position. A terminal pressed against the film without good mechanical contact behaves as an added series resistance of its own — a contact resistance distinct from the film's intrinsic resistance. Lilienfeld-era construction options include: a small metal clip or spring contact pressed against the film surface, a conductive paint or paste applied and dried, or a thin wire mechanically wound and pressed into the film during its still-soft growth stage. Each introduces a different, non-zero contact resistance, and this resistance adds directly in series with whatever the control electrode modulates — degrading the measured ratio between on-state and off-state current.
The two terminals cannot be symmetric with respect to the electrode and still measure the field effect cleanly. If both terminals are placed exactly over the electrode's center, there's no current path length over which the field can act — the terminals would short across the modulated region rather than passing current through it. The terminals must straddle the electrode's span, so current is forced to travel across the full field-affected region, not around it.
Terminal span should equal, not exceed, the buried electrode's span. If the terminals are placed wider than the electrode, the film between each terminal and the nearer electrode edge carries current but is never field-modulated — effectively adding unmodulated series resistance on both sides. If the terminals are placed narrower than the electrode span (inside it), the current path is shorter than the fully modulated region, which is not harmful in itself but wastes the electrode's full potential modulation range.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 5.1 | Determine terminal positions relative to buried electrode span | Film-coated assembly from Step 4, known electrode geometry from Step 3 | Marked terminal positions | Terminal-to-terminal span matches electrode span, ± 2 mm | Mismatch in either direction reduces the fraction of current path that is field-modulated |
| 5.2 | Select contact method (clip, paste, or embedded wire) | Marked assembly from 5.1 | Chosen contact method | Method selected per intended use: reversible (clip) for test articles, permanent (paste/wire) for final devices | Each method trades contact resistance against reversibility and durability |
| 5.3 | Prepare contact material (if paste or wire) | Chosen method from 5.2 | Prepared contact material | Conductive paste: silver-loaded, low cure temperature (< 80 °C to avoid film damage); wire: fine gauge, pre-cleaned | High-temperature cure methods risk damaging the copper-sulfide film or the electrode bond beneath |
| 5.4 | Apply terminal 1 (T1) at marked position | Prepared material from 5.3, position from 5.1 | T1 attached | Contact area: 2–4 mm² minimum for low contact resistance; mechanically secure | Undersized contact area raises contact resistance; loose contact drifts over time or vibration |
| 5.5 | Apply terminal 2 (T2) at marked position, opposite span end | Prepared material from 5.3, position from 5.1 | T2 attached | Same specification as 5.4, mirrored position | Asymmetric contact quality between T1 and T2 biases the measured current direction-dependence |
| 5.6 | Cure or set contacts per method (if applicable) | Both terminals from 5.4, 5.5 | Cured/set terminal assembly | Per material spec; avoid exceeding film-safe temperature limits | Overcuring risks oxidizing the film surface or embrittling the contact material |
| 5.7 | Verify terminal-to-terminal resistance at zero control bias | Cured assembly from 5.6 | Baseline-measured device | Resistance repeatable within 5% across 3 consecutive measurements | High variance indicates unstable or marginal contact, not yet ready for modulation testing |
| 5.8 | Verify terminal contacts are electrically isolated from the control electrode | Measured device from 5.7 | Isolation-verified device | Resistance between either terminal and the electrode: effectively open circuit (> 1 MΩ) | A low-resistance leak path between a terminal and the electrode shorts the control signal directly into the measurement path, corrupting any modulation reading |
Step 5 completes the three-terminal structure that every subsequent test in the 1925 series protocol depends on. The two film terminals (T1, T2) are what later measurement steps call the 11 and 12 terminals; the buried control electrode is terminal 13. Step 5.8's isolation check is the device-construction analog of the leakage measurement protocol established for the 1925 specimens — confirming before any bias is ever applied that the control path and the current-carrying path are genuinely separate circuits, joined only through the field, not through a direct electrical fault.
Why terminal attachment is the last purely mechanical step. After Step 5, the device has everything needed to be tested: two current-carrying contacts spanning a field-modulated region, and a control electrode isolated from both but capacitively coupled to the film beneath it. Every step from here forward is electrical characterization, not construction — the same baseline current-voltage, leakage, and bias-response protocols already established for the 1925 specimens can now be applied to this 1926 construction.