Point Contact Transistor Forward Bias Emitter Inject Holes
# Apply a Small Positive Emitter Voltage Relative to the Base: The Circuit Finally Does What Step 1 Chose It For
Step 17 decided which terminal would be common, but a topology with no voltage applied anywhere is still just wiring. Step 18 is the first instruction in this sequence that actually drives current through the device, and it drives it in the one direction this entire series has been built around since its first operation: a small positive voltage on the emitter, measured relative to the base Step 17 established as the shared reference, forward-biases that junction and injects holes into the N-type germanium. Step 1 chose N-type germanium specifically because it made holes, not electrons, the injected minority species. Every step since has prepared a path for holes to travel. This is the step where holes actually start moving.
"Relative to the base" is not a measurement convenience; it is the instruction that actually uses Step 17's decision. Before the base was established as a common reference, "the emitter voltage" had no single meaning — it could be measured against the collector, against some external ground, against anything. Step 17 fixed the base as the one node both loops share, and this step is the first to actually specify a quantity against that fixed point. Every voltage this device will ever be driven with, from this step through Step 22, inherits its meaning from the same reference, which is exactly why Step 17 had to be settled before this step could be written at all.
"Small" is bounded on both sides, and getting it wrong in either direction defeats a different earlier step. Too small a forward voltage injects too few holes to produce a current worth collecting, wasting every step from Step 1's diffusion-length argument through Step 14's forming pulse on a junction that is technically forward-biased but not usefully so. Too large a forward voltage pushes more current through the point contact than the alloy region Step 14's forming pulse created was calibrated for, risking the same kind of localized heating and degradation that made the forming current itself a bounded quantity rather than an unlimited one. There is also a quieter failure between those two extremes: this device is meant to operate as a small-signal amplifier, and a bias point that is technically safe but not actually small pushes the whole operating point into a region where the later small-signal modulation in Step 21 no longer produces a proportional response at the output in Step 22.
This step also commits the device to a continuous, standing condition rather than a momentary test. Step 14's forming pulse was brief and deliberately transient, applied once to shape the point contact and then removed. The bias this step applies is meant to hold steady for as long as the device operates, which means any marginal decision here — a voltage slightly too high, a connection slightly resistive — is no longer a one-time risk but a sustained one. A point contact that tolerated a brief, slightly excessive pulse during forming may not tolerate the same excess held indefinitely as an operating bias.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 18.1 | Determine the target emitter-to-base voltage from the device's forming current and intended small-signal operating range | Forming current record from Step 15.6, common-base topology from Step 17 | Target bias voltage | Voltage range that injects a useful hole current without approaching the forming current's upper bound | A target set without reference to the forming record risks a standing current comparable to the transient the point was only ever qualified to survive briefly |
| 18.2 | Confirm polarity: positive at the emitter relative to the base | Target bias voltage from 18.1 | Confirmed forward-bias polarity | Emitter measured positive relative to the common base node established in Step 17 | The opposite polarity reverse-biases this junction instead, which blocks injection rather than producing it |
| 18.3 | Apply the bias voltage and verify the resulting current against the target from 18.1 | Confirmed polarity from 18.2, sealed device from Step 16 | Biased emitter junction, current-verified | Measured injected current within the range specified in 18.1 | A voltage within the target range that produces a current outside it indicates a device-specific deviation worth investigating before proceeding |
| 18.4 | Verify the bias point sits within the device's small-signal linear region | Current-verified bias from 18.3 | Confirmed small-signal operating point | Bias point sits where the current-voltage relationship remains approximately linear over the range Step 21's signal will span | A bias point outside this region makes Step 22's amplification measurement meaningless, regardless of how correctly Steps 18.1 through 18.3 were followed |
| 18.5 | Hold the bias steady and monitor for drift over the measurement period | Confirmed operating point from 18.4 | Stable, sustained forward bias | No significant drift in current or voltage over the duration of later measurements | A bias that drifts during Steps 19 through 22 confounds those measurements with a variable this step was supposed to have fixed |
| 18.6 | Record the applied voltage, measured current, and verified operating point against the device's complete record | Stable bias from 18.5 | Documented forward-bias condition | Voltage, current, and operating-point verification recorded against every provenance record since Step 2.6 | Without this record, a later amplification result cannot be traced back to a specific, confirmed bias condition |
Step 18 does not introduce a new requirement; it is the step where every earlier requirement finally has to pay off at once. The diffusion length Step 1 verified, the point spacing Step 13 measured, the alloy region Step 14's pulse created, the base reference Step 17 established — none of it produced a single hole of injected current until this step actually applied a voltage. This is also the first step whose correctness cannot be fully judged by inspection alone, the way a spacing or a roughness could be measured directly; it has to be judged by whether the current it produces behaves the way the entire preceding sequence predicted it would.