Point Contact Transistor Reverse Bias Collector Attract Holes
# Apply a Negative Collector Voltage Relative to the Base: The Same Point, the Opposite Instruction
Step 18 made the emitter point inject by applying a small positive voltage relative to the base. Step 19 applies the opposite sign, a negative voltage, to the collector point — built the same way, formed by the same current-pulse process, mounted on a spring to the same tolerance, but now instructed to do the opposite job. Nothing about the collector's physical construction distinguishes it from the emitter; Steps 10 through 14 built both points by the same procedure. What separates them, electrically, is this single instruction: the sign of the voltage applied relative to the base Step 17 established as the shared reference. A forward-biased point injects. A reverse-biased point collects. The two points earn their different names from bias polarity, not from anything the forming process built differently into one versus the other.
Reverse bias does not pull current through the collector the way forward bias pushes current through the emitter; it widens a field that sweeps up whatever survives the trip. A positive emitter voltage forces majority carriers across a forward-biased junction, actively injecting holes into the bulk. A negative collector voltage does not inject anything — it widens the depletion region at the collector junction and strengthens the field across it, so that holes already diffusing through the bulk, carried there by Step 18's injection and surviving the crossing Step 1's diffusion-length argument was built around, get swept toward the collector rather than simply drifting past it. The collector's entire contribution to current is passive in this sense: it does not create the carriers it measures, it only captures a fraction of the carriers the emitter already put into motion.
The magnitude of this reverse voltage is bounded by a property Step 1 once called irrelevant, and is now the single most important number in this step. Step 1's own comparison table listed reverse breakdown voltage as "not directly relevant" to the transistor requirement, because at the material-selection stage nothing in the process was yet operating the point contacts in reverse bias at all. This step is where that changes. A collector voltage too small produces a weak depletion field that fails to sweep up carriers efficiently, which both undercounts the current that actually reaches the collector and weakens the high-impedance condition Step 17's power-gain argument depended on. A collector voltage too large approaches the point contact's actual reverse breakdown voltage, the exact property Step 1 set aside as irrelevant to initial material selection — irrelevant then, because nothing was reverse-biasing anything yet, and critical now, because this step is the first operation in the entire sequence to test it directly.
The two bias instructions together finish what Step 17 only set up as a possibility. Step 17 established that the base could serve as a shared reference without forcing either point's role to compromise; Step 18 and Step 19 are the two instructions that actually use that reference to put the emitter and collector into their intended, opposite operating regimes at the same time. Neither instruction makes sense evaluated alone against the device's purpose — a forward-biased emitter with no reverse-biased collector has nothing collecting what it injects, and a reverse-biased collector with no forward-biased emitter has nothing to collect. This step completes a pair that Step 18 began, not a step that stands on its own.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 19.1 | Determine the target collector reverse voltage with margin below the point contact's reverse breakdown voltage | Point contact properties verified since Step 1.3, common-base topology from Step 17 | Target reverse voltage range | Voltage magnitude comfortably below the measured or estimated reverse breakdown voltage for this specific point | A target set without margin below breakdown risks a collector that fails outright rather than merely collecting inefficiently |
| 19.2 | Confirm polarity: negative at the collector relative to the base | Target reverse voltage from 19.1 | Confirmed reverse-bias polarity | Collector measured negative relative to the common base node established in Step 17 | The opposite polarity forward-biases this junction instead, turning the collector into a second emitter rather than a collecting contact |
| 19.3 | Apply the reverse voltage with the emitter already forward-biased per Step 18 | Confirmed polarity from 19.2, forward-biased emitter from Step 18 | Both junctions biased simultaneously | Collector current measured with the emitter bias from Step 18 already established and stable | Reverse-biasing the collector before the emitter is actually injecting measures nothing but the collector's own leakage |
| 19.4 | Verify collected current is consistent with the emitter current and the device's expected current-transfer behavior | Simultaneously biased device from 19.3 | Verified collector current | Collector current a plausible fraction of the emitter current from Step 18, consistent with diffusion survival across the spacing from Step 13/14 | A collector current far outside the expected fraction indicates a problem upstream in forming, spacing, or bias, not in this step alone |
| 19.5 | Verify no indication of approaching reverse breakdown under sustained operation | Verified current from 19.4 | Breakdown-margin-confirmed collector bias | No current runaway or instability as the bias is held steady over the measurement period | A collector operating close to breakdown can appear stable briefly and fail only once held under sustained bias, the same risk Step 18 flagged for the emitter |
| 19.6 | Record the applied reverse voltage, measured collector current, and breakdown margin against the device's complete record | Confirmed bias from 19.5 | Documented reverse-bias condition | Voltage, current, and margin recorded against every provenance record since Step 2.6 and alongside Step 18.6's emitter record | Without both bias records together, a later amplification measurement cannot be attributed to a specific, confirmed pair of operating conditions |
Step 19 does not introduce a new kind of point contact; it assigns the opposite electrical meaning to a point built by the exact same process as the one Step 18 just activated. Everything that makes the collector a collector rather than a second emitter is contained in the sign of this one voltage, measured against the same reference Step 17 established and held alongside the emitter bias Step 18 already set. The property this step now treats as its tightest constraint — reverse breakdown voltage — is the same property Step 1 dismissed as irrelevant to selecting the material in the first place, which is less a contradiction than a reminder that a property's relevance depends entirely on what operation is finally being performed with it.