Silicon Grown Junction 1954 Test Transistor Action

# Test Transistor Action: Alpha Gain, Nanoampere Leakage, and Breakdown Verification

With the base lead wire pulse-alloyed in Step 31, the physical assembly of the silicon transistor is complete—but mechanical integrity and electrical continuity do not guarantee transistor action. A device can have three securely welded leads and exhibit two separate rectifying $p\text{--}n$ diodes, yet still produce zero current gain if the base contact failed to bridge the active $25\ \mu\text{m}$ base transport corridor. Step 32 is the decisive intermediate electrical screen where the device must prove that minority electrons injected by the emitter actually cross the base and are collected by the reverse-biased collector. Silicon's material properties create a starkly different electrical profile compared to 1951 germanium: breakdown voltages are double ($BV_{\text{CBO}} \approx 45\text{ to }60\ \text{V}$), collector reverse leakage drops by two orders of magnitude into the nanoampere domain ($I_{\text{CBO}} < 50\ \text{nA}$), while initial common-base current gain ($\alpha$) typically measures $0.90\text{ to }0.95$ due to shorter minority-carrier lifetime. Passing this test validates the device before it enters costly chemical passivation and hermetic can encapsulation.

## 1. Electrical Signature of Silicon Grown-Junctions: Silicon vs. Germanium

Silicon’s wider energy bandgap ($E_g = 1.12\ \text{eV}$ vs $0.66\ \text{eV}$ for Ge) transforms intermediate test expectations across every parametric dimension:
1. Collector Breakdown Voltage ($BV_{\text{CBO}} = 45\text{ to }60\ \text{V}$):
Because silicon's dielectric breakdown field ($E_{\text{crit}} \approx 3 \times 10^5\ \text{V/cm}$) is over twice that of germanium, collector junctions easily withstand reverse biases of $50\ \text{V}$ before impact ionization avalanche occurs. In 1951 germanium grown transistors, breakdown rarely exceeded $20\text{ to }30\ \text{V}$.
2. Sub-Microampere Reverse Leakage ($I_{\text{CBO}} < 50\ \text{nA}$ at 25°C):
Intrinsic carrier concentration in silicon ($n_i \approx 1.5 \times 10^{10}\ \text{cm}^{-3}$) is three orders of magnitude lower than germanium ($n_i \approx 2.4 \times 10^{13}\ \text{cm}^{-3}$). At room temperature, reverse saturation current is dominated by thermal generation within the space-charge layer, yielding leakage currents measured in low nanoamperes rather than the several microamperes typical of germanium.
3. Common-Base Current Gain ($\alpha_0 \approx 0.90\text{ to }0.95$):
Early silicon grown-junction transistors exhibited lower alpha values than germanium ($\alpha_{\text{Ge}} \approx 0.95\text{ to }0.98$):
* Minority Carrier Lifetime: Silicon grown from quartz crucibles contained trace oxygen and iron contaminants, limiting minority electron lifetime in the $p$-type base to $\tau_n \approx 5\text{ to }10\ \mu\text{s}$.
* Base Transport Factor: With $W_B = 25\ \mu\text{m}$, the transport factor $\beta_T \approx 1 - \frac{W_B^2}{2 L_n^2}$ and emitter injection efficiency $\gamma$ constrained common-base gain to $\alpha \approx 0.90\text{ to }0.95$ (corresponding to common-emitter gain $\beta = \frac{\alpha}{1 - \alpha} \approx 10\text{ to }20$).
* While modest by modern standards, $\alpha \ge 0.90$ provided robust, reliable signal amplification and met the stringent qualification thresholds for U.S. military avionics.

## 2. Real Diagram: Intermediate Probing Circuit and Collector Family Curves

## 3. The Functional Test Protocol: Diode Continuity vs. True Transistor Action

The intermediate test routine separates true minority-carrier transport from deceptive back-to-back diode conduction:
1. The Diode-Only Failure Mode:
A bar where the pulse bond missed the base corridor completely can still pass two separate two-terminal diode tests:
* Probing Emitter-to-Base checks the emitter-base diode.
* Probing Collector-to-Base checks the collector-base diode.
* If the base lead landed on dead, unmodulated bulk material, both diodes appear rectifying, but the device exhibits zero current gain ($\alpha \approx 0$). Injected emitter electrons recombine in dead crystal without reaching the collector.
2. Three-Terminal Operational Sequence:
* Stage 1 (Reverse Collector Screening): With emitter open ($I_E = 0$), $V_{\text{CB}}$ ramps to $45\ \text{V}$. Reverse current must satisfy $I_{\text{CBO}} \le 50\ \text{nA}$. Units exhibiting soft microampere leakage or premature breakdown below $40\ \text{V}$ are rejected.
* Stage 2 (Emitter Forward Turn-On): Emitter current is set to $I_E = 1.0\ \text{mA}$. The forward voltage drop $V_{\text{EB}}$ must measure between $0.65\ \text{V}$ and $0.75\ \text{V}$. A higher voltage indicates excessive base spreading resistance ($R_B$) or an incomplete alloy weld.
* Stage 3 (Differential Alpha Transfer): With $V_{\text{CB}} = 5.0\ \text{V}$, emitter current is stepped from $1.0\ \text{mA}$ to $2.0\ \text{mA}$. Collector current must increase by $\Delta I_C \ge 0.90\ \text{mA}$, confirming a common-base gain $\alpha \ge 0.90$.
3. Culling Before the Point of No Return:
Once a transistor enters Step 34 (Hermetic Sealing), it is enclosed in an expensive gold-plated Kovar can with lead glass seals and resistance-welded under inert gas. Culling non-functional bars at Step 32 eliminates scrap before incurring the material and labor costs of hermetic packaging.

## Test Transistor Action's Place in the Process Lineage

Testing transistor action is step thirty-two of this project's 1954 silicon grown-junction transistor series — following the pulse-bonding of the base lead wire, and preceding surface chemical passivation. It is the initial electrical gateway of Phase 5 (Testing, Encapsulation, & Qualification), separating viable amplifying devices from diode-only rejects. Step thirty-three, clean and passivate, takes the verified functional transistors through specialized chemical washes to stabilize junction surface states prior to hermetic sealing.