Silicon Gate 1967 Measure Switching Speed Improvement

# Step 8 — Measure the Switching-Speed Improvement This Self-Alignment Actually Buys: Ring Oscillators and the 5× Delay Breakthrough

## 1. What the Ring Oscillator Proves: A Greater Than 5× Stage Delay Collapse

When identical 11-stage ring oscillators fabricated side-by-side in metal-gate and self-aligned silicon-gate technologies are powered on an oscilloscope, the measurement delivers unambiguous physical proof of the self-aligned revolution: stage propagation delay drops from 12.6 nanoseconds to 2.4 nanoseconds—a greater than $5\times$ speedup. In the 1962 metal-gate process, every inverting logic stage was burdened by two compounding penalties: the massive Miller overlap capacitance ($C_{\text{gd}} \approx 105\ \text{fF}$) that multiplied input loading to over $1.2\ \text{pF}$, and the high threshold voltage ($V_{\text{th}} \approx 3.5\ \text{V}$) that starved the transistor of gate overdrive $(V_{GS} - V_{\text{th}})$. In the self-aligned silicon-gate process completed in Steps 1 through 7, both limitations are removed simultaneously: eliminating the mask alignment tolerance cuts effective stage input capacitance to $265\ \text{fF}$, while the $n^+$ degenerate work function drops threshold voltage to $1.2\ \text{V}$, boosting saturation drive current by over $6\times$ at standard 5-volt operation.

$$\tau_d = \frac{1}{2 N f_{\text{osc}}} \approx \frac{C_{\text{eff}} V_{DD}}{I_{\text{sat}}} = \frac{\left[C_{\text{ox}} W L + (1 + |A_v|) C_{\text{ox}} W x_{\text{lateral}}\right] V_{DD}}{\frac{1}{2} \mu C_{\text{ox}} \frac{W}{L} (V_{DD} - V_{\text{th}})^2}$$

where $N = 11$ is the number of inverter stages in the ring, $f_{\text{osc}}$ is the measured oscillation frequency, and $\tau_d$ is the propagation delay per logic gate. For the 1962 metal-gate ring oscillator, the circuit oscillated at $f_{\text{osc}} \approx 3.6\ \text{MHz}$, yielding $\tau_d = 1 / (2 \times 11 \times 3.6\times 10^6) \approx 12.6\ \text{ns}$. The self-aligned silicon-gate ring oscillator oscillates at $f_{\text{osc}} \approx 18.9\ \text{MHz}$, yielding $\tau_d \approx 2.4\ \text{ns}$. For the first time in history, MOS logic switches in the low single-digit nanoseconds, directly matching the speed of bipolar resistor-transistor and diode-transistor logic while consuming a fraction of the silicon area.

11-Stage Ring Oscillator Benchmark: 12.6 ns vs. 2.4 ns measured stage propagation delay on identical inverter ring architectures 11-STAGE CLOSED-LOOP INVERTER RING OSCILLATOR ... 11 Inverters in Loop ... Oscilloscope Probe 1962 METAL GATE RING OSCILLATOR fosc = 3.6 MHz τd = 12.6 ns / stage Throttled by 1,225 fF dynamic Miller load Sluggish switching edge; high gate delay 1967 SILICON GATE RING OSCILLATOR fosc = 18.9 MHz (5.25× faster) τd = 2.4 ns / stage Liberated by 265 fF dynamic load + high overdrive Crisp nanosecond transitions; bipolar-class speed τd,1962 / τd,1967 = 12.6 ns / 2.4 ns = 5.25× Speed Dividend Eliminating mask alignment overlap transforms MOS from a slow specialty device into a premier high-speed computing technology.

## 2. Real Diagram: Power-Delay Product and Supply Voltage Scaling

The speed improvement is accompanied by a revolutionary plunge in switching energy: the Power-Delay Product drops by more than $25\times$, while operation extends down to standard 5V rails.

Stage Delay vs. Supply Voltage and Power-Delay Product (PDP) low threshold voltage enables rapid switching on 5V supplies while reducing energy per transition PROPAGATION DELAY VS. SUPPLY VOLTAGE Supply Voltage VDD (V) → Stage Delay τd (ns) Fails below 5V (high Vth) 1962 Metal Gate Operates down to 3V 1967 Silicon Gate 5V Silicon gate delivers high speed at low operating voltages POWER-DELAY PRODUCT (PDP = C · V²) 176 pJ 1962 Metal Gate (1.22 pF @ 12V) 6.6 pJ 1967 Silicon Gate (0.265 pF @ 5V) >26× Reduction in Switching Energy The Definitive Figure of Merit: PDP collapses from 176 pJ to 6.6 pJ per transition. A circuit that switches 5× faster while burning 26× less energy per cycle changes the economics of digital computation.

## 3. Resolving the Long-Standing Speed Disadvantage of MOS

Since the invention of the point-contact transistor in 1947 and the planar bipolar transistor in 1959, bipolar technology reigned supreme in digital computers because minority-carrier charge injection and low base resistance enabled sub-10-nanosecond switching. When the MOSFET was demonstrated in 1962, it promised simpler fabrication and higher density, but engineers lamented its sluggish propagation speeds: at 12 to 15 nanoseconds per gate, MOS was too slow for mainframes and central processing units.

Step 8 documents the historical moment where that speed disadvantage was eliminated:
1. Measured 5× Latency Collapse: On identical 11-stage ring oscillator test structures, the self-aligned silicon-gate process reduces propagation delay from 12.6 ns to 2.4 ns.
2. The Overlap and Overdrive Compounding: This speed jump is not an incremental tuning of lithography; it is the compounding result of eliminating Miller feedback capacitance ($C_{\text{in}}$ drops by $4.6\times$) and lowering threshold voltage ($V_{\text{th}} \approx 1.2\ \text{V}$ delivers $6.4\times$ greater current drive at 5V).
3. The 26× Energy Dividend: Because switching energy scales as $C V^2$, cutting capacitance by $4.6\times$ and supply voltage from 12V to 5V reduces the Power-Delay Product from 176 pJ down to 6.6 pJ.

By proving that a MOS gate could switch in 2.4 nanoseconds while dissipating a fraction of bipolar power, Step 8 verified that the self-aligned silicon gate was ready to take over the digital computing world.

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