Fig. 1 — Top-down layout. Three shared-drain pairs (1,2 / 3,4 / 5,6). Drain M1 bus (gout) tapped at left; narrow bus segments add 2 Ω per hop. Sources strapped up to the M1 GND rail; PP/OD bulk tap ring at far left.
1.2 Layer legend
Layer
Color
Use in Fig. 1
M1
GND rail, source straps, gout drain bus + stubs
CO
Contact columns on every S/D stripe and bulk tap
PO
Gate fingers 1–6
OD
Active band (S/D diffusions) + bulk-tap active
NP
N+ implant enclosing the NMOS active
PP
P+ implant for substrate tap ring
2. Resistance ladder & current-share estimate
Assumes RRPO ≈ 8 Ω per finger (from prior sizing). Total per-pair ballast = RRPO,pair (≈ 4 Ω per shared-drain pair, two fingers in parallel) + M1 ladder R. Shares computed per pair by conductance ratio.
Rank
🖐️ Pair
🛣️ M1 R from tap
🧊 + pair RPO
Σ R to tap
⚡ Est. current share
📊
#1
Fingers 1, 2 (D₁₂)
0 Ω
4 Ω
4 Ω
~46%
#2
Fingers 3, 4 (D₃₄)
2 Ω
4 Ω
6 Ω
~31%
#3
Fingers 5, 6 (D₅₆)
4 Ω
4 Ω
8 Ω
~23%
3. Reading notes — what the asymmetry does during a zap
Fingers 1,2 are the new green pair. With the gout tap at the left, the M1 ladder steers ~2× the current of pair 5,6 into pair 1,2 (46% vs 23%). The nearest pair to the tap point runs hottest and filaments first — the layout reproduces exactly the mixed-ballast lean case (0 / 2 / 4 Ω on top of ~4 Ω pair RPO ≈ 4/6/8 Ω totals → ~60/40-style lean spread over three rungs).
Fixes that flatten the ladder: center-tap the gout bus between D₃₄ (drops worst-case to 2 Ω), widen the bus so each hop <0.5 Ω, or fishbone-tap all three drain stubs to a wide trunk. Alternatively exploit it: intentionally shorten RPO on pairs 3–6 by the metal delta so Σ R matches 8 Ω on all rungs.