MOSFET Scaling 1970s Manage Hot Carrier Degradation

# MOSFET Scaling (1970s): Step 14 — Managing Hot-Carrier Degradation

High-field carriers start damaging the gate oxide they pass beneath. The same high longitudinal fields that drive velocity saturation (step 13) can also accelerate a fraction of channel carriers to energies high enough to be injected into the gate oxide near the drain, where they create trapped charge and interface states that shift threshold voltage and degrade transconductance over the device's operating life. Step 14 covers this hot-carrier injection mechanism and the 1970s design responses to it.

## 1. Channel Hot Electrons and Oxide Injection

Near the drain end of a short channel operating at high field, some electrons gain enough kinetic energy through the accelerating field to overcome the silicon-to-oxide energy barrier and get injected into the gate oxide, where they either tunnel through (if near the interface) or become trapped in oxide defect states. Trapped charge near the drain locally raises or lowers threshold voltage at that end of the channel and degrades peak transconductance, with the damage accumulating over the operating lifetime of the part rather than appearing immediately — making it a reliability concern rather than an initial-test concern.

## 2. Real Diagram: Hot-Electron Injection Near the Drain Junction

Hot-Carrier Injection Near the Drain Junction high-field carriers near the drain gain enough energy to be injected into the gate oxide Channel (Source Side) low field, carriers thermalized, no injection risk Channel (Drain Side, Pinch-Off Region) peak longitudinal field, carriers accelerated Gate Oxide Above Drain Edge hot carriers injected, trapped charge accumulates Long-Term Effect local Vt shift, transconductance degradation over life damage accumulates over operating life, making this a reliability spec rather than an initial-test spec

## 3. Why Shrinking the Channel Makes This Worse, Not Better

Hot-carrier injection rate rises sharply with peak channel field, and peak field rises as channel length shrinks at a given drain voltage — exactly the generalized-scaling compromise from step 3, where voltage was held higher than pure Dennard scaling recommended to preserve board compatibility, directly increases hot-carrier stress on the shortest devices in a process generation. This is one of the clearest places where the step 3 compromise shows up as a measurable reliability cost rather than a theoretical one.

## 4. 1970s Mitigation Approaches

Design-level responses included keeping drain-side doping gradients less abrupt (a lower peak field for the same voltage drop) and, in some processes, lightly doped drain-like grading of the junction near the gate edge — an early precursor to the more systematic lightly-doped-drain structures that became standard practice in later scaling generations beyond this series' 1970s window. Process-level responses focused on oxide quality (fewer initial trap sites to fill) and on setting conservative maximum operating voltage specifications specifically to bound hot-carrier lifetime, rather than relying on the oxide-breakdown field margin from step 1 alone.

## 5. Companion Diagram: Stage Sequence and Trend Summary

MOSFET Scaling (1970s): Step 14 stage sequence and schematic outcome trend for this step Channel Hot Electro… 1 Real Diagram: Hot-E… 2 Why Shrinking the C… 3 1970s Mitigation Ap… 4 sequence progression outcome trend (schematic) companion diagram: stage sequence and schematic trend for this step

## Managing Hot-Carrier Degradation's Place in the Process Lineage

This is step 14 of 20 in the MOSFET Scaling (1970s) series, tracing planar NMOS scaling from early 1970s field pressure through the fixed-voltage-scaling break that opens the adjacent HMOS (1978) series.

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