MOSFET Scaling 1970s Shrink Source Drain Junction Depth

# MOSFET Scaling (1970s): Step 8 — Shrinking Source/Drain Junction Depth

Shallow junctions become necessary once channel length drops below a few micrometers. Dennard scaling calls for junction depth to shrink along with every other linear dimension, because a source/drain junction that stays deep while the channel shortens starts to dominate short-channel electrostatics — its depletion region reaches further laterally relative to the now-shorter channel. Step 8 covers how 1970s processes pushed junction depth down from several micrometers toward a few tenths of a micrometer, and the process limits encountered along the way.

## 1. Why Junction Depth Couples to Channel-Length Scaling

A source or drain junction's depletion region extends both vertically (into the substrate) and laterally (toward the channel) in rough proportion to junction depth and the doping gradient. If depth does not scale down with channel length, the two junctions' lateral depletion regions consume a growing fraction of the nominal channel, encouraging exactly the short-channel leakage and threshold-rolloff problems this series examines in steps 12-13. Shallow junctions are therefore not an optional refinement — they are a precondition for channel-length scaling to deliver its intended electrical benefit.

## 2. Diffusion-Based Junctions and Their Depth Floor

Traditional dopant-diffusion junction formation (drive dopant in from a doped-glass or gas source at high temperature for an extended time) has an inherent floor on how shallow a junction can be made reliably, because diffusion depth depends on the square root of time at temperature, and very short diffusion times become difficult to control precisely and reproducibly across a full wafer lot. By the mid-1970s this floor was becoming a real constraint on how far channel length could be pushed using diffusion alone for the source/drain.

## 3. Real Diagram: Junction Depth vs. Depletion Encroachment on Channel

Junction Depth vs. Lateral Depletion Encroachment on Channel deeper junctions project depletion further into a short channel, eroding effective channel length junction depth (normalized to channel length) depletion encroachment (fraction of channel) diffusion-era junctions shallow implant-era junctions shallow junctions keep depletion encroachment low even as nominal channel length keeps shrinking

## 4. The Implant Answer, Previewed

Ion implantation (covered in depth in this knowledge base's dedicated Ion Implantation series, and introduced for threshold-voltage control specifically in this series' steps 9-10) offered a way around the diffusion depth floor: implant energy and dose can be set independently and precisely, placing dopant at a controlled shallow depth before a shorter, lower-thermal-budget anneal activates it without driving it much deeper. This decoupling of placement from thermal diffusion is a large part of why implantation displaced pure diffusion for source/drain formation as channel lengths kept shrinking through the decade.

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

MOSFET Scaling (1970s): Step 8 stage sequence and schematic outcome trend for this step Why Junction Depth … 1 Diffusion-Based Jun… 2 Real Diagram: Junct… 3 The Implant Answer,… 4 sequence progression outcome trend (schematic) companion diagram: stage sequence and schematic trend for this step

## Shrinking Source/Drain Junction Depth's Place in the Process Lineage

This is step 8 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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