lightly doped drain

**Lightly Doped Drain (LDD) / Halo Implants** — carefully engineered doping profiles around the transistor channel that control short-channel effects and optimize the tradeoff between drive current and leakage. **LDD (Lightly Doped Drain)** - Problem: Abrupt, heavily doped source/drain junctions create intense electric fields at the drain edge → hot carrier injection (HCI) damages gate oxide - Solution: Grade the junction with a lightly doped extension - Process: Implant shallow, light dose extension → form spacer → implant deep, heavy dose source/drain - Result: Smoother field distribution, reduced HCI **Halo / Pocket Implant** - Problem: Short-channel effects — as gate length shrinks, source/drain depletion regions merge → loss of gate control (punch-through) - Solution: Implant opposite-type dopant right next to source/drain - For NMOS: p-type halo implant at angled angle near source/drain edges - Effect: Locally increases channel doping, raises $V_{th}$, prevents punch-through **Process Sequence** 1. Gate patterning complete 2. Halo implant (angled, 4 rotations) 3. LDD/extension implant (low energy, low dose) 4. Spacer formation (SiN/SiO₂) 5. Deep source/drain implant (high energy, high dose) 6. Activation anneal **LDD and halo implants** are essential junction engineering techniques — without them, modern short-channel transistors would simply not function correctly.