Ldmos Transistor
LDMOS (Laterally Diffused Metal-Oxide-Semiconductor) is the power transistor architecture where the channel region is formed by lateral diffusion of the body (p-type) into an n-drift region, creating a transistor with high breakdown voltage, excellent RF linearity, and sufficient gain to amplify signals from MHz to multi-GHz frequencies — making LDMOS the dominant technology for base station power amplifiers, broadcast transmitters, industrial RF, and high-voltage power management ICs that require simultaneous high power (10 W to multi-kW), high gain (10–18 dB), and rugged reliability.
LDMOS Structure
- Key feature: Source and body are shorted (same potential) → eliminates substrate bias effect → stable operation.
- N-drift region: Lightly doped n-region between channel and drain → supports high breakdown voltage by spreading the depletion region.
- RESURF (Reduced SURface Field): P-substrate and n-drift doping chosen so the vertical junction between them depletes in conjunction with the horizontal drain junction → surface field is reduced → higher breakdown at same drift region length.
LDMOS vs. Standard MOSFET
| Parameter | Standard MOSFET | LDMOS |
|---|---|---|
| Breakdown voltage | 2–5 V | 28–65 V (RF), 100–800 V (power) |
| On-resistance | Low | Higher (drift region adds Ron) |
| Frequency | DC–10 GHz | DC–6 GHz (RF LDMOS) |
| Linearity | Moderate | Excellent (smooth Gm vs. Vgs) |
| Die size | Small | Larger (long drift region) |
LDMOS Process Flow
1. P-type substrate
2. N-buried layer (optional, for isolation)
3. P-well / P-body diffusion (lateral diffusion defines channel)
4. N-drift implant (sets breakdown voltage, Ron tradeoff)
5. RESURF optimization: Adjust P-substrate / N-drift charge balance
6. Gate oxide growth (thin, 5–10 nm)
7. Poly gate deposition + etch
8. P-body extension (lateral diffusion under gate → sets Leff)
9. N+ source in P-body; N+ drain on drift edge
10. Source metal connected to P-body (source-body short)
11. Drain metal over field oxide (with field plate)Field Plate
- Metal extension over thick field oxide on drain side.
- Redistributes electric field peak → more uniform field distribution → higher breakdown voltage.
- RF LDMOS: Gate field plate + drain field plate → +20–30% breakdown improvement.
RF Performance Metrics
| Metric | Typical LDMOS | Definition |
|---|---|---|
| Pout | 5–100 W/die | Output power |
| Gain | 12–18 dB | Power gain at 3.5 GHz |
| PAE | 50–65% | Power Added Efficiency |
| ACPR | −50 to −55 dBc | Adjacent Channel Power Ratio (linearity) |
| Ruggedness | 10:1 VSWR | Withstands severe load mismatch |
Applications
- 5G base station (sub-6 GHz): LDMOS dominates at 700 MHz – 3.5 GHz (NXP, Wolfspeed, STM).
- Broadcast: FM/AM transmitters, MRI RF amplifiers (high power CW operation).
- Industrial ISM: 915 MHz and 2.45 GHz cooking, plasma generation.
- Defense: Radar transmitters (pulsed high-power LDMOS from 1–6 GHz).
- Smart power ICs: High-side switch, motor driver (automotive 28V systems).
LDMOS is the workhorse of high-power RF amplification worldwide — its unique combination of RESURF-enabled high breakdown voltage, source-body shorted topology for stability, and smooth transconductance for linearity makes it the go-to power transistor for infrastructure, broadcast, and industrial RF applications where GaN's higher cost or reliability questions make silicon LDMOS the preferred choice.