LDMOS Transistors: What They Are and How They Fit into RF and Microwave Circuit Design

Updated

LDMOS (laterally diffused MOSFET) is a silicon power MOSFET structure in which the channel is formed by lateral diffusion of dopants under the gate, with a lightly doped drain drift region that sustains high voltage while keeping resistance low. That combination—high breakdown voltage, low on-resistance, and a planar structure with the source connected to the substrate—is why LDMOS became the dominant RF power device for base-station and broadcast amplifiers in the roughly 1–4 GHz range. Choose LDMOS when you need high output power, ruggedness under mismatch, and good thermal spreading through a substrate-connected source; look elsewhere (GaN, SiC, or RF bipolars) when you need very high frequency, very high power density, or specific linearity behavior.

What the device structure does for RF

The "lateral diffusion" refers to how the channel length is set: dopants diffuse sideways under the gate rather than being defined purely by lithography. This gives short, reproducible channels without advanced lithography. The lightly doped drain (LDD) drift region between channel and drain contact drops most of the drain voltage, so the device can run at 28–50 V supplies—much higher than typical RF bipolars—which directly raises achievable output power per unit of current.

The source is typically tied to the bottom of the die (source-to-substrate connection). In RF packages this means the source inductance is minimized and heat flows straight into the package flange and heatsink. That is a structural advantage over bipolar devices, where the collector is usually the backside contact and the emitter is on top.

LDMOS vs. RF bipolars and other microwave transistors

Dimension LDMOS RF bipolar (Si BJT/HBT) GaN HEMT
Typical supply voltage High (tens of volts) Lower High
Power per device High Moderate Very high power density
Linearity / distortion Good with proper bias; behavior differs from bipolars Well-understood intermodulation behavior Excellent but bias-sensitive
Ruggedness under VSWR mismatch Generally strong More sensitive Strong
Thermal path Source-to-substrate, good spreading Collector backside Substrate-dependent
Frequency sweet spot Roughly 1–4 GHz Extends to higher frequencies Higher frequency and power

The practical takeaway: LDMOS is a workhorse for high-power, moderate-frequency amplification where ruggedness and thermal handling matter. RF bipolars remain relevant where their gain and linearity characteristics are already designed in, and GaN is the choice when frequency or power density pushes beyond what silicon LDMOS delivers.

Where LDMOS is commonly used

  • Cellular base-station power amplifiers, especially in the 1–3 GHz bands
  • Broadcast transmitters (FM, DTV) where high power and efficiency dominate
  • Industrial, scientific, and medical RF generators
  • Radar and avionics transmitters in the lower microwave range

In each case the common thread is a requirement for substantial continuous output power with a manageable thermal design.

Selection parameters that actually decide the part

  1. Frequency range and gain. Check gain at your exact operating frequency, not just the datasheet's peak. Gain falls with frequency, so margin at the band edge matters.
  2. Output power and compression point. Specify P1dB and saturated power for your modulation; for multi-carrier signals, back off from saturation and check intermodulation (IMD) performance at that back-off.
  3. Efficiency. Drain efficiency at your operating point drives heatsink sizing and supply design.
  4. Supply voltage. LDMOS devices are designed around specific drain voltages; matching your rail to the device's intended operating point keeps efficiency and linearity in the specified range.
  5. Package and thermal resistance. The junction-to-case thermal resistance plus your heatsink determines junction temperature. Power flanged packages and heat-sink sub-mounts are common in microwave assemblies.
  6. Matching and bias. LDMOS input impedance is largely capacitive and low at high power, so input matching networks are essential; gate bias networks must be decoupled carefully to avoid low-frequency instability.

Integration concerns in thin-film MIC assemblies

In a thin-film microwave integrated circuit environment, LDMOS devices are typically mounted as discrete die or packaged parts alongside thin-film matching networks, chip capacitors, spiral inductors, and resistors. Practical points:

  • Biasing: provide a clean, well-decoupled gate bias; sequence gate and drain supplies so the device is never operated without proper gate voltage.
  • Matching: design input and output matching for the actual operating band and power level; verify with load-pull or measured S-parameters rather than relying on small-signal data alone.
  • Heat sinking: use flanged packages, aluminum nitride sub-mounts, or heat-sink carriers matched to the thermal load; the source-to-substrate path is only as good as the mounting interface.
  • Stability: check for low-frequency oscillation with the full bias network and realistic source/load impedances.

US Microwaves lists LDMOS among its RF power transistor offerings alongside RF bipolars, thin-film MIC circuits, spiral chip inductors, ceramic and MOS/MNOS capacitors, and chip resistors—useful when a single assembly needs the transistor plus its passive matching and bias components sourced together. For current part numbers, frequency coverage, and pricing, request a quote or cross-reference through the site rather than assuming catalog availability.