GaN HEMT for RF/Power is the high-electron-mobility transistor in AlGaN/GaN heterostructure exploiting 2DEG formation — enabling high-power RF amplification and efficient power switching with superior breakdown voltage and thermal performance versus silicon.

AlGaN/GaN Heterostructure:

• Material system: GaN channel layer with AlGaN barrier layer; lattice-mismatched heterostructure
• Bandgap engineering: AlGaN wider bandgap than GaN; creates potential well for electrons
• Spontaneous polarization: inherent material property creates fixed polarization charge; induces 2DEG
• Piezoelectric polarization: strain in heterostructure creates additional polarization; enhances 2DEG
• Total polarization: sum of spontaneous and piezoelectric polarization creates very high 2DEG density

2DEG (Two-Dimensional Electron Gas) Formation:

• Electron confinement: electrons confined to 2D layer at heterointerface; quantum mechanical confinement
• High density: polarization-induced 2DEG density ~10¹³ cm⁻² (vs doping ~10¹⁶ cm⁻³ in Si)
• High mobility: electron mobility ~1000-2000 cm²/Vs at room temperature; scattering limited
• Channel characteristics: very thin channel (~nm scale); depletion mode normally-on transistor
• Electron transport: ballistic transport possible; high velocities enable high-frequency operation

HEMT Device Structure:

• Gate/source/drain: gate electrode above AlGaN barrier; controls 2DEG channel
• Gate-induced depletion: negative gate voltage depletes 2DEG channel; turns off transistor
• Schottky gate: metal gate forms Schottky junction; controls channel via depletion
• Channel access: electrons flow laterally through 2DEG channel; vertical device not feasible
• Barrier thickness: thin barrier (~20-30 nm); controls gate modulation efficiency

High Critical Electric Field:

• GaN critical field: ~3.3 MV/cm (vs 0.3 MV/Vs for Si); enables thick drift region for same breakdown
• Breakdown voltage: 600 V, 1200 V, 3300 V rated devices; superior to Si/SiC
• Voltage scaling: thickness scales as 1/E_c; thin GaN drift enables low on-resistance
• Efficiency advantage: lower on-resistance at same voltage → better efficiency
• Power density: enables higher power density in compact devices

GaN-on-SiC Substrate:

• Thermal conductivity: SiC substrate (~3.3 W/cm·K) vs Si (~1.4 W/cm·K); superior heat spreading
• Lattice match: better lattice match reduces defects vs GaN-on-Si; improved device quality
• Cost: SiC more expensive than Si; justified for thermal-demanding RF applications
• Integration: full GaN epitaxy on SiC enables monolithic integration; no backside processing
• Power amplifier advantage: excellent thermal spreading critical for high-power RF amplifiers

GaN-on-Si Substrate:

• Cost advantage: Si substrate much cheaper than SiC; enables cost-competitive GaN devices
• Integration: CMOS drivers on Si; enables monolithic integration
• Substrate conductivity: Si conductive; substrate coupling issues require isolation
• Vertical leakage: vertical component through Si; affects isolation and leakage current
• Practical success: GaN-on-Si achieves good performance with careful design; now mainstream

RF Power Amplifier Applications:

• 5G base station: 3.5 GHz / 28 GHz / 39 GHz amplifiers; replacing LDMOS and GaAs
• Efficiency advantage: 70-80% power-added efficiency (PAE); superior to Si technologies
• Waveform capability: dynamic voltage scaling enables efficient modulated signal amplification
• Linearity: GaN HEMT high gain and linear response; predistortion enables linear operation
• Integration: monolithic integration of driver + power stage on single chip

Current Collapse and Trapping:

• Trapping: charge trapping in AlGaN or at interface reduces available charge; dynamic on-resistance increase
• Collapse mechanism: current suddenly drops under transient conditions; reduced current drive
• Performance degradation: dynamic on-resistance higher than static (DC measured) resistance
• Recovery time: charges slowly released after removal of stress; tens to thousands of milliseconds
• Mitigation: surface passivation (SiN), gate engineering, substrate engineering reduce trapping

Reliability and Temperature:

• Operating temperature: GaN transistors operate >200°C junction temperature; Si limited to ~150°C
• Thermal management: GaN inherent advantage; can handle higher temperature
• Degradation mechanisms: gate dielectric stress (PBTI), hot-carrier injection, surface degradation
• Qualification: automotive and military qualification available; increasingly reliable
• Electromigration: metallization must handle high current density; careful interconnect design

Device Modeling and Simulation:

• Large-signal models: account for nonlinear capacitance, trapping, thermal effects
• Dynamic effects: frequency-dependent behavior; dispersion of S-parameters with bias
• Thermal models: junction-to-case thermal resistance; temperature-dependent parameters
• Harmonic balance simulations: nonlinear RF circuit simulation; predicts intermodulation
• Measurement validation: model fitting to measured S-parameters, load-pull measurements

Power Electronics Switching:

• Hard-switching capability: high voltage rating enables 400-800 V bus operation
• Efficiency gains: low on-resistance reduces conduction losses; fast switching reduces switching losses
• Inverter topology: three-phase inverter for motor drives, EV chargers, renewable energy
• Cascode structure: GaN HEMT + Si MOSFET cascode; matches characteristics for gate driving
• Enhancement-mode option: normally-off devices via hybrid or p-channel designs

Gate Driver Requirements:

• Threshold voltage: GaN normally-on threshold ~-2 to -4 V; requires negative off-voltage for turn-off
• Drive voltage: typical +6V on, -2V off; different from Si standard +15V/0V
• Gate charge: lower total gate charge than comparable Si MOSFET; faster switching possible
• EMI considerations: fast switching edges enable smaller filters but increase EMI; careful layout required
• Driver integration: monolithic GaN+driver chips simplify system integration

Thermal Characteristics:

• Thermal resistance: θ_JC ~0.5 K/W typical; junction-to-case; improves with larger die
• Temperature coefficient: R_ON increases with temperature (~+0.5%/°C); positive feedback
• Self-heating effects: high current causes temperature rise; reduces current capability (thermal stability)
• Heatsinking: critical importance; thermal interface material and mounting essential
• Derating curves: maximum current decreases with temperature; operating point must satisfy both constraints

GaN HEMTs deliver superior RF power and switching performance through polarization-induced 2DEG and high critical field — enabling efficient 5G amplifiers, radar, and power converters versus conventional silicon technologies.