GaN HEMT

GaN HEMTs achieve power densities of 5-10 W/mm of gate periphery, significantly exceeding GaAs and Si LDMOS technologies. This stems from the high critical electric field of GaN (3.3 MV/cm), which allows higher operating voltages without breakdown. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN heterojunction provides high sheet carrier concentration and electron mobility, enabling compact transistor geometries that deliver substantial RF power from small die areas.

GaN-on-SiC HEMTs exhibit thermal conductivity up to 400 W/m·K when fabricated on silicon carbide substrates, enabling efficient heat extraction from the transistor channel. This superior thermal management reduces self-heating effects that cause gain and phase variations with signal envelope changes. Lower channel temperature rise minimizes long-term memory effects, simplifying digital predistortion linearization requirements and improving overall amplifier reliability.

GaN HEMTs operate at drain voltages of 28-50V, substantially higher than GaAs devices typically limited to 8-12V. This high-voltage capability directly translates to higher load-line impedance for a given output power, simplifying impedance matching network design. In Doherty amplifier architectures, the higher operating voltage enables wider bandwidth impedance transformers and reduces current handling requirements in the output combiner network.

The lateral device structure of GaN HEMTs results in intrinsically low parasitic capacitances compared to vertical silicon MOSFETs. Key advantages include:

• Reduced Cgd (gate-drain capacitance) minimizing feedback and improving gain
• Higher gain-bandwidth product enabling operation at mmWave frequencies
• Faster switching transitions reducing dynamic power loss
• Simplified broadband matching network design for wideband Doherty amplifiers

GaN HEMTs exhibit gradual, soft gain compression as they approach saturation, unlike the abrupt hard compression of silicon LDMOS devices. This smooth nonlinearity profile is more amenable to digital predistortion linearization, as the AM-AM and AM-PM distortion curves are continuous and well-behaved. The soft compression characteristic reduces the order of memory polynomial models required for effective linearization, simplifying DPD implementation complexity.

GaN HEMTs are susceptible to charge trapping phenomena at surface states and buffer layers that cause low-frequency dispersion. Key effects include:

• Gate lag: Slow drain current recovery after gate voltage switching
• Drain lag: Current collapse under high drain voltage stress
• Knee voltage walkout: Dynamic increase in knee voltage with signal history These trapping mechanisms introduce complex long-term memory effects that require sophisticated behavioral models and adaptive predistortion algorithms for effective compensation.

GaN is a wide-bandgap semiconductor with a bandgap of 3.4 eV, compared to 1.1 eV for silicon. This fundamental material property enables:

• High breakdown voltage: GaN HEMTs operate at 48V or higher drain voltages, directly enabling high power density
• High-temperature operation: Reduced intrinsic carrier concentration at elevated temperatures allows reliable operation beyond 200°C junction temperature
• Superior Johnson figure of merit: GaN's high critical electric field and saturation velocity make it ideal for high-frequency power applications The wide bandgap directly translates to the high output impedance and reduced parasitic capacitance that make Doherty combiners more realizable.

The 2DEG is the conductive channel in a GaN HEMT, formed at the AlGaN/GaN heterojunction without intentional doping. Key characteristics:

• Piezoelectric polarization: Strain at the AlGaN/GaN interface induces a high-density sheet charge exceeding 1×10¹³ cm⁻²
• High electron mobility: Electrons confined in the quantum well experience reduced impurity scattering, achieving mobility >1500 cm²/V·s
• High saturation velocity: 2.5×10⁷ cm/s enables excellent high-frequency gain This spontaneous formation of a highly conductive channel is why GaN HEMTs are inherently depletion-mode (normally-on) devices, requiring careful gate bias sequencing in Doherty amplifier designs.

GaN HEMTs exhibit a soft compression behavior where gain gradually decreases as output power approaches saturation, unlike LDMOS devices that show abrupt hard compression. This characteristic:

• Reduces AM-AM distortion severity: The gradual gain roll-off generates lower-order nonlinearities that are more amenable to digital predistortion correction
• Simplifies Doherty linearization: The smooth transition through the Doherty back-off region reduces the DPD model complexity required
• Improves ACLR margin: Soft compression naturally produces less spectral regrowth, providing 2-3 dB better raw ACLR before linearization The soft compression mechanism stems from the gradual reduction in 2DEG carrier density under high electric fields rather than abrupt channel pinch-off.

Trapping effects are the dominant source of low-frequency dispersion and memory in GaN HEMTs, caused by charge capture at surface states and buffer defects:

• Gate lag: Surface traps between the gate and drain deplete the 2DEG during high-voltage swings, causing slow recovery of drain current after RF pulses
• Drain lag: Buffer traps in the Fe-doped or C-doped GaN buffer capture electrons during high-field conditions, modulating the channel conductivity with time constants from microseconds to seconds
• Current collapse: The combined effect reduces the available RF output power compared to DC characteristics, requiring dynamic trap-aware models for accurate DPD Advanced GaN processes use field plates and optimized passivation to mitigate trapping, but residual effects still demand memory polynomial DPD models with long time constants.

GaN HEMTs achieve power densities of 5-8 W/mm of gate periphery, but this concentrated heat flux demands sophisticated thermal management:

• Self-heating effect: Channel temperature rises dynamically with instantaneous dissipated power, causing transient gain reduction and phase shift that manifests as long-term memory in DPD models
• SiC substrate advantage: GaN-on-SiC leverages SiC's high thermal conductivity (370 W/m·K) to spread heat, reducing peak junction temperature by 30-40% compared to GaN-on-Si
• Thermal memory time constants: Typically 1-100 μs, requiring DPD coefficient update rates fast enough to track thermal dynamics The self-heating-induced phase shift is particularly critical in Doherty amplifiers where the carrier and peaking devices experience different thermal trajectories during signal envelope peaks.

The knee voltage (Vknee) is the minimum drain-to-source voltage at which the HEMT enters current saturation. GaN HEMTs exhibit exceptionally low knee voltage due to:

• Low on-resistance (Rds-on): The high 2DEG conductivity minimizes resistive voltage drop before saturation
• High drain efficiency: A low Vknee maximizes the RF voltage swing relative to the DC supply, enabling drain efficiency exceeding 70% in Doherty carrier amplifiers
• Harmonic engineering: The sharp knee characteristic supports Class-F and inverse Class-F harmonic terminations that further shape voltage and current waveforms for minimum overlap Typical GaN HEMT knee voltages of 3-5V at 48V operation represent less than 10% of the supply voltage, a key advantage over LDMOS for high-efficiency Doherty designs.