Thermal Crosstalk

The physical pitch between adjacent transistor fingers is the dominant geometric factor. Closer spacing reduces the thermal resistance path, intensifying crosstalk.

• Narrow pitch: High thermal coupling; heat from one finger rapidly diffuses to neighbors.
• Wide pitch: Greater thermal isolation but increases die area and cost.
• Gradient: Unequal spacing creates asymmetric temperature profiles, complicating phase compensation.

The base material's ability to conduct heat laterally dictates the spread of thermal energy. High-conductivity substrates act as a thermal short, increasing crosstalk.

• Silicon Carbide (SiC): High conductivity (~370 W/mK) in GaN-on-SiC HEMTs promotes rapid lateral heat spreading.
• Silicon (Si): Lower conductivity (~150 W/mK) provides slightly better natural isolation.
• Diamond: Extreme conductivity (~2000 W/mK) maximizes crosstalk but minimizes peak temperatures.

The localized heat flux (W/mm²) generated by each finger directly drives the temperature gradient. Higher power density creates steeper thermal gradients across the device.

• Peak-to-average ratio: Signals with high PAPR cause transient hotspots that propagate to adjacent fingers.
• Biasing class: Deep Class-AB or Class-A operation sustains higher quiescent dissipation, elevating the baseline thermal floor.
• Non-uniform drive: Unequal power splitting between fingers in a corporate combining structure creates asymmetric heating.

The thermal interface between the die and the heat sink governs vertical heat extraction. A high-resistance die attach forces heat to spread laterally, exacerbating crosstalk.

• Sintered silver: Low void content and high conductivity (~250 W/mK) improves vertical extraction, reducing lateral spread.
• Conductive epoxy: Higher thermal resistance (~2-5 W/mK) traps heat in the die, increasing lateral diffusion.
• Thermal interface material (TIM): Degradation over time increases junction temperatures and alters crosstalk patterns.

The modulation bandwidth of the transmitted signal determines whether thermal crosstalk manifests as a static gradient or a dynamic memory effect.

• Low envelope frequencies (< 1 MHz): Temperature can track the instantaneous power envelope, creating dynamic AM-AM and AM-PM distortion across fingers.
• High envelope frequencies (> 10 MHz): Thermal capacitance filters the transient, resulting in a quasi-static temperature gradient.
• Multi-carrier signals: Independent carriers create intermodulation products that fall within the thermal bandwidth.

In Doherty or outphasing architectures, the carrier and peaking amplifiers operate at different efficiency points, generating asymmetric heat profiles.

• Doherty carrier amplifier: Operates near saturation with high dissipation, heating adjacent peaking paths.
• Peaking amplifier: Turns on only at high power, creating a transient thermal disturbance for the carrier.
• Combined output: The phase mismatch caused by thermal crosstalk between paths degrades the load modulation and efficiency.

A multi-port representation that extends scalar thermal impedance to capture mutual heating between adjacent transistor fingers. The off-diagonal elements of the Zth matrix quantify the temperature rise in one finger due to power dissipated in another.

• Essential for compact electro-thermal modeling of multi-finger GaN HEMTs
• Extracted from 3D thermal finite element analysis or transient thermal measurements
• Enables prediction of thermal gradients across the die surface

Self-heating is the temperature rise in a transistor finger caused by its own power dissipation. Mutual heating (thermal crosstalk) is the additional temperature rise caused by adjacent fingers. In dense multi-finger layouts, mutual heating can dominate the total junction temperature.

• Self-heating responds to the local envelope power of a single finger
• Mutual heating introduces spatial and temporal lag dependent on inter-finger spacing
• The combined effect creates a distributed thermal memory that distorts the combined output

A nonlinear phase shift in the output signal caused by temperature-dependent transistor capacitances. Thermal crosstalk creates spatially non-uniform phase distortion across the amplifier periphery, which cannot be corrected by a single memoryless predistorter.

• Phase shift varies with the envelope history of all coupled fingers
• Produces thermal-induced spectral asymmetry in the combined output
• Requires thermal-aware predistortion with per-finger or distributed compensation

A co-simulation technique that couples semiconductor device physics with dynamic heat generation and dissipation equations. For thermal crosstalk analysis, the thermal solver must resolve the full 3D temperature field across the multi-finger structure.

• Combines harmonic balance or transient circuit simulation with finite element thermal analysis
• Captures the interaction between GaN trapping and temperature-dependent effects
• Used to generate training data for thermal-aware behavioral models

A mathematical operation that models the junction temperature of each finger as the convolution of the instantaneous power dissipation waveform with the device's thermal impulse response. For crosstalk, a multi-input convolution is required.

• Each finger's temperature is the sum of self-heating convolution and cross-heating convolutions from neighbors
• The thermal impulse response is derived from the thermal impedance matrix
• Enables efficient time-domain simulation without full 3D thermal solvers

A digital linearization technique that incorporates real-time temperature sensing or electro-thermal models into the predistorter. For thermal crosstalk, the predistorter must compensate for dynamically shifting nonlinearities that vary across the device.

• Uses temperature-compensated LUTs indexed by both amplitude and estimated junction temperature
• May employ thermal-induced memory polynomial terms to capture long-duration thermal lag
• Critical for maintaining ACLR performance in Doherty amplifier and massive MIMO arrays