Self-Heating

The magnitude of self-heating is directly proportional to the instantaneous power dissipated in the transistor channel. As the signal envelope drives the amplifier into compression, DC power consumption rises, and the efficiency drop converts more energy into heat. This creates a signal-dependent thermal profile where high-power symbols generate more localized heating than low-power symbols, establishing the fundamental link between the modulation scheme and thermal dynamics.

Elevated junction temperature degrades carrier mobility in the semiconductor channel. In GaN HEMTs, increased lattice scattering reduces electron velocity, leading to lower transconductance and gain compression. This mechanism creates a slow-varying thermal AM-AM distortion that cannot be corrected by memoryless predistorters. The mobility degradation follows a power-law relationship with temperature, typically proportional to T^(-n) where n ranges from 1.5 to 2.5 depending on the material system.

Self-heating causes a negative shift in the transistor's threshold voltage (V_th). As junction temperature rises, the Fermi potential decreases, requiring less gate voltage to invert the channel. This drift alters the amplifier's quiescent bias point, shifting the conduction angle and modifying the gain expansion characteristics. The resulting quiescent bias shift introduces a slow-memory nonlinearity that evolves over the thermal time constant, typically in the microsecond to millisecond range.

Junction temperature variations modulate the transistor's parasitic capacitances, particularly the gate-to-source capacitance (C_gs) and drain-to-gate feedback capacitance (C_dg). These capacitance shifts alter the phase response of the amplifier as a function of the signal envelope history. The resulting thermal AM-PM distortion introduces asymmetric spectral regrowth that appears as an imbalance between upper and lower adjacent channel power, a signature that distinguishes thermal memory from electrical memory effects.

In multi-finger transistor layouts, self-heating is non-uniform across the device structure. Center fingers experience higher thermal resistance to the heat sink than edge fingers, creating thermal gradients that cause unequal current distribution. This phenomenon, known as thermal crosstalk, means each finger operates at a different effective temperature and bias point, distorting the combined output and complicating behavioral modeling. The effect is particularly severe in wide-bandgap devices like GaN where power density is high.

Self-heating responds primarily to the low-frequency envelope components of the modulated signal. When the envelope frequency falls within the thermal bandwidth of the device (typically DC to a few MHz), the junction temperature can track the signal variation, creating dynamic distortion. This envelope frequency heating means wideband signals with high peak-to-average ratios produce more complex thermal memory than narrowband constant-envelope signals, requiring predistorters with extended memory depth to compensate.

A distortion mechanism where the device's temperature history—driven by signal envelope variations—alters its instantaneous electrical behavior. Unlike electrical memory effects, thermal memory operates on microsecond to millisecond timescales, creating long-term nonlinear memory that cannot be corrected by memoryless predistortion. The effect manifests as a dynamic shift in gain and phase that depends on the average power level over the preceding thermal time constant.

The operating temperature at the semiconductor die level of a transistor, critically governing carrier mobility, threshold voltage, and instantaneous nonlinear characteristics. In GaN and GaAs power amplifiers, junction temperature can swing tens of degrees Celsius within a single transmission burst. This dynamic thermal state directly modulates:

• Transconductance and gain
• Input and output capacitances
• Leakage currents and quiescent bias point

A measure of a material's resistance to heat flow, defining the dynamic relationship between power dissipation and the resulting temperature rise. Thermal impedance is frequency-dependent: at high frequencies, thermal capacitance attenuates temperature swings, while at low envelope frequencies, the full thermal resistance path determines the junction temperature. This frequency-dependent behavior is the root cause of thermal memory dispersion in power amplifiers.

The characteristic time required for a device's junction temperature to reach approximately 63.2% of its steady-state value following a step change in power dissipation. Power amplifiers exhibit multiple thermal time constants corresponding to different layers in the heat dissipation path:

• Die-level: nanoseconds to microseconds
• Die attach: microseconds to milliseconds
• Package and heat sink: milliseconds to seconds These overlapping constants create the complex thermal memory profile that predistorters must model.

A co-simulation technique that couples semiconductor device physics with dynamic heat generation and dissipation equations. This approach solves the self-consistent problem where electrical behavior depends on temperature, and temperature depends on electrical power dissipation. Modern electro-thermal models combine:

• Compact transistor models (Angelov, Curtice)
• Thermal RC network representations
• Envelope-domain simulation for modulated signals These models are essential for designing thermal-aware predistortion algorithms.

A nonlinear phase shift in the output signal that varies as a function of the input signal's envelope history. Temperature-dependent transistor capacitances—particularly the gate-source and drain-source capacitances—alter the device's phase response dynamically. This creates a dispersive phase distortion that cannot be corrected by amplitude-only predistortion. Thermal AM-PM is especially problematic in high-order QAM and OFDM systems where phase integrity is critical for demodulation.