Junction Temperature

As junction temperature rises, phonon scattering increases dramatically, reducing electron and hole mobility in the semiconductor channel. This directly degrades the transistor's transconductance (gₘ) and current drive capability.

• In GaN HEMTs, mobility can decrease by 30-50% from 25°C to 150°C
• Reduced mobility lowers the amplifier's available gain, creating thermal AM-AM distortion
• The effect is instantaneous with temperature change but the temperature itself lags behind power dissipation

The threshold voltage (Vₜₕ) of a FET exhibits a negative temperature coefficient, decreasing approximately -2 to -4 mV/°C in GaN devices. This shift alters the quiescent bias point of the amplifier.

• A 100°C temperature rise can shift Vₜₕ by 200-400 mV
• This causes quiescent current drift, moving the amplifier's operating class
• The resulting gain variation creates a slow-memory effect that memoryless predistortion cannot correct

Junction temperature does not respond instantaneously to power changes. The thermal response is governed by multiple RC time constants corresponding to different physical layers in the heat dissipation path.

• Die-level: Microsecond-scale heating within the semiconductor itself
• Die attach: Millisecond-scale thermal diffusion through the bonding layer
• Package/heat sink: Second-scale thermal equilibration with the cooling solution
• These distributed time constants create thermal memory effects spanning many orders of magnitude in frequency

The junction temperature fluctuates dynamically with the envelope of the modulated RF signal, not the carrier. The thermal bandwidth of most power amplifier devices is in the kHz to low MHz range, which overlaps with the modulation bandwidth of modern communication signals.

• A 100 MHz 5G NR signal with 1 MHz envelope components will induce dynamic thermal modulation
• This creates envelope frequency heating that tracks the signal's instantaneous power profile
• The resulting temperature ripple modulates gain and phase, producing thermal AM-AM and AM-PM distortion

The junction temperature rise above ambient is determined by the product of instantaneous power dissipation and the device's thermal impedance Zₜₕ(jω). This is a complex, frequency-dependent quantity that defines the dynamic thermal behavior.

• Static thermal resistance (Rₜₕ): Steady-state temperature rise per watt
• Thermal capacitance (Cₜₕ): Heat storage capacity creating the transient response
• Zₜₕ is typically modeled using Foster or Cauer RC ladder networks extracted from transient thermal measurements
• Accurate Zₜₕ modeling is critical for predicting the temperature waveform from the power dissipation envelope

In GaN HEMTs, junction temperature directly modulates charge trapping dynamics. Traps in the buffer and surface states have thermally activated time constants, meaning their capture and emission rates are strong functions of temperature.

• Higher temperatures accelerate detrapping, reducing gate lag but potentially increasing drain lag
• The combined thermal-trapping memory creates complex, inseparable nonlinear dynamics
• Electro-thermal models must couple temperature-dependent trap kinetics with self-heating for accurate behavioral prediction
• This interaction is a primary reason why simple memory polynomial models fail to fully linearize GaN amplifiers

A distortion mechanism where the device's temperature history—driven by signal envelope variations—alters instantaneous electrical behavior. Unlike electrical memory effects (nanosecond scale), thermal memory operates on microsecond to millisecond time constants, creating long-term nonlinear memory that standard memoryless predistorters cannot correct. The effect manifests as a dynamic shift in gain and phase that depends on the cumulative thermal energy stored in the semiconductor lattice.

The process by which power dissipation within the transistor channel directly increases its own junction temperature. Key characteristics:

• Instantaneous power multiplied by thermal impedance determines temperature rise
• Creates a dynamic feedback loop: higher temperature → reduced carrier mobility → lower drain current → altered gain
• In GaN HEMTs, self-heating can cause 20-30% drain current collapse within microseconds
• Distinguished from ambient temperature changes by its signal-dependent, transient nature

A complex, frequency-dependent measure defining the dynamic relationship between power dissipation and junction temperature rise. Unlike static thermal resistance (Rth), Zth captures transient behavior:

• Magnitude decreases with frequency due to thermal capacitance smoothing
• Typically represented as a Foster or Cauer network of RC stages
• Critical parameter extracted from transient thermal response measurements
• Governs the bandwidth of thermal memory effects—low Zth at high frequencies means envelope-frequency heating dominates

The characteristic time required for junction temperature to reach ~63.2% of steady-state after a step change in power dissipation. Multiple time constants exist in real devices:

• Die-level: nanosecond to microsecond range (small thermal mass)
• Package-level: millisecond range (larger thermal capacitance)
• Heat sink-level: seconds to minutes (system-level cooling)

The dominant time constant within the signal envelope bandwidth determines the severity of thermal memory distortion in a given application.

A co-simulation technique coupling semiconductor physics with dynamic heat equations to predict temperature-dependent nonlinearities. The approach:

• Solves Poisson and continuity equations self-consistently with the heat diffusion equation
• Captures the bidirectional coupling: electrical behavior affects heating, heating affects electrical behavior
• Used to generate training data for thermal-aware predistortion algorithms
• Enables prediction of thermal AM-AM and AM-PM distortion before hardware prototyping

An augmented behavioral model that extends standard memory polynomials with low-frequency thermal terms. Structure includes:

• Conventional memory terms: capture electrical memory (nanosecond scale)
• Thermal envelope terms: convolve |x(n)|² with thermal impulse response
• Cross terms: model interaction between instantaneous nonlinearity and thermal state
• Enables single-model compensation of both short-term electrical and long-term thermal memory effects in a unified predistorter architecture