How does self-heating affect the accuracy of a transistor model at high power levels?

Self-heating is the temperature rise of the transistor junction caused by its own power dissipation. At high power levels (PA operation), self-heating significantly affects the transistor performance and the accuracy of the model: (1) Mechanism: the transistor dissipates power: P_diss = V_DS × I_DS - P_RF_out. For a Class AB PA: P_diss is approximately 50-70% of the DC power (the efficiency is 30-50%). The dissipated power heats the junction: T_j = T_ambient + R_th × P_diss. R_th (thermal resistance): depends on the device geometry, substrate material, die attach, and package. For a GaN HEMT on SiC: R_th ≈ 5-20°C/W (device-level). For a GaN HEMT on Si: R_th ≈ 15-60°C/W. For a GaAs HBT: R_th ≈ 200-600°C/W (HBTs have higher R_th due to the vertical current flow through resistive layers). (2) Effect on performance: as T_j increases: I_DSS decreases (by approximately 0.2-0.5%/°C for GaN, 0.3-0.7%/°C for GaAs). g_m decreases (the gain drops). V_th shifts (slightly). The net result: the gain decreases with temperature (negative gain slope: -0.01 to -0.03 dB/°C). The P1dB decreases. The PAE decreases (both output power and gain are reduced). (3) Model accuracy: if the model does not include self-heating: the model operates at a fixed temperature (the extraction temperature, typically 25°C). At high power: the actual T_j may be 50-150°C above ambient. The model will over-predict the gain by 1-4 dB and over-predict P_sat by 0.5-2 dB (because the model does not account for the reduced I_DSS at the elevated junction temperature). If the model includes self-heating (thermal sub-circuit): the model dynamically calculates T_j from P_diss and R_th. The current source and g_m are temperature-adjusted in real time during the simulation. The result is much more accurate (within ±0.5 dB of measured P_sat and ±2% of measured PAE).