Distributed and multiple time-constant electro-thermal modeling and its impact on ACPR in RF predistortion

Abstract

The performance of power amplifier linearization techniques is often impaired by thermal and electrical memory effects when the signal bandwidth is changed. Effective design of linearizers requires transistor models to be able to correctly reproduce these effects. We are presenting a distributed and multiple time-constant electro-thermal LDMOSFET model which is more accurate in predicting distributed and transient thermal effects. The distributed temperature field across the fingers is obtained using a recently reported 3D image method [l]. A distributed equivalent electrethermal model is then constructed to accurately and efficiently predict the average thermal resistance of large devices and is therefore particularly useful for device scaling. The electro-thermal model is further used to more accurately predict the transistor's transient thermal response so that the thermd memory effects in RF predistortion linearization be investigated separately from electricd memory effects. For this purpose an improved hybrid image method in the time-domain is derived and compared to FEM to find the LDMOSFET's transient thermal response. The transistor's transient thermal response is fitted using a multiple time-constant RC network to implement this time-dependent thermal model in an harmonic balance simulator. The complete distributed electrethermal LDMOSFET model is then used in a predistortion linearization system. It is verified that the impact of thermal memory effects on adjacent-channel power-ratio for various modulation bandwidth is only dominant over electrical memory for signals with modulation bandwidth below I MHz for the device considered featuring a thin Si layer. Under such low modulation bandwidth, the multi-stage thermal model is found to be more accurate than the single-stage thermal model. The methodology should also be usefu1 for optimizing the device layout as it establishes a relationship between thermal memory effects and the device structure.

Frequency Domain Dynamic Thermal Analysis in Gaas HBT for Power Amplifier Applications

“…First order equivalent thermal circuit with low pass behavior is used to predict T j . Mutual heating between fingers of a power HBT is also analyzed and modeled as a thermal resistance matrix [4]. These models gave good results in predicting steady state thermal effects.…”

Design and analysis of ultra-broadband distributed power amplifiers for multi-platform communications

“…This model has low-pass behavior. The model is being integrated with the standard bipolar compact models such as VBIC, HiCUM, MEXTRAM, and Agilent HBT [50, 56-58].The mutual heating among fingers of power HBTs is also analyzed and modeled using thermal resistance matrix[59]. Under steady state condition, the first order model predicts the junction temperature accurately.…”