InP DHBT Optimization for mm-Wave Power Applications

This work presents the optimization of an InP Double Heterojunction Transistor (DHBT) technology for Power Amplifier (PA) applications in the millimeterwave frequency range. Starting from an existing InP DHBT technology for high-speed mixed-signal applications, the epitaxial structure of a single-finger DHBT has been designed to fulfill the requirements set for the design of the power cell in terms of maximum cutoff frequency fMAX and breakdown voltage BVCEO . A 2D TCAD modeling approach is proposed to investigate device high-frequency performances for different collector structures. The static and high-frequency performances of devices with different geometrical layout dimensions have been investigated to select the unit finger device with 0.7×10 µm2 emitter area having fMAX = 400 GHz and BVCEO > 7 V for the unit power cell. Single-finger devices are combined in multi-finger structures to increase total output power. The electrical performances of multi-finger devices are investigated with respect to number of fingers and geometrical dimensions. The thermal characterization of multi-finger devices is performed to understand the impact of self and mutual-heating. An approach based on 3D thermal simulations of multi-finger devices is proposed to study heating effects and to extract thermal parameters for the device large-signal model. A 4-finger device with 0.7x10 µm2 unit finger emitter area is finally selected for the PA power cell. The 4-finger device has fMAX =370 GHz and can deliver POUT = 16 dBm to an optimal load under class-A operation. As a further improvement to reduce thermal effects in multi-finger devices, the ballasting resistor approach is investigated. The performances of DHBTs with different ballasting resistor networks are compared in terms of static and high-frequency performances. A ballasting solution is finally proposed as a trade-off between the improvement in device Safe Operating Area (SOA) and the degradation of high-frequency performances.