Analysis of electrothermal effects in devices and arrays in InGaP/GaAs HBT technology

In this paper, the dc electrothermal behavior of InGaP/GaAs HBT test devices and arrays for power amplifier output stages is extensively analyzed through an efficient simulation approach. The approach relies on a full circuit representation of the domains, which accounts for electrother-mal effects through the thermal equivalent of the Ohm’s law and can be solved in any commercial circuit simulator. In particular, the power-temperature feedback is described through an equivalent thermal network automatically obtained by (i) generating a realistic 3-D geometry/mesh of the domain in the environment of a numerical tool with the aid of an in-house routine; (ii) feeding the geometry/mesh to FANTASTIC, which extracts the network without performing simulations. Non-linear thermal effects adversely affecting the behavior of devices/arrays at high temperatures are included through a calibrated Kirchhoff’s transformation. For the test devices, the thermally-in-duced distortion in I–V curves is explained, and the limits of the safe operating regions are identified for a wide range of bias conditions. A deep insight into the electrothermal behavior of the arrays is then provided, with particular emphasis on the detrimental nonuniform operation. Useful guidelines are offered to designers in terms of layout and choice of the ballasting strategy.