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Power limiting mechanisms in InP HEMTs

The InAlAs/InGaAs High Electron Mobility Transistor (HEMT) fabricated on InP has proven to be the fastest transistor ever produced. It also appears to be the best candidate for power amplification at mm-wave frequencies. To date, however, very good high-frequency power performance has not been realized in these devices, and the lack of a clear understanding of the power-limiting mechanisms has hampered success in pushing the devices farther. In an effort to understand the dominant power-limiting mechanisms in InA1As/InGaAs HEMTs, we performed extensive electrical characterization including temperature-dependent measurements on state-of-the-art HEMTs fabricated by Lockheed Martin. Our experiments focused on the mechanisms determining the maximum drain current and the off-state breakdown voltage (BV) since these are the important parameters setting the power limits of a device. We conclude that the maximum current appears to be limited by velocity saturation in the extrinsic channel. We also find that the breakdown voltage primarily is set by tunneling and thermionic-field emission of electrons from the gate, and our findings are consistent with a new predictive model for the temperature dependence of BV based on this mechanism.