Gallium nitride (GaN) based high electron mobility transistors (HEMTs) out-perform the silicon-based conventional transistors in high frequency and high voltage applications. However, GaN-based HEMTs are normally ON or depletion mode devices. In power electronic switching applications, normally-off devices with low on-resistance are desirable to avoid standby power loss and to get rid of additional circuitry. Techniques such as recessed-gate, gate-injection transistor, or p-type cap layer are used to realize normally-off devices. Recently, some research groups have demonstrated tri-gate structures with better electrostatic control on the channel as potential candidates for enhancement-type devices. In such devices, down-scaling of lateral fin-width induces a positive shift in the threshold voltage of the device due to the side gate depletion. In order to reliably use these transistors for circuit design, robust compact models that accurately capture the behavior of the device are needed. In this talk we present a compact model to predict the capacitance and threshold voltage for fin-shaped HEMTs.

First, a physics-based closed-form model for the gate capacitance in fin-shaped HEMTs will be discussed. Later, the threshold voltage for this device is formulated by incorporating the gate capacitance model in the threshold voltage expression. A major contribution of this work is that conformal mapping and principle of least action have been used for the first time for modeling fin-shaped HEMTs. The proposed model is completely physics-based with no additional fitting parameters in contrast to the available state-of-the-art models. Finally, the model validation with experimental data will be presented.