Pulsed-IV Pulsed-RF Cold-FET Parasitic Extraction of Biased AlGaN/GaN HEMTs Using Large Signal Network Analyzer

A new pulsed-
IV
pulsed-RF cold field-effect transistor (cold-FET) technique is presented to extract the parasitics of AlGaN/GaN HEMTs under various quiescent dc-biasing points. The measurement system implemented with a large signal network analyzer applies the technique of multiple recording to acquire pulsed-RF small-signal
S
-parameters with no loss of dynamic range as the pulse duty cycle decreases. These cold-FET measurements are performed on unpassivated and silicon nitride (SiN) passivated devices by turning the device off for 1 ?s with a 1% duty cycle to analyze the impact of slow thermal and trapping effects on the device parasitics. The parasitic fringe capacitances extracted are found to be bias independent, except for the gate to drain capacitance in devices without SiN passivation. In unpassivated devices, the drain parasitic resistance is found to rapidly increase with increasing drain bias at negative gate to source voltages. On the contrary, in devices with SiN passivation, the dependence of the resistance with the drain bias voltage is much less significant. A simple physical model is used to fit the functional dependence of the 2-D electron gas (2DEG) concentration upon the gate-to-source and gate-to-drain voltages, which is then proposed for fitting the measured data. The analysis indicates that the variation of the resistance with bias voltage in the device studied with SiN passivation and also for the unpassivated device at
V

_GS
=0 V is well accounted for by the reduction of the mobility with increased temperature due to self-heating, whereas for the device studied without SiN passivation, the increase of the drain resistance with drain voltages at negative gate bias principally arises from the decrease of the 2DEG population in a narrow region near the gate contact. An equivalent circuit is also introduced to explain the decrease of the source and drain parasitic inductances with increasing drain voltages at large negative gate bias, which is observed in unpassivated devices.