Abstract
Current conduction and saturation mechanisms in ungated AlGaN∕GaN∕SiC transfer length method test structures with contact-to-contact distance L varying from 2to32μm are investigated. Current-voltage (I-V) characteristics are measured in the pulsed and in the direct current (dc) regimes. The pulsed characteristics were evaluated for the pulse duration of 50ns, and together with the dc ones show that the current saturation value depends on the contact distance and the current saturation occurs at an electrical field ⪡150kV∕cm. This behavior contradicts the theoretical expectation valid for the ungated structures. An analysis of the I-V characteristics shows that the dc saturation current values are up to 5.5 times lower than those under the pulsed conditions for the same L. If the self-heating effect was supposed to be responsible for this observation, an unrealistic temperature rise would be required. This together with the observed ∼1s long transition time into the steady state, which is several orders of magnitude longer than what one can expect for the thermal transient, excludes the dominant role of the thermal effects in the current saturation mechanism. A model of the current conduction and saturation mechanism is suggested where the charge injection from the contacts and charging of the AlGaN surface are responsible for a AlGaN∕GaN channel depletion. Thus the saturation mechanism is similar to that occurring in gated structures. The model explains well the observed dependencies and is further validated by a transient interferometric mapping method. This thermooptical method operated under the pulsed conditions allows to evaluate the free carrier concentration profiles along the structure channel and to make the comparisons with the concentration below Ohmic contacts. A strong channel depletion effect has been observed for the structure with L=32μm in comparison to the negligible surface depletion effect found in the L=2μm structure. This is in full agreement with the suggested model of the current saturation mechanism and with the measured I-V characteristics.
Highlights
AlGaN / GaN high electron mobility transistorsHEMTsare supposed to deliver high power and work at millimeterwave frequencies
We suggest that the AlGaN surface potential is not floatingi.e., following the channel potential, as expected for the ungated structure,12 but is biased by a charge injected from contacts
We have investigated the current conduction and saturation mechanism in the AlGaN / GaN ungated TLM structures using electrical and optical mapping methods
Summary
AlGaN / GaN high electron mobility transistorsHEMTsare supposed to deliver high power and work at millimeterwave frequencies. Experiments show unexpected low values of Ecr and sat, and the questionable mechanism of the current saturation. The authors assumed that the pronounced temperature rise due to Joule heating and the carrier mobility decrease, and not the carrier velocity saturation, account for the current saturation This explanation cannot be applied for experiments with nanosecond-range pulses, where substantial elimination of the self-heating can be expected.. In this work we analyze mechanisms of current saturation in AlGaN / GaN TLM structures combining dc and pulsed I-V characterization with transient interferometric mappingTIMoptical method.. The potential of the AlGaN surface is not floating as expected for the ungated structure, and we suggest a different current saturation mechanism resembling the operation of gated structures. We show that TIM results provide an explanation of the current saturation mechanism and directly support conclusions of the I-V characterization
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