PACS 72. 70.+m, 73.40.Kp, 73.50.Td γ-ray radiation effect has been studied on transport and noise properties of high electron mobility transistors (HEMTs) with gate lengths in the range from 350 to 150 nm at room temperature. Current-voltage (I -V) characteristics of the devices demonstrate higher radiation hardness to 60 Co γ-rays up to doses of 10 9 Rad at larger gate lengths. This confirms the very important role of surface passivation for channel transport of the HEMTs. The deviation of the I -V characteristics parameters saturated current, transconductance, channel conductance, and threshold voltage does not exceed 20% at highest radiation dose. The noise spectra of pre-irradiated devices and after γ-irradiation show different frequency dependences corresponding to different fluctuation processes in the HEMTs. The results are confirmed by dynamic current measurements of the channel conductivity.
We address experimental and theoretical study of a two-dimensional electron gas transport at low and moderate electric fields. The devices under study are group-III nitride-based ͑AlGaN/GaN͒ gateless heterostructures grown on sapphire. The transmission line model patterns of different channel lengths, L, and of the same channel width are used. A strong dependence of the device I-V characteristics on the channel length has been found. We have developed a simple theoretical model to adequately describe the observed peculiarities in the I-V characteristics measured in steady-state and pulsed (10 Ϫ6 s) regimes. The effect of the Joule heating of a heterostructure is clearly distinguished. The thermal impedance and the channel temperature rise caused by the Joule self-heating have been extracted for the devices of different L at different values of dissipated power. The current reduction due to both self-heating and hot-electron effects is determined quantitatively as a function of the electric field.
In this work we present steady-state characteristics and low-frequency noise spectra of AlGaN/GaN based high electron mobility transistors (HEMTs) exposed to gamma ray radiation. The devices with a variety of gate length (150-350 nm) and width (100-400 νm) were irradiated by 60Co gamma rays with doses in the range of 104-109 Rad and flux of 102 Rad/s. Dose dependencies of basic operating parameters of the transistors, such as saturation current (Isat), transconductance (gm), channel conductance (gc), and threshold voltage (VT) are analysed. Our study show that visible changes of above mentioned parameters are observed under relatively small doses (105 Rad) and strongly depend on the HEMT's topology. The transconductance decreases and threshold voltage becomes more negative for all devices while deviation of these parameters from its initial values does not exceed 20% at highest irradiation dose. At the same time variation of the channel conductance as well as saturation current depends to a high extent on the gate voltage (Vg). At |Vg| < |Vcr|, both Isat and gc show a reversal proportional to the cumulative dosage of radiation. However, at |Vg| > |Vcr|, drain saturation current and channel conductance increase with the cumulative dosage of radiation. The effect is more pronounced in short-length-gate devices.
Room temperature current-voltage and noise measurements have been made before and after gamma irradiation on AlGaN/GaN high electron mobility transistors (HEMTs) grown on sapphire. The saturation current due to radiation-induced defects shows a nonlinear dependence on radiation dose. The deviation of the device parameters does not exceed 20% at highest radiation dose 10 9 Rad and the devices with larger gate lengths demonstrate a higher radiation hardness to the 60 Co gamma rays. The noise spectra of devices after gamma irradiation follow the flicker noise (1/f g ) dependence with the exponent g close to one. The Hooge parameter estimated for the HEMTs after gamma irradiation dose does not show a gate-bias dependence and increases by approximately three to five times at radiation dose 2 Â 10 8 Rad. The analysis of noise spectra allowed to identify the major sources of noise generation in the investigated HEMT heterostructures.Introduction AlGaN/GaN heterostructures, in spite of a lattice mismatch between epilayers of different alloy composition as well as the substrate which may be a reason for structural defect formation, demonstrate unique noise properties [1]. Besides, due to a strong chemical bonding they are expected to be promising for broad applications in radiation environment [2]. Investigation of noise properties gives important information about defect structure in multiplayer semiconductor heterostructures, especially if additional traps are introduced under external irradiation. Up to now, noise properties in irradiated HEMTs have not been investigated. At the same time, such investigations are very important not only from fundamental point of view for obtaining substantial information on the physical properties of semiconductor materials and devices but also it gives limiting values of the device performance for application in space environment. It is essential that we introduce basic point defects by gamma irradiation in a controlling way. The changes in defect structure under gamma irradiation are usually accompanied by a charge carrier removal. Additionally, radiation induced transfer of the elemental components stimulated by ionisation processes takes place. In our previous work gated influence of gamma irradiation on basic operating parameters of the transistors, such as transconductance, channel conductance, and threshold voltage. In this paper, the effect of gamma irradiation on low-frequency noise spectra of AlGaN/GaN HEMTs with nanoscale gate length are reported. These measurements provide information about radiation hardness of the HEMTs and their high potential in satellite system applications.Experimental Details Transport and noise properties in AlGaN/GaN grown by organometallic vapour phase epitaxy (OMVPE) on sapphire substrates have been studied before and after gamma irradiation. The device layer structure was as follows: a 40 nm AlGaN (16% Al) nucleation layer grown on the substrate was followed by a 1.1 mm nominally undoped GaN buffer layer and a 23 nm n-AlGaN (33% Al) undoped bar...
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