a b s t r a c tThe temperature dependent capacitance-voltage (C-V) and conductance-voltage (G/x-V) characteristics of (Ni/Au)/Al 0.22 Ga 0.78 N/AlN/GaN heterostructures were investigated by considering the series resistance (R s ) effect in the temperature range of 80-390 K. The experimental results show that the values of C and G/x are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point ($2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/x vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/x values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. R s values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C-V curves and the increase in R s with temperature can be explained by the lack of free charge carriers, especially at low temperatures.
We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiN x -passivated Al 0.83 In 0.17 N/AlN/GaN heterostructures. C-V and G/x-V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal-AlInN surface. The density (D t ) and time constant (s t ) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E c À E t ). The D st and s st values of the surface trap states for the unpassivated samples were found to be D st ffi ð4 À 13Þ Â 10 12 eV À1 cm À2 and s st % 3 ls to 7 ls, respectively. For the passivated sample, D st decreased to 1:5 Â 10 12 eV À1 cm À2 and s st to 1.8 ls to 2 ls. The density of surface trap states in Al 0.83 In 0.17 N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiN x passivation, indicating that the SiN x insulator layer between the metal contact and the surface of the Al 0.83 In 0.17 N layer can passivate surface states.
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