Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AlN/GaN heterostructures

The temperature dependent capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated by considering the series resistance (Rs) effect in the temperature range of 80–390 K. The experimental results show that the values of C and G/ω 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/ω 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/ω values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. Rs 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 Rs with temperature can be explained by the lack of free charge carriers, especially at low temperatures.