Impedance spectroscopy has been proven to be a useful tool to understand the interface processes and the transport mechanisms in devices’. Then, we have used this technique to understand the charge transport and study the conductance mechanism in the AlGaN/GaN/Si HEMTs. At an applied frequency, the complex impedance of a system can be written as [19] :

Z = ZR + j Zi = Z’ + j Z’’ (1)

where Z’ and Z” are real and imaginary part components of the impedance. Fig.2 shows the real part of impedance as a function of radial frequency at different bias voltage. As can be seen, the magnitude of Z’ decreases with the frequency increase. On the other hand, the real part of passivated AlGaN/GaN HEMTs shows the presence of constant region and a decreasing tendency at high frequency. In addition, the values of the real part of impedance decreases after passivation with N2O pretreatment. This means that the quality of the Schottky contact is improved by passivation, however, the decrease in barrier height [8]. Fig.3 shows the imaginary part of impedance signal as a function of radial frequency at different bias voltages. As can be noticed, the spectrum of an unpassivated and passivated AlGaN/GaN/Si HEMTs is composed of one overlapped peak. The variation of the imaginary part of impedance as a function of radial frequency presents a maximum peak at particular frequencies. It is found that the imaginary part of the impedance increases with the radial frequency and reaches a maximum value before decreasing again. In addition, the peak position shifts to higher frequencies and the peak intensity decreases with decreasing bias voltages. This indicates a relaxation time process. It should be noted that the contribution of relaxation process may possibly be explained by the presence of defects in AlGaN/GaN heterointerface. Fig.4 shows the complex impedance plot of an unpassivated and passivated AlGaN/GaN/Si HEMTs at different bias voltages. It is worth noticing that the single semi-circle in complex plane is derived from the ac reponse with frequency. In addition, all the semicircles exhibit some depression instead of a semicircle centered on the x-axis. It is found that the impedance spectrum is displayed a single semi-circle at bias voltages and the diameter of the semi-circle decreases with decreasing applied bias voltages. It should be noted that the impedance results were analyzed by the Z view Software [20]. However, all spectra could excellently be fitted to the equivalent circuit. Fig.5 shows the complex impedance spectrum at Vgs = 0V and the equivalent electrical circuit. This clearly shows that the best fit for the impedance data consisting of an additional series resistance Rs connected in series with two parallel circuits CPEb/Rb and CPEt/R2DEG. The impedance of the equivalent circuit at the AlGaN/GaN heterointerface can be expressed as [19] :

where Rs, Rb and R2DEG are the resistance of the ohmic contact, cap/barrier layers and 2DEG respectively. CPEb is the capacitive element of the barrier/cap layer, Qb is the charge of the barrier/cap layer, CPEt is the frequency dependent trapping of 2DEG charge carriers and Qt is the charge of 2DEG. Let CPE be the constant phase element, it is given according to [19] :

with 0 ≤ α ≤ 1. For α = 0, the system is described by a resistor, while for α = 1 an ideal capacitor is described.

It can be noticed that the CPE behavior include varing thickness or composition, surface roughness, or non-uniform current distribution [21-23]. The CPE accounts is used to successfully model an equivalent circuit of AlGaN/GaN heterointerface including effects due to traps and 2DEG depletion and frequency dispersion. The parameters obtained for the electrical equivalent circuit are summarized in Table 1. As can be seen, the resistance (Rs, Rb and R2DEG) decreases after passivation. However, the CPEt and CPEb increases due to the defect levels in the AlGaN/GaN HEMTs which create a distribution of charge generation/recombination time constants [19]. It should be noted that the passivation gives rise to a more improved electron transport. This improvement is assigned to the reduction of electron traps.