In this paper, we present some results on low frequency noise of GaN HEMTs grown either on sapphire, SiC or Si. The evolution of LF drain and gate current noise is analysed in ohmic and saturation regime. Devices on sapphire grown either by MOCVD or by MBE present αH value in ohmic regime as low as 10-4, whereas for devices grown by MOCVD on SiC, αH extends from 5x10-4 to 10-2. Low frequency gate current noise and coherence function have been measured to discuss possible correlation between drain and gate current noise. Devices with high reverse gate current exhibit high gate current noise. The coherence function increases when the Ig/Id ratio becomes higher than 10-5 which results in increase of the drain current noise due to contribution of gate current fluctuations.
The low frequency noise level is one of the key parameters, which determines the device potential for microwave applications. The fabrication of AlGaN/GaN HEMT on Si produces much more dislocations than on sapphire or SiC substrates. Therefore, it is important to analyse the influence of traps on the LF noise level of such devices.
Devices under test are undoped AlGaN/GaN HEMT on silicon substrate. These non-passivated devices present gate geometries of 3.5μm by (2x75)μm and 1.5μm by (2x50)μm. At 300K, output characteristics Id-Vds present kink effect occurring at Vds around 6V and high values of Vgs. As this effect vanishes for temperature above 300K, deep traps are activated. LF drain noise measurements were performed in the ohmic regime. We have measured the low frequency drain noise spectral density :- for drain current of 2mA with Vgs varying from -2V to 0V,
- for Vgs of 0V and -1V with Id from 2 to 6 mA. The dispersion of the drain noise level is about one order of magnitude difference. A typical spectrum is composed of 1/f noise which level is in the range of the ones measured on AlGaAs/GaAs HEMT and several generation-recombination components. Cut-off frequencies of G-R noise are typically lower than 100Hz. Therefore, the 1/f noise is screened by G-R noise at low frequencies and becomes preeminent for frequencies higher than 500Hz. The 1/f noise level verifies the Hooge relation. It is proportional to the square of Id. We have found that the αH/N parameter (that is the normalized drain current spectral density fSid/Id²) is in the range of 3×10-11 to 3×10-10.
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