The effect of gate leakage on the noise figure of AlGaN/GaN high electron mobility transistor (HEMTs) is explored. It is shown that these devices have a sizable amount of gate leakage that cannot be ignored when measuring their noise performance. Measurements across a single sample have more than 1 dB of variation in minimum noise figure. We will show this variation is because of gate leakage. A modified van der Ziel model is used to predict this large variation and allows easy noise figure prediction of HEMT and MESFET devices.

The effect of noise figure of different AlGaN/GaN high electron-mobility transistor (HEMT) epitaxy structures is reported. The addition of a thin AlN layer between the barrier and channel gives better performance at biasings other than the best for minimum noise figure. However, varying Al composition in the HEMT barrier does not change the noise performance, contrary to a 2003 study by Lu et al. The measurements are checked with both the Pospieszalski and van der Ziel (Pucel) models. The models are used on six different samples, helping to reinforce the measurements and showing the strengths and weaknesses of each.

The first AlGaN/GaN HEMT based differential oscillator is reported. The MMIC oscillates at a frequency of 4.16 GHz and provides 22.9 dBm of power from one side at a biasing of Vgs -1 V and Vds 20 V. The HEMTs each have a 0.7 um x 200 um gate. The second harmonic is 45 dB below the carrier and the third harmonic is more than 70 dB below the carrier. To our knowledge, this is the best reported harmonic performance for a GaN oscillator. The oscillator efficiency is between 4% and 9.4% depending on bias. The measured phase noise is -86.3 dBc and -115.7 dBc at offsets of 100 kHz and 1 MHz respectively. The phase noise at a 1 MHz offset is similar to the noise performance of FET based differential oscillators in other technologies.