A low phase-noise X-band MMIC VCO using high-linearity and low-noise composite-channel Al0.3Ga0.7N/Al0.05Ga0.95 N/GaN HEMTs

A low phase-noise X-band monolithic-microwave integrated-circuit voltage-controlled oscillator (VCO) based on a novel high-linearity and low-noise composite-channel Al
_0.3
Ga
_0.7
N/Al
_0.05
Ga
_0.95
N/GaN high electron mobility transistor (HEMT) is presented. The HEMT has a 1 μm x 100 μm gate. A planar inter-digitated metal-semiconductor-metal varactor is used to tune the VCO's frequency. The polyimide dielectric layer is inserted between a metal and GaN buffer to improve the Q factor of spiral inductors. The VCO exhibits a frequency tuning range from 9.11 to 9.55 GHz with the varactor's voltage from 4 to 6 V, an average output power of 3.3 dBm, and an average efficiency of 7% at a gate bias of -3 V and a drain bias of 5 V. The measured phase noise is -82 dBc/Hz and -110 dBc/Hz at offsets of 100 kHz and 1 MHz at a varactor's voltage (V
_tune
)=5 V. The phase noise is the lowest reported thus far in VCOs made of GaN-based HEMTs. In addition, the VCO also exhibits the minimum second harmonic suppression of 47 dBc. The chip size is 1.2 x 1.05 mm
²