Low and high frequency noise properties of heterojunction transistors.

Abstract

This thesis studies: (i) the noise properties of AlGaAs/GaAs Heterojunction Bipolar Transistors (HBT's) and novel High Electron Mobility Transistors (HEMT's) and (ii) the necessary measurement techniques. The baseband noise of AlGaAs/GaAs power HBT's and its upconversion to the operating frequency by non-linear device operation were studied. Comparison of direct and indirect measurement techniques for baseband noise demonstrated that direct measurements were the best to use. In all cases, the collector noise was greater than the base noise. Each contained different spectral components indicating that at least two intrinsic noise sources exist. The frequency and bias dependences of the spectra indicated that the origins of the noise were recombination based mechanisms, demonstrating that these devices are not at their technological limit. The role of baseband noise upconversion in the phase noise of HBT based dielectric resonator oscillators (DRO's) was studied for the first time and it was shown that in most cases the oscillator noise can be approximated by the HBT's baseband noise and the DRO's upconversion coefficient. The impact of base termination, collector current, and frequency on baseband noise, upconversion coefficient and phase noise are reported. The Y-factor and the noise power measurement approaches were compared to determine which is the best for determining HEMT microwave noise properties. Experiments demonstrated that the Y-factor approach has greater uncertainty than the noise power approach. Simulations showed that the Y-factor approach is sensitive to tuner magnitude errors, while, the noise power approach is sensitive to tuner phase errors. The microwave noise properties of double-doped strained InP HEMT's were analyzed and reported for the first time. The measured minimum noise figure, $F\sb{min}$, is as low as 0.3dB at 10GHz. The intrinsic drain noise current, $\overline{i\sbsp{dsn}{2}},$ however, is $>$7dB larger than that of longer gate GaAs MESFET's. The excellent $F\sb{min}$ is due to the reduced $\sqrt{\overline{i\sbsp{dsn}{2}}}/f\sb{T}\ (f\sb{T}$ is the cutoff frequency) ratio. Stability studies of HEMT's indicated that all electrical parameters decreased rapidly with applied thermal stress. $f\sb{T}$ decreased from 141GHz to 90GHz and $f\sb{max}$ decreased from 164GHz to 120GHz. This was primarily due to degradation of the device gain. Measurements indicated that increased trapping below the channel is a possible cause of the degradation.