Research On Noise Mechanisms And Modeling For New Types Of Microwave Transistors

The performance of microwave low noise amplifier (LNA) determines, to a largeextent, the sensitivity and the dynamic range of the receiving system. As the core deviceof LNA, the transistor plays a very important role, therefore, doing researches on thenoise characteristics of the transistor is really necessary and meaningful for improvingthe performance of the entire receiving system. As the continuous developing of thesemiconductor technologies, many new material and new structure transistors come tothe fore, keeping improving the devices’ and the circuits’ performance. Nowadays, thetraditional Silicon metal-oxide-semiconductor field-effect-transistors (MOSFETs) havescaled down to nanometer regime, a lot of new types of transistors based on whichemerge, such as multi-gate structure, junctionless transistor, etc. Because the processingtechnic of Si MOSFETs is compatible with that of the baseband and digital signalprocessing module, doing researches on the noise characteristics of nanoscale SiMOSFETs has great significance for designing and realizing the system on chip (SoC)based on the newest node of complementary metal-oxide-semiconductor (CMOS)technology, which has the advantages of low cost, high performance and highintegration density. On the other side, GaN high electron mobility transistors (HEMTs),which is one kind of wide bandgap semiconductors, have many merits, such as highworking voltage, high power density, high thermal conductivity and high electronsaturate velocity. Thus doing researches on the noise characteristics of GaN HEMTs issignificant for designing and realizing the LNA with strong points of ultra-wideband,high robustness, high dynamic range and high output power. Therefore, it is urgent to doresearches on the noise mechanisms and modeling for these new types of transistors toboost their application in the microwave low noise area, and to provide the theories andtechniques for designing and fabricating high performance LNA. The noisecharacteristics of nanoscale Si MOSFETs and GaN HEMTs have been studied in thisthesis by employing physical model, semi-physical model and empirical modelseparately.1. Research on the noise characteristics of nanoscale double-gate Si MOSFETs usingphysical model based on Monte Carlo methodAs the noise mechanics of the nanoscale Si MOSFETs have not been clearly discovered yet, the quantum Monte Carlo (MC) method is employed, which can mimicthe microscopic noisy movement of carriers inside the device, therefore, it includes themicroscopic noise sources inherently and without the need of making any previousassumption about their definition. The noise spectral densities are calculated from theanalysis of instantaneous current fluctuations at the drain and gate terminals in the MCsimulation by utilizing the formalism of the autocorrelation functions. Then the noiseparameters are deduced based on the noise theory. The accuracy of the MC method hasbeen verified by comparing the simulated results with those from other literature.Finally, the static, small-signal and noise performances of a30nm gate-length doublegate (DG) Si MOSFET have been studied by using the MC method. A detailedcomparison with conventional single gate (SG) Si MOSFETs is presented. The resultsshow that the DG structure provides better small-signal characteristics and superiornoise performance, owing to its better gate control ability, higher channel electrondensity and lower channel noise.2. Research on the noise characteristics of nanoscale junctionless Si MOSFETs usingphysical model based on Monte Carlo methodBoth the small-signal and noise performances of junctionless (JL) MOSFETs havebeen analyzed using the MC method for the first time, and the results of which havealso been compared with those of the conventional Si MOSFET with junction. Thestatic, small-signal and noise performances of the JL MOSFET have been studied basedon the inherent scattering mechanisms. The results indicate that, compared with thetraditional Si MOSFET with junction, the JL transistor exhibits lower electron velocitydue to the much stronger impurity scattering in the high doping channel, resulting inlower drain current (Ids), lower transconductance (gm), lower cut-off frequency (ft), andhigher minimum noise figure (NFmin). However, the JL MOSFET presents animprovement in the intrinsic voltage gain (Avo) and maximum frequency of oscillation(fmax) owing to a reduced output conductance (gds).3. Research on the noise characteristics for the novel nanoscale junctionless SiMOSFETsTo improve the small-signal and noise performances of nanoscale junctionless SiMOSFETs, three new structures of JL MOSFETs have been proposed for the first time,which optimize the gate electrode material, the oxide layer under the gate and thechannel doping concentration separately. They can improve the carriers transport efficiency in the channel, thus improve the transistor’s performance, by adjustingpotential and electrical field distributions in the channel. The MC method is alsoemployed to analyze the static, small-signal and noise performances of a30nmgate-length JL MOSFET with laterally graded-doping channel, named Grade-channelPseudo-junctionless (GPJL) MOSFET. The results show that, compared with thecommon JL MOSFET, the GPJL MOSFET exhibits better small-signal characteristics,the maximum values of ftand fmaxincrease11.6%and61%separately; meanwhile, italso presents an improvement in noise performance, its NFminat40GHz, for example,decreases about30%.4. Research on the noise characteristics of GaN HEMTs using semi-physical modelbased on drift-diffusion modelThe working mechanism of GaN HEMTs is studied by paying particular attentionon the characters of GaN material. A semi-physical model based on drift-diffusion (DD)model for GaN HEMTs has been set up, which can mimic the noise performance oftransistors by using Impedance Field method (IFM). The accuracy of the model hasbeen verified by comparing its simulated DC current with the measured results. Afterthen, the performances of a recessed-gate GaN HEMT and an InAlN/GaN HEMT havebeen analyzed using the semi-physical model separately. The results show that,compared with the common gate GaN HEMT, the recessed-gate GaN HEMT presentssuperior small-signal and noise characteristics as it improves the gate control ability, themaximum values of ftand fmaxfor recessed-gate GaN HEMT increase18%and27%respectively, and its NFminat12GHz decreases0.2dB; The noise results of theInAlN/GaN HEMT show that the value of NFminbecomes larger than that of thecommon GaN HEMT in the low frequency range as the gate leakage current ofInAlN/GaN HEMT is much larger than that of the common GaN HEMT.5. Research on the noise characteristics of GaN HEMTs using empirical model basedon equivalent circuitA simplified noise equivalent circuit model (SNECM) for GaN HEMTs has beenproposed to represent the noise performance of GaN HEMTs. The induced gate noisecurrent source in the equivalent circuit has been replaced by a thermal noise voltagesource caused by the intrinsic channel resistor, the shot noise caused by the gate leakagecurrent has been included. The method of calculating the noise parameters using theSNECM has been deduced theoretically, its accuracy has been verified by comparing its simulated noise results with the measured data of a GaN HEMT. The contribution ofeach noise source in the GaN HEMT as well as the effects of the gate leakage current onthe noise parameters have been analyzed by using the SNECM. The SNECM for arecessed-gate GaN HEMT from local GaN processing technic has been set up, based onwhich a LNA working from2to8GHz has been designed and fabricated, the measuredresults show that the power gain of the LNA is higher than11.5dB and the noise figureis lower than2.1dB.