| dc.description.abstract | In this dissertation, the dc, ac and noise characteristics of InGaP/GaAs HBTs with various base contact size are investigated. According to the discussions of dc and ac measurement results, the difference of noise performance between the HBTs with various base contact size can be attributed to the ac current crowding effect. Besides, we proposed new analytical expressions for four noise parameters of InGaP/GaAs HBT based on the complete small-signal equivalent circuit, which takes base contact capacitance and ac current crowding effect into account. Consequently, this new noise model shows excellent experimental agreement. It can describe the noise behavior of InGaP/GaAs HBT at high frequencies accurately including ac current crowding effect.
In chapter 1, the overview of the HBTs has been introduced briefly. The origin of dc and ac emitter current crowding of the HBTs and the introduction of base contact impedance are also described. Besides, the overview of noise characteristics of the HBTs is described as well. In final, the noisy circuit analysis procedure is introduced for the noise parameters expression derivation of HBTs in Chapter 3.
The dependence of dc, ac and noise characteristics of InGaP/GaAs HBTs on the base contact size is presented in Chapter 2. The measured results show that the dc performances are not significantly dependent on WB but on LB. In the ac performances, the fT is not apparently dependent on WB and LB but the fmax is significantly degraded with reduced WB and LB due to the increased RB×CBC product. In the noise performance, the measured NFmin values increase slowly with reduced WB and LB at high frequency ranges due to ac current crowding effect. In the studied devices with fT of ~35 GHz, the measured minimum noise figure (NFmin) increases slowly with reduced base contact width (WB) while the operating frequency is over than 10 GHz. The corresponding decrease in noise resistance (Rn) of HBTs while increasing operating frequency indicates a significant decrease in the base resistance (RB) from the ac current crowding effect. The NFmin of device with WB of 2.0 μm is lowest at medium frequencies but higher than that of devices with WB < 2.0 μm at high frequencies. Based on the experimental results, the ac current crowding effect decelerates the increase in the NFmin at high frequencies.
In Chapter 3, a complete HBT high frequency noise model including the influences of the base contact capacitance and ac current crowding effect. Based on the proposed noise model, new expressions for the noise parameters of InGaP/GaAs HBTs are derived to describe the high frequency noise behavior in the presence of ac current crowding effect. The validity of the new noise model is presented by analyzing the four noise parameters of the HBTs with various base contact size. Good agreement is obtained between measured and calculated data for HBTs with various base contact size. The effect of ac current crowding on high frequency noise is well described. For device with high equivalent noise resistance (Rn), the proposed noise model with frequency dependent intrinsic base thermal noise describes the drop of Rn at high frequencies accurately. In addition, the base contact capacitance (CB) shows its significant influence on the noise parameters only when the base resistance is high.
However, current crowding effect is distributive. Therefore, the method in the noise model that we use Cbi//Rbi to describe the frequency dependent ac current crowding effect may not appropriate. Therefore, it still needs better method to describe the distributive base resistance to meet physical meaning.
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