The results of a comprehensive experimental and modelling investigation of high-frequency noise (HFN) of Si/SiGe:C and InP/InGaAs heterojunction bipolar transistors (HBTs) demonstrate that the key parameters to correctly model HFN for both HBTs are the self-heating (SH) effect at high collector injection levels, i.e. J C > 2-3 mA μm −2 , and the noise transport time (τ ) which must be included at all bias levels. In contrast, thermal noise contribution of the base-emitter junction of the Si/SiGe:C HBT, associated with the diffusive nature of electron transport at this junction because of the lack of conduction band discontinuity, is only important at operation frequencies lying in the frequency range f > f T /2. The noise analysis indicates that in order to correctly model HFN performances at any injection level and operation frequency, τ must be constant with bias and equal to the addition of base and collector transit times (τ B + τ C ). On the other hand, at high J C levels if SH is neglected, then the amplitude of the equivalent noise resistance (R n ) will be the most impacted noise parameter for both HBTs. Concerning the InP/InGaAs HBT, if SH is off at J C = 5.5 mA μm −2 , the minimum noise figure (NF min ) of the InP HBT will be underestimated by 10%.

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We present an original and reliable technique to elucidate the different contributions to the apparent base resistance (R B = R Bx + XR Bi ) of double heterojunction bipolar transistors (HBTs) designed by Alcatel-Thales III-V Lab. The extrinsic base resistance (R Bx ) is quantified using small-signal measurements. The base-collector junction distribution factor (X) and the intrinsic base resistance (R Bi ) are extracted from high-frequency noise (HFN) measurements. This method was applied to three InP/InGaAs HBTs having different emitter surfaces (S E ). The correct determination of R Bx , X and R Bi may be a useful tool for compact and/or linear electrical modelling and may give some guidelines to designers to improve operation frequencies. Moreover, this strategy can be applied to any layout and technological variation of HBT; it can be also applied to homojunction bipolar transistors. Our results show that HFN analysis should be included to fully characterize bipolar transistors.

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Propagation of sound waves and pulses through a weak inhomogeneous medium with a van der Waals interface is studied in the non-dissipative approximation by using a perturbative technique. It turns out that in some cases the resonance frequency of sound standing waves in a cavity will change from one resonant mode to another according to the detailed shape of the interface. Also, important differences between the standing waves and the pulse techniques are found. These results may provide a promising way to probe weak inhomogeneous interfaces.

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