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Performance predictions for A1GaN/GaN heterojunction bipolar transistors Fathpour, Sasan


Heterostructure transistors made from wide band-gap, wurtzite, nitride semiconductors are candidates for high-power, and high-temperature electronics. AlGaN/GaN heterojunction bipolar transistors (HBTs) have been studied since 1998, and the performance of fabricated devices has been improving steadily. However, the limits to the performance of these devices are not known. The main objective of this work is to achieve theoretical performance predictions for the characteristics of abrupt-emitter, and graded-emitter, n-p-n Al[sub x]Ga₁[sub –x]N/GaN HBTs. As a direct band-gap material, GaN has a high radiative recombination rate, which tends to dominate the base current. Therefore, the direct recombination probability is readily recognized as the limiting factor of the current gain. According to the employed semi-empirical model for the recombination lifetime, gains as high as 300 are predicted at room-temperature, while the figure decreases to around 100 at 600 K. There is a high spontaneous polarization in Al[sub x]Ga₁[sub –x]N nitrides. It is shown that the related intrinsic charges can degrade the gain considerably for layers which are N-faced, but there is no impact on the gain for Ga-faced layers. The incomplete ionization of magnesium, the presently employed acceptor in these materials, is another limiting factor of the devices; it leads to high-level injection in the base, which limits the ideal operating of the collector current density to below a few kA cm⁻². Regarding the high-frequency performance of HBTs, a comparison between the traditional analytic expressions and numerical regional signal-delay times has been accomplished. Partitioning the device has been performed by means of a simple phenomenological scheme. The study has been mostly performed on Al₀.₃Ga₀.₇As/GaAs HBTs, where the modeling parameters are mature, high-level injection is not a limitation, spontaneous polarization is not an issue, and complete ionization is a fair approximation. It is shown that the available expressions are inadequate to account for the mobile charge in the base-emitter space-charge region, as well as in the quasi-neutral emitter. It is suggested to simply multiply the base-emitter junction capacitance, calculated by the depletion approximation, by a correction factor of about two as a practical approach for accounting for these storage phenomena in graded-emitter HBTs. For the base-delay time, it is demonstrated that choosing the appropriate collection velocity is crucial in estimating the delay time analytically, if band-gap narrowing, quasi-ballistic transport, or hot-electron injection are important. For the collector-delay time, it is shown that the effect of the mobile charge in the collector-base space-charge region cannot be neglected, even at biases below the onset of the Kirk effect. The effects of incomplete ionization and high-level injection in AlGaN/GaN HBTs are discussed separately, and a practical cut-off frequency of 10 GHz is predicted for abrupt-emitter transistors.

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