Spontaneous and Piezoelectric Polarization Effects on the Frequency Response of Wurtzite Aluminium Gallium Nitride / Sil
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1069-D07-19
Spontaneous and Piezoelectric Polarization Effects on the Frequency Response of Wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors Choudhury Praharaj Cornell University, Ithaca, NY, 14853 ABSTRACT We present theoretical calculations for the effect of spontaneous and piezoelectric polarization on the base resistance and frequency response of wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors. Heterojunction Bipolar Transistors ( HBTs ) built using wide band gap semiconductors with AlGaN emitter and SiC base/collector hold the promise of high-power and high-frequency operation due to lower impact ionization coefficients and higher breakdown voltages. Further, Silicon Carbide has an indirect bang gap, and a large lifetime of minority carriers compared to most other compound semiconductors, which tend to have direct band gaps. This reduces the base recombination factor when the base is made from SiC, and helps to achieve higher overall current gain. Spontaneous and piezoelectric polarizations of the order of 1013 electrons per cm2 exist in wurtzite wide band-gap semiconductors. This has a non-trivial effect on band profile, charge transport and overall device characteristics since the polarization-induced charges are of the same order of magnitude as the total dopant charge content of critical device layers, and can significantly affect the amount of mobile charge depletion or accumulation in these layers. We calculate the effect of this polarization for both very thin pseudomorphic emitters and for relaxed emitter structures. We present calculations for the cases of Si-face and C-face SiC, since the signs of polarization-induced charges are different for the two cases. The intrinsic base resistance near emitter flat-band conditions is changed by about a factor of 10 depending on the alloy composition of the emitter and the polarity of growth. The maximum frequency of oscillation under emitter flat-band conditions can also be modulated by the polarizationinduced charges by up to 60%. Our calculations show that the technologically less prevalent C-face SiC can give a higher advantage for frequency response, especially when the emitter thickness is larger than the critical thickness. INTRODUCTION The most common crystalline form for the wide band gap group III nitrides and SiC is the hexagonal wurtzite structure. These semiconductors have spontaneous polarization in the absence of externally applied electric fields [ 1, 2 ]. Further, the piezoelectric coefficients are very large [ 3 ] in these materials. The resulting polarization discontinuity at abrupt heterojunctions leads to sheet charges of the order of 1013 electrons per cm2. For forward active operation of Heterojunction Bipolar transistors based on wurtzite wide band-gap semiconductors, the resistance of the base layer is modified due to this sheet charge. For base layer widths required to maintain high current gains, this affects the frequency response of the device. Modulation
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