Design of AlGaN/GaN Heterojunction Bipolar Transistor Structures
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Design of AlGaN/GaN Heterojunction Bipolar Transistor Structures Yumin Zhang, Cheng Cai, and P. Paul Ruden Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455 ABSTRACT The potential of III-nitride materials for the fabrication of bipolar transistors is investigated theoretically. Several different pseudomorphic AlGaN/GaN n-p-n heterojunction bipolar transistor structures are examined through calculations of their band profiles and majority carrier distributions in equilibrium. Spontaneous and piezoelectric polarization charges are utilized to create large hole sheet carrier densities in the base layer, thus minimizing the base spreading resistance. At the same time, a large accelerating field in the base can help reduce the base transit times of the electrons and, hence, increase the current gains of these devices. The effect of strain due to substrate mismatch is also investigated. INTRODUCTION With the development of suitable epitaxy and processing techniques for III-nitride material systems, 1 their use for the fabrication of electronic devices is under active investigation. Due to their large bandgaps and high electron drift velocities, these materials appear to be well suited for high power and high temperature applications. Heterojunction bipolar transistors (HBTs) generally have distinguishing advantages. Among these are high transconductance, good linearity and threshold uniformity, high power density, and low flicker noise. 2,3 The main limiting factors for HBTs in the III-nitrides appear to be associated with the relatively large acceptor ionization energies and the rather low hole mobility that together give rise to a high base spreading resistance. The high base resistance imposes severe limitations on the maximum oscillation frequency and is expected to cause considerable current crowding. Also associated with the high base spreading resistance is a lateral electric field in the base layer that attracts the injected electrons to the base contact, possibly increasing the recombination rate and thus reducing the gain. Wurtzite structure semiconductors such as the III-nitrides typically have relatively ionic bonds leading to large stress-dependent electric polarization parallel to the hexagonal crystal axis. 4,5 One may refer to the polarization in these materials as consisting of an equilibrium spontaneous polarization and a stress dependent piezoelectric polarization. It has been suggested previously 6,7 that polarization charges may be employed to induce mobile charge carriers in specified regions of a device structure with modulated material composition. SELF-CONSISTENT BAND MODEL III-nitride materials can be grown epitaxially along the hexagonal axis with two distinct polarities. 8 For an emitter-up HBT design, the cation-capped, (000-1) growth direction is advantageous. This polarity yields negative polarization charges in the region where the AlN composition increases from the base to the emitter layer. The integrated (two-dimensional)
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