High current gain InGaN/GaN HBTs with 300°C operating temperature

Abstract

Material growth and device fabrication: The materials used in this study are grown in a Thomas Swan close-coupled showerhead (CCS) metal organic chemical vapour deposition (MOCVD) reactor with a 7 � 2 inch diameter wafer capacity. Trimethylgallium (TMGa), trimethylindium (TMIn), and high-purity ammonia (NH3 )a re used as the primary sources, while disilane (Si2H6) and bis-cyclopenta- dienylmagnesium (Cp2Mg) are the n-type and p-type dopant sources, respectively. All devices are grown on a 2.5 mm-thick high- temperature (Tg ¼ 1050 � C) GaN layer, which is grown with an � 20 nm low-temperature (Tg ¼ 550 � C) GaN buffer layer on a (0001) sapphire substrate. The layer structure of the InGaN=GaN HBT used in this study is shown in Table 1, and includes compositional grading in the emitter to eliminate any conduction band spikes, as well as to enhance the valence band barrier to holes. The graded region between the base and collector is n-type doped as a means of compensating polarisation charges that arise from compositional grading. The indium mole fraction was determined using X-ray diffraction rocking curve measurements. The hole concentration in the p þ InGaN base layer, as determined from Hall measurements, was found to be 2.5 � 10 18 cm � 3 , with a mobility of 3c m 2 =V-s at room temperature. SIMS profiles of this wafer show a Mg concentration of (Mg) � 3 � 10 19 cm � 3 . It should be noted that the base layer is designed with a slight reverse grade, with 3% indium mole fraction at the collector side of the base, graded to 4% at the emitter, in order to suppress defect formation and improve the epitaxial material quality. This grade is in the direction opposite to that desired to increase current gain.