Design and Fabrication of GaN-based Permeable-Base Transistors
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Y10.21.1
Design and Fabrication of GaN-based Permeable-Base Transistors Jasper S. Cabalu1, Liberty L. Gunter3, Ian Friel2, Anirban Bhattacharyya1, Yuri Fedyunin1, Kanin Chu3, Enrico Bellotti1, Charles Eddy1 and Theodore D. Moustakas1 1 Department of Electrical and Computer Engineering, Boston University, Boston, MA, U.S.A. 2 Department of Physics, Boston University, Boston, MA, U.S.A. 3 BAE Systems, Nashua, NH, U.S.A. ABSTRACT In this work, we present the first successful fabrication of an etched grooved GaN-based permeable-base transistor structure. Growth of the device structures was done by Molecular Beam Epitaxy (MBE) on thick HVPE GaN quasi-substrates. The fabrication process took advantage of isolations pads via He implantation and SiN deposition, as well as submicron ICP etching of collector fingers patterned via e-beam lithography. SEM of the finished devices shows smooth etched finger structures and base layer surface with finger sidewall angles of ~85o for 1:1 and 1:3 finger spacing. Specific contact resistivities of ~ 3 X 10-6 Ω•cm2 for the ohmic contacts were achieved with Ti/Al/Ni/Au metallization scheme. Preliminary DC testing of the devices show a collector current IC = 140 mA/mm at VCE of 5V and VBE of +0.5V. The maximum transconductance gm is ~111 mS/mm in the measured collector-emitter bias range. These values are comparable, within the measurement tolerance, to physics-based modeling results. INTRODUCTION The permeable base transistor (PBT) is a viable alternative to the current generation of planar GaN based devices for high power, high frequency and high temperature applications. PBTs are essentially vertical MESFETs that can be fabricated with equivalent gate lengths shorter than 0.10 µm extending the frequency response beyond that of conventional lateral devices. Thus, a GaN PBT technology combines the high breakdown field of GaN and the good frequency response of PBT and thus can be potentially designed for millimeter-wave power applications. In this device, the current flowing vertically between the collector (drain) and the emitter (source) is controlled by the height of the electrostatically induced potential barrier under the collector. Although PBTs have already been demonstrated in Si, GaAs and SiC [1-3], to date there is very little being done to develop these devices in the III-Nitride material system. The high carrier vertical mobility [4] and high power characteristics of GaN makes it a suitable material for the fabrication of PBTs. Theoretical investigations of a PBT, also known as static induction transistor (SIT), in this material system have previously been reported [5, 6]. In this work we present the design and fabrication of Permeable-Base Transistors on GaN.
Y10.21.2
500 Α
o
0.2 µm
Collector Channel
1.3 µm
n+-GaN
o
n--GaN
Gate
600 Α
0.2 µm
EXPERIMENTAL DETAILS
6-10µm
Base
n+-GaN Emitter SAPPHIRE
(a) (b) Figure 1. (a) A 3-D illustration and (b) a schematic of the cross section of the completed PBT device. The PBT device fabricated in this research program ha
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