Short gate length AlGaN/GaN HEMTs
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Short gate length AlGaN/GaN HEMTs O. Breitschädel, L. Kley, H. Gräbeldinger, B. Kuhn, F. Scholz, and H.Schweizer 4. Phys. Institute, University of Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany Phone: +49-711-685-4961, Fax: +49-711-685-5097, [email protected] ABSTRACT We report on our progress on the fabrication of AlGaN/GaN HEMTs with extremely short gate length. AlGaN/GaN HEMTs with different gate length from 6 PGRZQWRQP were fabricated to investigate DC- and high frequency behavior as well as short channel effects. We have found that the transistors with gates in the 100 nm range can be improved in the device performance with respect to transconductance and high frequency but shows also short channel effects as the loss of saturation in the output characteristics and a strong dependency of the threshold voltage on the gate length. INTRODUCTION Significant progress has been made in the last view years in the development of GaNbased transistors. Compared with other III-V semiconductor heterostructures like AlGaAs/GaAs, the AlGaN/GaN heterostructures provides numerous advantages for transistor performance, such as high breakdown voltage, high peak electron velocity and high sheet electron concentration resulting in high drain current density up to 1.43 A/mm [1]. Especially GaN based high electron mobility transistors (HEMTs) [2] have demonstrated excellent high frequency performance at high power with gate length down to 120 nm [3]. To achieve high speed operation with transistors the time constant of the RC-circuit, where R represents the gate resistance and C the gate capacitance, should be as low as possible. The value of the capacitance is essentially determined by the gate length of the device. Therefore the gate resistance has to be kept low while simultaneously reducing the gate length. This can be done by forming a T-shaped gate. EXPERIMENTAL All device structures investigated in this contribution were grown by low pressure MOVPE using standard precursors on (0001) sapphire substrate. The vertical structure consists from bottom to top of a 100 nm thick AlN nucleation layer and a 1 - PWKLFN*D1 layer followed by a 36 nm and 50 nm thick AlGaN top layer, respectively. The Al concentration was adjusted to about 20 %. We have also investigated doped and undoped AlGaN top layer. From Hall measurements at room temperature in Van der Pauw geometry we obtained a sheet electron concentration of about 9.3×1012 cm-2 and 2.8×1013 cm-2, respectively, dependent on the doping level. The electron mobility values are between 1000 cm2/Vs and 500 cm2/Vs. The measured sheet electron concentration at 77 K was nearly the same as at room temperature. The measured mobility increased between a factor of two to three, which indicates the formation of a 2DEG at the AlGaN/GaN interface. In order to fabricate HEMTs, mesas were defined by optical lithography and formed by ion beam etching (IBE) [4]. As opposed to other dry etching techniques like ECR-reactive ion beam etching, IBE allows to con
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