High temperature operation of n-AlGaN channel metal semiconductor field effect transistors on low-defect AlN templates

Abstract

We report room-temperature to 200 °C operation of n-Al0.65Ga0.35N channel metal semiconductor field effect transistors (MESFET) grown over high-quality AlN/sapphire templates. For this temperature range, the source-drain currents, threshold voltages, and dc-transconductance values remain nearly unchanged with an estimated field-effect mobility of ∼90 cm2/V-s at 200 °C and currents of >100 mA/mm. The analysis of the temperature dependent current-voltage characteristics of the gate-source Schottky barrier diode reveals that the leakage currents arise from Frenkel-Poole emission. The capacitance-voltage data show no hysteresis, indicating a high quality Schottky barrier interface. These MESFET's have excellent potential for use as a high temperature power electronic or a solar-blind ultraviolet sensing device.

Design of compositionally graded contact layers for MOCVD grown high Al-content AlGaN transistors

“…For Sample A (Figure 1(a)), the contact layer was graded from x=0.7 to x=0 over 50 nm, like previous reports, which requires [Si + ] > 8×10 18 cm -3 for polarization charge compensation ( Figure 1(b)). 22,23 For sample B (Figure 1(c)), the Al-composition was graded from x=0.7 to x=0.3 over 150 nm such that a lower [Si + ] concentration, can compensate the negative polarization charge density (Figure 1(d)). While a lower x at the top surface of the contact layer yields a lower metal-semiconductor resistance, the contact layer resistance will be higher due to a higher negative polarization charge density due to larger compositional grading and vice versa.…”

“…Equation 1 predicts that for a 50 nm linearly down-graded contact layer with x graded from 0.7 to 0, like previous reports, an activated dopant density needs to be higher than 8×10 18 cm -3 , throughout the contact layer, to achieve low-resistance films -a challenging problem for MOCVD due to the reasons mentioned above. 22,23 This indicates that the contact layer resistance is sensitive to the dopant incorporation, and it is critical for the activated dopant density to be higher than the negative polarization charge density. Thus, to achieve lower contact resistance in MOCVD grown graded contact layers, it is necessary to reduce the negative polarization charge density in the graded contact layers, ρ graded , by reducing the factor, ∇ 𝑑𝑑 𝑃𝑃[𝑥𝑥(𝑧𝑧)] -gradient of the polarization charge.…”

Ultra-wide band gap AlGaN polarization-doped field effect transistor

“…Despite achieving ohmic contacts, the I ds,max value for the PolFET is lower than the best previous reports for UWBG AlGaN HEMTs 2,3,9,10) and MESFETs. [14][15][16] Lower I ds,max for the PolFET is due in part to a larger source-to-drain spacing but primarily to a lower Al contrast in the heterostructure that results in a higher R sh and smaller knee voltage. Nonetheless, the PolFET compares well against prior HEMT and MESFET results in important aspects.…”

“…6,12,13) However, achieving ohmic contacts and simultaneously high μ and sheet density (n s ) remain significant challenges for AlGaN transistors. Al x Ga 1−x N metal-semiconductor field effect transistors (MESFETs) with ohmic source and drain contacts have been demonstrated for x = 0.65, 14) and 0.70, 15,16) but μ has yet to exceed 100 cm 2 V −1 s −1 for devices with current density >100 mA=mm. Channel μ of highly conductive UWBG Al x Ga 1−x N MESFETs is likely to remain <100 cm 2 V −1 s −1 because significant improvement requires increasing x > 0.8 to reduce alloy scattering, 5,17) but n-type impurity doping becomes inefficacious for x > 0.8.…”

Study of Nonradiative Recombination Centers in n-GaN Grown on LT-GaN and AlN Buffer Layer by Below-Gap Excitation

“…It has been reported that the insertion of thin AlN buffer layer between GaN epilayer and sapphire substrate can reduce tensile growth stress and dislocation density which in turn improve crystalline quality compared to that of the LT-GaN buffer layer [12] [13] [14] [15]. Recently, the high temperature AlGaN MSFET with AlN buffer layer and better surface morphology and crystalline quality of thick AlGaN have been realized for the growth on the AlN buffer layer [16] [17]. However, for further improvement of GaN based device performance, it is still insufficient to understand the formation mechanism of defect states and structural optimization for eliminating them during the growth process.…”