Breaking Through the Multi-Mesa-Channel Width Limited of Normally Off GaN HEMTs Through Modulation of the Via-Hole-Length
© The Author(s). 2017
Received: 3 March 2017
Accepted: 8 June 2017
Published: 17 June 2017
We present new normally off GaN high-electron-mobility transistors (HEMTs) that overcome the typical limitations in multi-mesa-channel (MMC) width through modulation of the via-hole-length to regulate the charge neutrality screen effect. We have prepared enhancement-mode (E-mode) GaN HEMTs having widths of up to 300 nm, based on an enhanced surface pinning effect. E-mode GaN HEMTs having MMC structures and widths as well as via-hole-lengths of 100 nm/2 μm and 300 nm/6 μm, respectively, exhibited positive threshold voltages (V th) of 0.79 and 0.46 V, respectively. The on-resistances of the MMC and via-hole-length structures were lower than those of typical tri-gate nanoribbon GaN HEMTs. In addition, the devices not only achieved the E-mode but also improved the power performance of the GaN HEMTs and effectively mitigated the device thermal effect. We controlled the via-hole-length sidewall surface pinning effect to obtain the E-mode GaN HEMTs. Our findings suggest that via-hole-length normally off GaN HEMTs have great potential for use in next-generation power electronics.
Wide-bandgap III–V nitrides are promising semiconductor materials for frequency and voltage operation because of their excellent material properties, including large band gaps, high critical electric fields, high-saturation electron velocities, and high conductivities [1, 2]. Accordingly, they are widely used in various applications, including light emitting diodes (LED) and transistors . Furthermore, aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructures form two-dimensional electron gases (2DEGs) suitable for the development of high-performance devices, taking advantage of the spontaneous and piezoelectric polarization of III-nitride compounds [4–6]. The quantity of a 2DEG is influenced by the proportion of polarization-induced doping, which directly affects the device characteristics [7–9]. Although they have many attractive properties, AlGaN/GaN high-electron-mobility transistors (HEMTs) have not found universal utility because their electronic characteristics can require complex circuit configurations for digital, power, RF, and microwave circuit applications. Accordingly, normally off operation would be essential for any future III–V semiconductor devices [10, 11]. Although some special fabrication techniques have been tested (e.g., use of recessed gates [12–14], insertion of p-type capping layers under the gate [15, 16], tunnel junction structures , fluoride ion implantation into the barrier under the gate , and inclusion of thin AlGaN barrier layers with a special metal gate and rapid thermal annealing (RTA) treatment ), they can worsen device performance and cause stability issues through processing-induced material damage and increased thermal and electric field effects.
Alternatively, a team at Hokkaido University found that AlGaN/GaN HEMTs fabricated with fin-nanochannels exhibited a shift in the threshold voltage (V th) in the positive direction [20, 21]. A group at Soochow University reported that the value of V th underwent a systematic positive shift when the nanochannel width was less than 90 nm . Researchers at Kyungpook National University considered the partial strain relaxation of the channels’ sides to explain the behavior . A team at the Massachusetts Institute of Technology simulated the threshold voltage after surface passivation of GaN-based HEMTs and determined that positive values occurred when the width of the channel was less than 100 nm , the result of sidewall effects and increased tensile stress that decreased the electron concentration in the channel. Fin-shaped structures not only shift the threshold voltage but also improve gate controllability, due to the 3-D structure, which induces on-state performance while improving the off-state characteristics. The normalized maximum drain current (I D/mm) in an AlGaN/GaN HEMT having a fin-shaped structure is higher than that in a corresponding planar structure . Although these methods have been used to fabricate E-mode HEMTs, it remains very challenging to develop high-performance normally off GaN power transistors. First of all, the combination of a low on-resistance (R on) and a low device total power is to achieve when the width of the channel is limited to be less than 100 nm. Although the value of R on of the channel can be decreased by shrinking the length of the normally off gate, controlling the off-state drain leakage current poses another challenge because the gate width influences the transconductance and gate leakage through polarization coulomb field scattering and gate leakage paths [26, 27]. Deposited films can be used as gate dielectrics to improve these issues .
In this letter, we describe a breakthrough in the width limitation of tri-gate channels and propose a method for modulating the via-hole-length of the channels. Our device achieved the E-mode with a MMC structure width of 300 nm and a via-hole-length of 6 μm and exhibited a threshold voltage of 0.46 V. This approach not only decreased the device on-resistance (R on) but also could mitigate the Joule heating effect. By combining a 3-D tri-gate with various channel widths and via-hole-lengths, we achieved normally off GaN HEMTs having positive values of V th of 0.79 and 0.46 V when the channel widths/via-hole-lengths were 100 nm/2 μm and 300 nm/6 μm, respectively.
Results and Discussion
To date, most technological developments in GaN high-voltage transistors have been based on AlGaN/GaN HEMTs, which are intrinsically depletion-mode (D-mode) devices because of the polarization-induced 2-D electron gas at the AlGaN–GaN interface . Nevertheless, normally off GaN transistors will be required if the power electronics industry is to adopt GaN technologies widely.
Threshold voltages of HEMTs having MMC structures of various lengths and widths, measured at a drain current of 1 mA/mm
We have prepared E-mode GaN HEMTs having a multi-mesa-channel (MMC) structure; they exhibited a positive threshold voltage of 0.46 V when the channel width and via-hole-length were 300 nm and 6 μm, respectively. We infer that the effects of both lateral channel depletion and via-hole-length surface bending. When containing a tri-gate having a MMC via-hole-length structure, the new normally off GaN HEMTs exhibited very low on-resistance, even when increasing the MMC structure width to 300 nm (formerly limited to less than 100 nm). In addition, modulation of the via-hole-length MMC structure provided normally off GaN HEMTs improving excellent power performance, as a result of increasing the MMC structure device width.
We thank Nano Device Labs (NDL), Hsinchu, Taiwan, for performing the low-frequency noise and load-pull measurements. This study was supported financially by the National Science Council (NSC) of Taiwan (contract no. NSC-102-2221-E-182-060) and Chang Gung Memorial Hospital (BMRP 591).
YHY and RML conceived the idea and project. CYC and CHK designed the experiments. CYC and YHY optimized the MOCVD epitaxy. JHL prepared the mesa and multi-mesa-channel structures using e-beam lithography. WHW and CYL prepared the ohmic and Schottky region using photolithography. CYC and JHL recorded the SEM and optical microscopy images. YHY performed the material analyses. WHW and JHL performed the device’s electrical measurements. CHK provided the instruments for SEM and e-beam lithography. RML provided the MOCVD system. CYC wrote the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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