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High-frequency Performance of GaN HEMTs at Cryogenic Temperatures

• Authors: D. S. Lee, T. Palacios
• Sponsorship: DARPA NEXT, NSF CAREER Award

The high-frequency performance of GaN-based high-electron-mobility transistors (HEMTs) has been significantly improved over the last two decades. State-of-the-art devices have demonstrated current gain cutoff frequencies over 300 GHz^[1][2][3] and power gain cutoff frequency of 400 GHz^[4]. In this study, we characterize for the first time the low-temperature performance of 300-GHz-class devices and, based on the temperature-dependent small-signal circuit parameters, we provide a roadmap for future improvements in frequency performance.

The heterostructure used in this study consisted of a 10.8-nm In_0.13Al_0.83Ga_0.04N layer near lattice-matched to GaN and 0.5-nm AlN interlayer as the top barrier, and 1.8-µm GaN buffer. Devices with gate lengths between sub-30 nm and 120 nm and total on-resistance below 0.5 Ω·mm were fabricated. As the temperature goes down, the on resistance decreases by 15-20 % while the extrinsic g_m increases by 10-13 % due to the increase of the mobility as shown in Figure 1. In RF performance, the improvement in f_T with decreasing temperature is larger in the shorter gate length devices. In the 29-nm gate length device, the f_T increases from 313 GHz at room temperature to 347 GHz at 77 K. According to a delay analysis based on the small-signal circuit parameters, the improvement results mainly from a reduction of the parasitic delay and a marginal increase of the electron velocity (v_e).

Based on the small-signal circuit parameters extracted as a function of temperature, we estimated the high-frequency performance of GaN HEMTs as the gate length is scaled below 30 nm. To account for the short-channel effects, linear and parabolic trends are assumed for output resistance (R_ds) and intrinsic transconductance (g_m,_int), respectively. Our estimates show that to push f_T above 400 GHz. it is very important to significantly reduce the electron velocity degradation caused by the short-channel effects and fringing gate capacitance as shown in Figure 2.



Figure 1: DC characteristics of 29-nm gate length device at 77 and 300 K.



Figure 2: Comparison between the measurement data and model and estimation of the frequency performance of GaN HEMTs by improving each component.




1. K. Shinohara, D. Regan, A. Corrion, D. Brown, S. Burnham, P. J. Willadsen, I. Alvarado-Rodriguez, M. Cunningham, C. Butler, A. Schmitz, S. Kim, B. Holden, D. Chang, V. Lee, A. Ohoka, P. M. Asbeck, and M. Micovic, “Deeply-Scaled Self-Aligned-Gate GaN DH-HEMTs with Ultrahigh Cutoff Frequency,” IEDM Tech. Dig., 2011, paper 19.1, pp. 453-456. [↩]
2. D. S. Lee, X. Gao, S. Guo, D. Kopp, P. Fay, T. Palacios, “300-GHz InAlN/GaN HEMTs With InGaN Back-Barrier,” IEEE Electron Dev. Lett., vol. 32, no. 11, pp. 1525-1527, Nov. 2011. [↩]
3. Y. Yue, Z. Hu, J. Guo, B. Sensale-Rodriguez, G. Li, R. Wang, f. Faria, T. Fang, B. Song, X. Gao, S. Guo, T. Kosel, G. Snider, P. Fay, D. Jena, H. Xing, “InAlN/GaN HEMTs With Regorwn Ohmic Contacts and fT of 370 GHz,” IEEE Electron Dev. Lett., vol. 33, no. 7, pp. 988-990, July 2012. [↩]
4. K. Shinohara, A. Corrion, D. Regan, I. Milosavljevic, D. Brown, S. Burnham, P. J. Willadsen, C. Butler, A. Schmitz, D. Wheeler, A. Fung, and M. Micovic, “220 GHz f_T and 400 GHz f_max in 40-nm GaN DH-HEMTs with Re-grown Ohmic,” IEDM Tech. Dig., 2010, paper 30.1, pp. 672-675. [↩]

High Linearity GaN HEMTs with Nanowire Channel

• Authors: D. S. Lee, T. Palacios
• Sponsorship: ONR YIP, DARPA NEXT, NSF CAREER Award

The performance of GaN-based high-electron-mobility transistors (HEMTs) has been significantly improved over the last two decades through extensive research^[1][2][3]. However, the demonstrated device characteristics are still below theoretical expectations based on GaN material properties, and several unsolved problems remain for large-signal RF operation of these devices. One of the critical issues is the non-linear behavior of short-channel GaN HEMTs. In conventional GaN devices, the extrinsic transconductance (g_m) drops quickly with increasing drain current after reaching its peak value, and this issue becomes more serious as the gate length scales down. Another important issue is that a maximum drain current density in conventional short-channel GaN devices is still far below the theoretical value given by q·n_s·v_sat, where q is electron charge, n_s is electron sheet charge density, and v_sat is saturation velocity.

To address these two issues, we have demonstrated a HEMT with a nanowire channel structure as shown in Figure 1. We found that the increase of source access resistance with drain current severely limits the linearity of GaN HEMTs. When the ratio of the source access region width to that of the channel increases, the transistor access region has a larger current drivability than the channel region, which enables the source access region to behave more like an ideal source. The suppression of the increase in source access resistance with current provides a flat transconductance (g_m) at high drain current even in sub-100-nm short-channel devices as shown in Figure 2. Moreover, the constant source resistance allows a higher effective gate voltage overdrive, which enables a higher drain current density (over 3.5 A/mm) in the intrinsic device. The new devices also show very flat current-gain cut-off frequency (f_T) as a function of gate voltage. These results highlight the importance of the source access region in limiting the maximum current density and the linearity of GaN HEMTs.



Figure 1: Device structure of nanowire channel HEMT and its fabrication process.



Figure 2: Comparison of DC (a) transfer and (b) output characteristics of Lg=70-80-nm nanowire channel device with planar device.




1. K. Shinohara, D. Regan, A. Corrion, D. Brown, Y. Tang, J. Wong, G. Candia, A. Schmitz, H. Fung, S. Kim and M. Micovic, “Self-Aligned-Gate GaN HEMTs with Heavily-Doped n+-GaN Ohmic Contacts to 2DEG, ” IEDM Tech. Dig., 2012, paper 27.2, pp. 617-620. [↩]
2. Y. Yue, Z. Hu, J. Guo, B. Sensale-Rodriguez, G. Li, R. Wang, f. Faria, T. Fang, B. Song, X. Gao, S. Guo, T. Kosel, G. Snider, P. Fay, D. Jena, H. Xing, “InAlN/GaN HEMTs with Regorwn Ohmic Contacts and fT of 370 GHz,” IEEE Electron Dev. Lett., vol. 33, no. 7, pp. 988-990, July 2012. [↩]
3. D. S. Lee, X. Gao, S. Guo, D. Kopp, P. Fay, T. Palacios, “300-GHz InAlN/GaN HEMTs with InGaN Back-Barrier,” IEEE Electron Dev. Lett., vol. 32, no. 11, pp. 1525-1527, Nov. 2011. [↩]