{"id":1101,"date":"2013-07-25T18:26:29","date_gmt":"2013-07-25T18:26:29","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/?p=1101"},"modified":"2013-08-13T21:49:26","modified_gmt":"2013-08-13T21:49:26","slug":"high-frequency-performance-of-gan-hemts-at-cryogenic-temperatures","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/high-frequency-performance-of-gan-hemts-at-cryogenic-temperatures\/","title":{"rendered":"High-frequency Performance of GaN HEMTs at Cryogenic Temperatures"},"content":{"rendered":"

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<\/a>]<\/sup>[2<\/a>]<\/sup>[3<\/a>]<\/sup> and power gain cutoff frequency of 400 GHz[4<\/a>]<\/sup>. 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.<\/p>\n

The heterostructure used in this study consisted of a 10.8-nm In0.13<\/sub>Al0.83<\/sub>Ga0.04<\/sub>N layer near lattice-matched to GaN and 0.5-nm AlN interlayer as the top barrier, and 1.8-\u00b5m GaN buffer. Devices with gate lengths between sub-30 nm and 120 nm and total on-resistance below 0.5 \u03a9\u00b7mm were fabricated. As the temperature goes down, the on resistance decreases by 15-20 % while the extrinsic gm<\/sub> increases by 10-13 % due to the increase of the mobility as shown in Figure 1. In RF performance, the improvement in fT<\/sub> with decreasing temperature is larger in the shorter gate length devices. In the 29-nm gate length device, the fT<\/sub> 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 (ve<\/sub>).<\/p>\n

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 (Rds<\/sub>) and intrinsic transconductance (gm<\/sub>,int<\/sub>), respectively. Our estimates show that to push fT<\/sub> 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.<\/p>\n\n\t\t