InAs/InGaAs composite-channel HEMT on InP: Tailoring InGaAs thickness for performance

Maximizing In composition in the channel structures of high-electron-mobility transistors on InP is one important aspect of achieving devices capable of operating beyond 300 GHz. In this article, we compare dc and rf performance results from two variations of one such device design, incorporating a composite-channel structure comprised of InAs clad by InP-lattice-matched InGaAs. The only difference between these two variations is the thickness of the bottom InGaAs cladding layer. The thicker gave extremely high performance, with current-gain-cutoff frequency (f_T) exceeding 500 GHz, enabled by room-temperature channel-electron Hall mobility (mu_e) as high as 15,400 cm²/V/s and dc transconductance (g_m) exceeding 2700 mS/mm; but it also incurred significant impact ionization. The thinner incurred less of this short-channel effect and yet gave very high performance, with f_T exceeding 440 GHz, enabled by mu_e as high as 14,800 cm²/V/s and g_m exceeding 2200 mS/mm, initially indicating that such a tradeoff might be the more overall beneficial. However, from a subsequent process iteration, in which the gate-recess etch was deepened for reduced short-channel effects, both of these same composite-channel design variations gave even better performance results. In that process iteration, the thicker variation not only achieved f_T exceeding 500 GHz, but also achieved the recently-published new record maximum frequency of oscillation (f_MAX) exceeding 1 THz. Therefore, the thicker bottom InGaAs cladding layer has indeed proven to be the more optimal composite-channel design variation for performance beyond 300 GHz.