Effect of optical phonon scattering on the performance limits of ultrafast GaN transistors

As GaN HEMTs are scaled down to push performance into 100's of GHz range, it is timely to investigate their performance limits. Unlike Si MOSFETs and most other III–V semiconductor based HEMTs, the electron - polar optical phonon interaction is exceptionally strong in GaN. As a result, the mean free path of hot electrons in GaN is λop ∼ 3.5nm, far shorter than typical HEMT gate lengths (Lg). Thus while Si MOSFETs and other III-V HEMTs can approach near ballistic behavior by reduction of parasitic delays and Lg, the situation is starkly different for GaN HEMTs. Here, we investigate the intrinsic performance limits of GaN HEMTs by incorporating the effect of polar optical phonon backscattering into a quasi-ballistic model. Then, we include parasitic elements and quantitatively investigate the degradation in performance. The method used is semi-analytical, and will prove very helpful in designing future generations of devices. The work not only sets a roadmap for scaling to high speeds, it also offers clear physical reasons for a number of unexplained features observed in state-of-the-art GaN HEMTs.