Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

Abstract

Abstract:We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10 7 cm/s at a low sheet charge density of 7.8 x 10 11 cm -2 . A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.a)

Amplified spontaneous emission of phonons as a likely mechanism for density-dependent velocity saturation in GaN transistors

“…The measurements performed in [6] agree qualitatively with the theory developed in [8] but cannot be approximated by the simple power dependence over the entire range of electron densities.…”

“…The situation is strikingly similar to the "spectral hole burning" [25] occurring in lasers (the "spectrum" here is of course in the spatial rather than the frequency domain). As a result, the distribution of LO phonons (Fig.5b) is expected to saturate and broaden, and the drift velocity is expected to continue to increase slowly, as indicated by the dashed lines in Fig.5a and had been indeed measured in [6].…”

“…therefore the saturation velocity exhibits a strong dependence on electron density (as well as on phonon lifetime) but at really high densities (in excess of 2×10 19 cm -3 ) the drift velocity is expected to saturate around 10 7 cm/s for a wide range of conditions. This can be best seen in Fig.4a where the density dependence of saturation velocity is compared with the experimental values obtained in [6]. Since our model is three-dimensional, the values of Ne were obtained from the measured sheet density Ns by a self-consistent solution of the Poisson and Schrödinger equations.…”

Ultrawide‐Bandgap Semiconductors: Research Opportunities and Challenges

“…These high-field transport properties have been investigated experimentally in WBG GaN, but not yet in UWBG AlGaN. [91,172] Saturation velocities of the order 10 7 cm s −1 are measured at electric fields above 0.1 MV cm −1 in GaN. High-field carrier saturation in the oxides and diamond are expected to follow similar trends since the effective mass is similar to that of GaN.…”

Electron transport in a coupled GaN/AlN/GaN channel of nitride HFET

“…The cut-off frequency is related to the average carrier velocity in a two-dimensional electron gas (2DEG) channel. The experimental reports show that the room temperature electron velocity values in GaN-based 2DEG channels lie between (1-3)×10 7 cm/s [3][4][5][6][7][8][9]. While the low-field electron mobility tends to decrease as the density of 2DEG increases, the high-field drift velocity is not a monotonous function of the density [10].…”