Polar optical phonon scattering of two-dimensional electron gas in AlGaN/GaN high electron mobility transistor

Abstract

<sec>With the increasing demand for high-frequency, high-power and high-temperature microwave applications, AlGaN/GaN high electron mobility transistors have received much attention due to their promising material features such as wide band gaps, high-concentration two-dimensional electron gas (2DEG), strong electric field, at which the electron velocity is saturated, and high operating temperature. The 2DEG mobility at AlGaN/GaN interface is a key parameter to describe characteristics of high electron mobility transistor, and the mobility of 2DEG in AlGaN/GaN high electron mobility transistor is determined by a variety of scattering mechanisms in which the polar optical phonon scattering caused by electrostatic field between uneven polar positive and negative charges is responsible for mobility limitation in a 2DEG at high temperature. </sec><sec>Calculation of polar optical phonon scattering is carried out by the analytical model in which Fang-Howard variational wave function and Fermi’s golden rule are used. The interaction between 2DEG and phonon is described by scattering matrix element for the transition, in which phonon occupation number is given by Bose-Einstein statistics. The scattering time is derived by neglecting the in-scattering, and the numerically calculated energy-dependent scattering time is averaged according to Fermi statistics. </sec><sec>At temperatures in a range of 200–400 K and two-dimensional electron gas concentration in a range of 6 × 10¹¹–1 × 10¹³ cm^–2, the mobility varying with temperature is analyzed. It is found that the mobility limited by polar phonon scattering decreases monotonically with the temperature rising and their dependence is well approximated by a function of <inline-formula><tex-math id="Z-20200719010430-1">\begin{document}$\mu_{\rm PO} = AT^{-\alpha} ~ (\alpha = 3.5)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20200250_Z-20200719010430-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20200250_Z-20200719010430-1.png"/></alternatives></inline-formula> as the phonon occupation number increases with temperature rising. Furthermore, the polar optical phonon scattering is enhanced by greater electron concentration as a result of increased interaction between phonon and 2DEG. The mobility limit is calculated separately by emission phonon and absorption phonon, the results indicate that absorption phonon is predominant, which is attributed to high optical phonon energy in GaN. The mobility of polar optical phonon scattering is further studied by changing the optical phonon energy, which shows that the room temperature mobility of 2DEG can be improved by increasing the energy of polar optical phonon.</sec>