Algan/gan High Electron Mobility Transistor Device Modeling And Characteristic Research

Compared with recently semiconductor materials, GaN has some features, such as large direct bandgap, high saturation drift velocity, high thermal stability, high breakdown electric field and so on, which indicates it can be used in radiation resistant、 high frequency、power microwave and optoelectronics field. AlGaN/GaN high electron mobility transistor (HEMTs) is a kind of GaN device based on the AlGaN/GaN heterojunction, and they own the outstanding performances of high transconductance, high output saturation current and high cutoff frequency. This paper has a very detailed introduction of the structure and operation principle of AlGaN/GaN HEMT. Fermi energy versus two-dimensional electron gas (2DEG) density and the charge control model are investigated from the point of the view of device, and we also make a thorough inquiry for current collapse effect as well as the methods of decreasing current collapse.(1) After reading lots of related materials, we get the operation principle of AlGaN/GaN HEMT. It is to control the current by altering the density of the two-dimension electron gas (2DEG) near the heterostructure through changing the Schottky barrier. So, the most important factor is the heterostructure between doped AlGaN and undoped GaN. The confinement of carriers in the2DEG of the unintentionally doped GAN, and the spatial isolation from their parent impurity atoms on the AlGaN side reduce their scattering, increasing their mobility and enhance other characters of the device. The main origins of the electrons of2DEG are transferring from doped AlGaN layer, contributed by impurities in the GaN channel layer and induced by spontaneous polarization as well as piezoelectric polarization.(2) Current collapse is a major factor of reliability and output power which effects the application of AlGaN/GaN HEMTs on microwave power area. We made a thorough inquiry for current collapse effect and summarized the main two mechanisms which can give reasonable interpretation of the current collapse effect. Especially, we described a few methods which can decrease current collapse, such as surface treatment, field plate structure, growing capped layer and so on. And, we made some research with the methods of decreasing current collapse from some experimental results and simulation results.(3) Two new nonlinear expressions of Fermi-level variation with two-dimensional electron gas (2-DEG) density have been proposed. It was found that our expressions have a better fit with the numerical results of Ef versus ns than other approximate functions in all regions of operation of interest. On the basis of our expressions, two analytical expressions for ns in terms of the applied gate voltage is developed. Compared with other previous approximations, the solutions of our expressions of ns versus VG-Voff have a better agreement with the exact numerical results over the entire range of interest, and particularly, a great improvement is made in the subthreshold region. Our second expression has more simple structure than the first one, and it can be used more convenient. Besides, for an AlGaN/GaN system, the solutions of the second expression of ns versus VG are compared with the experimental data and shown to be in good agreement over a wide range of bias conditions. This single analytical expression for ns as a function of VG, valid from subthreshold to high conduction, can be used in the development of an improved accurate analytical model for the HEMT.