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This paper is mainly dedicated to understand the phenomena governing the formation of two-dimensional electron gas (2DEG) confined in the quantum well which hold the role of the channel in the high electron density transistors (HEMT) based on AlGaN / GaN heterojunction. The theory takes into account: the crystal structure, the spontaneous and piezoelectric polarization concept, the formation mechanism of two-dimensional electron gas at the AlGaN / GaN interface, the approximate resolution of the Poisson and Schrödinger equations to determine the density of Two-dimensional electron gas after the analytical formula of the current-voltage characteristic is established. Our study is also concerned with the dependence of the twodimensional electron gas density on the following technological parameters: Aluminum molare fraction, AlGaN layer thickness and AlGaN layer doping, In order to control the influence of these parameters on the device performance. Finally, the current-voltage characteristic which reflects the variation of the drain-source current as a function of the modulation of the gate voltage has been discussed. Keyword: 2DEG INTRODUCTIONThe area of microelectronics has been marked by an increase in the demand for components which can operate in frequency increasingly higher [1][2][3]. Emerging needs in this area are not only related to the telecommunications market, but in other areas of application such as embedded electronics at bound for the automotive and aeronautic. The largest concerns in semiconductor integrated circuits are high-speed operation and low power consumption [4]. Since the 90s, a new generation of semiconductor components (HEMT) is under study and evaluation thanks to the advent of wide band gap materials such as GaN [5], GaN based devices are very useful for high frequency high temperature microwave applications such as radar systems [6]. Transistors AlGaN / GaN HEMTs results are very promising for power electronics and high frequency due to their two-dimensional electron gas 2DEG high density and high mobility as well as to their high breakdown field [7].To develop a reliable model of HEMT, an accurate estimate of the two-dimensional electron gas density at AlGaN / GaN interface is of considerable importance. A number of charge control models for HEMTs have been developed to characterize the 2-DEG concentration. These models are useful as they provide an insight into the physical operation of the device, but they generally require simplifying assumptions for the 2DEG density. Some models assume a linear dependency of 2DEG density at the gate bias, but the values of 2DEG density are underestimated near the threshold [8], [9]. Other models offer a

In a high electron mobility transistor (HEMT), the density of the twodimensional electron gas (2DEG) channel is modulated by the application of a bias to a Schottky metal gate. These devices are depletion mode (D-mode), which means that a negative bias must be applied to the gate to deplete the electron channel and turn. The most challenging aspect in the present research activity on based-GaN devices is the development of a reliable way to achieve an enhancement-mode (E-mode) HEMT. Enhancement-mode GaN HEMTs would offer a simplified circuitry by eliminating the negative power supply. In this work, the aim is to investigate the different techniques which can influence the threshold voltage and shift it to a positive value. A novel E-mode metal-insulator-semiconductor (MIS) AlInN/GaN HEMT with an Au-T-gate has been investigated. The impacts of window-recess and deep-recess have been discussed, it was found that for dp=28 nm and wn=1.8 µm the threshold voltage achieves 0.7 V and the transconductance (Gm) peak value of 523 mS at Vgs=3.5 V. The drain current characteristic has been demonstrated.