Design considerations for normally-off operation in Schottky gate p-GaN/AlGaN/GaN HEMTs

Design criteria for normally-off operation in Schottky gate p-GaN/AlGaN/GaN HEMTs have been investigated. The threshold voltage (V-th) of this structure is determined by the following five parameters: (i) acceptor concentration of p-GaN layer (N-a), (ii) thickness of p-GaN layer (d(p-GaN)), (iii) Al composition of AlGaN layer (Alx), (iv) thickness of AlGaN layer (d(AlGaN)), and (v) Schottky barrier height at gate metal/p-GaN interface (phi(B)). Analytical equations are derived to obtain the quantitative relations of these parameters and dependences of normally-off condition on these parameters are discussed. To derive the analytical equation, two cases are considered in this study, i.e., case 1; thickd(p-GaN)case, and case 2; thind(p-GaN)case. For case 1, the normally-off condition is determined by Alx andd(AlGaN), and independent ofN(a),d(p-GaN), and phi(B). While for case 2, it is strongly affected byN(a),d(p-GaN), and phi(B). It is found that a thickerd(p-GaN)is needed for a lowerN(a), and with the increase ofd(AlGaN)at a certain Alx to achieve a normally-off operation. Also, it is found that Alx has to be kept less than 0.26 because higher Alx will cause the electric field across AlGaN layer to exceed 3 MV cm(-1), which may lead to breakdown in the AlGaN layer.