Lattice matched InGaAs/InAlAs HEMTs have lower noise figure, higher gain and higher cut-off frequency over GaAs-based FETs. So, InP-based HEMT technology is the mainstream of high speed microwave and millimeter wave circuits. Recently, the performance of InP-based HEMT is improved by higher In mole fraction channel and reduction of the gate-length while maintaining a high aspect ratio.
In this work, the epi-structure simulation based on quantum mechanics and the fabrication development of InGaAs/InAlAs HEMT have been studied and discussed. $Schrödinger$ equation and poisson equation are solved by the conventional finite-difference method. Self-consistent solution of $Schrödinger$ equation and poisson equation is successfully obtained using the subsequent iteration method. Based on the self-consistent solution, charge control in quantum-well channel is analyzed. From the calculated gate capacitance-voltage behavior, a simple estimation for the device performance has been presented. Experimental results on InGaAs/InAlAs HEMTs are in good quantitative agreement with the simulated results.
Performance development of HEMTs is achieved by lower ohmic contact resistance and lower gate leakage current. Small signal intrinsic transconductance extracted from the measured S-parameters is increased of 13\% over the previous result. Cut-off frequency($f_T$) and maximum oscillation frequency($f_{max}$) are also improved. The gate leakage current is suppressed by a sidewall etching and a selective gate recess. The selective etching over InGaAs/InAlAs is successfully achieved by the solution of a succinic acid and a hydrogen peroxide. Excellent RF performance($f_T$=25.7GHz, $f_{max}$= 53.1GHz) is obtained by the optimized fabrication process. For reliable HEMT operation, passivation effects of various materials have been studied. Benzocyclobutene(BCB) passivated HEMTs exhibited very stable performance for thermal stress of 200℃. Moreover, this material has the advantages of a low dielectric permittivity(2.7) and a low loss tangent(0.0008). So, this passivation technology has great potential for HEMT passivation.
