Performance analysis and design of FET-embedded capacitive micromachined ultrasonic transducer (CMUT)

In this paper, we present design guideline and simulation results of capacitive micromachined ultrasonic transducer (CMUT) embedded field effect transistor (FET) for high frequency mode operation. The CMUT-FET consists of a mechanical CMUT part and an electrical sensing FET part. The device has a characteristic of a shared gate dielectric between a CMUT and an FET; hence, the gate capacitance change in the CMUT part can be reflected to the FET current as pressure changes. With an FET's current amplifying characteristic, the CMUT-FET makes it possible to achieve higher sensitivity and high resonant frequency with smaller element pitch and smaller active cell area than existing CMUTs. Combination of a 3-D finite element analysis (FEA) model of the CMUT part and a commercial technology computer aided design (TCAD) simulation of the FET device was used for inspection of the CMUT-FET device performance. In the 3-D FEA simulation, we focused on investigating pull-in voltage and electrical potential of a rectangular CMUT plate which have 20MHz resonant frequency in immersion with different dimensions. The FET part simulation was performed with variation of gate oxide thickness, source/drain doping concentration, channel doping concentration, substrate doping concentration. We combined data and inspected overall device performance with pressure variation at specific gate voltage and optimized the factor above-mentioned to get high sensitivity. Simulation results show that the optimized model has the collapse voltage of 13.4V and the pressure sensitivity of 11.34μA/Pa at gate voltage of 11V.