Study On GaN-based Enhancement-mode MIS-HEMT And E/D-mode Circuits

AlGaN/GaN high electron mobility transistors?HEMTs?are highly desirable for high voltage RF power applications owing to their outstanding physical properties.In recent years,enhancement-mode?E-mode?devices have been attracting great interest for achieving E/D mode logic circuit to simplify the drive circuit design and reduce the circuit size.The MIS-HMETs with an inserted dielectric layer can effectively decrease the gate leakage current,moreover,enlarge the forward gate voltage swing.E-mode Al₂O₃/AlGaN/GaN MIS-HEMTs were fabricated,furthermore,the models of the D-mode HEMTs and E-mode MIS-HEMTs were built.The level shifter circuit based on the corresponding E-mode MIS-HEMTs were simulated and fabricated.The measurement and analysis of the circuit were conducted further.E-mode Al₂O₃/AlGaN/GaN MIS-HEMTs with different recess depth were fabricated using gate recess and ALD-Al₂O₃ process by controlling the gate recess etching time.The DC and small-signal performances were compared and analyzed respectively.The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of+1.08 V,a saturation drain current of425.3 mA/mm and a peak transconductance of 69.7 mS/mm.The fabricated devices show RF performances which consist of a cut-off frequency of 15.3 GHz and a maximum oscillation frequency of 43.8 GHz.The frequency/temperature-dependent capacitance-voltage measurement are employed for the extraction of the interface trap state density between Al₂O₃ and AlGaN.And the interface trap state density decreased effectively after the post-dielectric annealing process.The EE-HEMT1 model was utilizing to fit the DC curves which comprise of the output and transfer characteristics of the D-mode HEMTs and E-mode MIS-HEMTs.The E/D-mode inverter and voltage level shifter circuit was simulated and designed using the D-mode HEMT and E-mode MIS-HEMT models fitted above.The performances of the inverters with different size and the inverter under different bias were compared.Furthermore,the size of the devices in the voltage level shifter circuit was determined.When the input voltage of the voltage level shifter circuit is 0 V,the output voltage Vout1is high voltage level of-0.137 V and Vout2 is low voltage level of-4.420 V at the supply voltage of 7 V and-7 V.When the input voltage is high voltage level 7 V,Vout1 outputs low voltage level of-4.416 V and Vout2 outputs high voltage level of–0.137 V.The simulation results show that the voltage level shifter circuit can control the on and off state of the switch device in the digital phase shifter.The E/D-mode inverter and voltage level shifter circuit based on the E-mode MIS-HEMTs were fabricated and measured.The inverter can work well under the bias voltage 7 V,8 V or 9 V,which suggests that the E-mode recessed-gate MIS-HMETs with an inserted dielectric layer can effectively enlarge the forward gate voltage swing to improve current drive.The output Vout1 of the voltage level shifter circuit outputs high voltage of-0.35 V and Vout2 outputs low voltage-4.34 V when the input voltage is low voltage level 0 V.When the input voltage is high voltage level 7 V,the voltage level shifter circuit outputs a low voltage of-4.23 V and a high voltage-0.42 V.The measurement results demonstrate that the voltage level shifter circuit can control the work state of the switch device in the digital phase shifter effectively.Furthermore,the voltage level shifter circuit can work well under different supply voltage,which shows the application advantages of the MIS-HEMTs.It can enlarge the forward gate voltage swing furthermore weaken the influence of the load circuit by changing the supply voltage.