Study On Performance Of AlIn(Ga)N/GaN Heterostructure HEMTs And PH Sensor

GaN-based HEMTs is the third generation semiconductor device from III nitride heterostructrue materials.GaN-based HEMTs had demonstrated excellent applications in power devices,photo detectors,sensors and radio-frequency devices by its advantages of such as wide band gap,large critical electric field,and strong spontaneous polarization creating high-density two-dimensional electron gas(2DEG).Performance degradation is an urgent obstacle for the practical high temperature applications of AlInN/GaN high electron mobility transistors(HEMTs);in addition,self-heating effect of materials is one of reason for devie degradation.1)In this work investigates theoretically the feasibility of suppressing the high temperature performance degradation of AlInN/GaN HEMTs by inserting an InGaN back barrier between buffer layer GaN and channel layer GaN.The simulation results show that inserting a InGaN back barrier to form AlInN/GaN/InGaN/GaN heterostructure and the thickness of InGaN and Indium component had been optimizied,which can substantially decrease the off-state leakage current,increase the saturate drain current and the transconductance,no matter at room temperature or higher temperature,and meanwhile,the performance degradation of the HEMT at higher temperature is also effectively suppressed.For exsample,when the circumstance temperature is 300 K,the saturated drain current and transconductance(gm)of the WBB(the device with InGaN back barrier)are larger than the WOBB(the device without InGaN back barrier).The saturated drain current increased 41%,the gm increased 35.5%;when the circumstance temperature is 400 K,the saturated drain current increased 213%,the gm increased 285.1%.2)It is difficult to realize enhance model AlInN/GaN HEMTs due to higher 2DEG density in channel.We calculate the relationship between barrier layer AlInN thickness and 2DEG density,the critical thickness of AlInN barrier when 2DEG is zero had been calculated,and the enhancement mode of Al0.83In0.17N/GaN HEMTs with ultrathin barrier layer had been realized and verified it by simulation.The self-heating stability of enhancement mode(E-mode)lattice-matched Al0.83In0.17N/GaN HEMTs with different heat dissipative substrates(tranditonal device with sapphire substrate,AIN flip-chip structure and freestanding GaN substrate)had been evaluated and analysed.The DC characteristic,device temperature,lattice temperature and its distribution,current cutoff frequency(fT),unilateral power-gain-cutoff frequency(fmax)and transient response were investigated comparatively.The results indicate that the HEMTs with a freestanding GaN substrate shows the best performances compare with the other two structure.This is benefit by the free-standing GaN structure device has same material with barrier layer,avoiding the defects owing to lattice mismatch and thermal mismatch.In addition,GaN-based device has attracted much attention for many different sensor applications also.The wide band-gap and strong chemical hardness of the?-nitride materials ensure that the GaN-based heterostructure can be used as a chemical sensor in harsh application environments such as high temperature and acid or alkali solution.As one of ion sensor,pH sensor is very important in various fields,and has triggered many types of devices based on different materials and mechanisms.How to improve the performances of the sensors is one of major challenges now.In this thesis,GaN-based pH sensor had been studied.And the open gate geometry had been optimized based on the formula of drain current.We fabricated many device with different open gate width and length,characterized thire performance in different pH value solution.Based on experiment results,no matter AlInN/GaN or AlGaN/GaN,which have higher sensitivity whe W/L equal to 7.5.In addition,compare with AlGaN/GaN HEMT device,AlInN/GaN HEMTs demonstrates excellent pH sense functionality and higher drain current;for example,the device sensitivity is-30.83?A/pH for AlInN/GaN,-4.6?A/pH for AlGaN/GaN.In the other hand,we developed an idea to improve the sensitivity of pH sensors based on AlGaN/GaN high-electron-mobility transistors(HEMTs)by introducing multi-sensing segments,and experimentally fabricated prototype devices and then investigated their responses to aqueous solutions under different pH values;in contrast,the other two device with single sensing area has been fabricated with same process.The optimized sensor exhibits a very high linear sensitivity of 1.35 mA/pH at drain-source voltage of 1.5 V,which is much higher than those of conventional pH sensors with single open gate sensing area.The other two sensors-21.89 and-7.08?A/pH for sensor B and A,respectively.This indicates that the multi-sensing segments is very effective in increasing the sensitivity of the pH sensor,rather than simply increasing the sensing area,and avoided background noise sharp rise.Our finding paves new strategy for the future design of high sensitive and stable pH sensors based on HEMTs.In this thesis,open gate AlIn(Ga)N/GaN HEMTs pH sensor had been fabricated and characteristiced.The sensor with the multi-sensing segments does not require any additional amplifying circuit during the measurement and so reduce the complexity of the integrated circuit when the sensor is integrated with other functional electronic devices on the same chip.So this design can reduce power consumption of sensing system and manufacturing cost in the furture.