Study On The Characteristics Of High Electron Mobility GaN-based Double-heterostructure Materials And Devices

For high frequency, high power and anti-radiation applications, GaN-based highelectron mobility transistors (HEMT) have shown excellent performances due to theirwide band-gap, high electron mobility, large breakdown field strength, and strongspontaneous and piezoelectric polarization. In recent years, the performances ofconventional AlGaN/GaN heterostructure materials and devices have obtained greatimprovement with the progress of the material quality and device technology. However,the conventional AlGaN/GaN heterostructure still remains some weaknesses, such asthe poor two dimensional electron gas (2DEG) confinement, the large buffer layerleakage current, the bad off-state characteristics at high drain voltage and so on. In orderto meet the requirements of the national defense electronic communication field and therapidly developing mobile communication field, AlGaN/GaN double heterostructureHEMT devices have been proposed. In the new structure,2DEG confinement has beengreatly enhanced due to the introduction of the back barrier layer below the channel.Therefore, the buffer layer leakage current has been reduced and the breakdowncharacteristic, off-state characteristic and power characteristic have been improved aswell, making the AlGaN/GaN double heterostructure more suitable for being used underhigh temperature and high voltage environment.The paper presented the material growth, structure optimization, and devicefabrication and measurements of the AlGaN/GaN double heterostructure HEMTs. Themain results are as follows:1. The energy band diagrams and carrier distributions of AlGaN/GaN/AlGaN/GaN,AlGaN/GaN/AlGaN, AlGaN/InGaN/GaN, AlGaN/GaN/InGaN/GaN structures weresimulated by solving1D Poisson equation and the principals and characters of the fourstructures were also discussed.2. The AlGaN/GaN/AlGaN double heterostructure with the2DEG mobility of1862cm~2/Vs was successfully grown on sapphire substrate by low pressure MOCVDwith three kinds of optimization schemes. First, the composite structure of GaN bufferlayer and AlGaN buffer layer was used with innovation to improve the crystal qualityand electrical characteristics of the AlGaN/GaN/AlGaN double heterostructure.Secondly, the2DEG mobility was improved by increasing the channel thickness todecrease the interface roughness scattering. Moreover, the ultrathin AlN interlayer wasintroduced between the GaN channel layer and the AlGaN buffer layer to decrease the alloy disorder scattering for the first time. The Hall Effect measurements of theconventional heterostructure and four double heterostructure materials at differenttemperatures showed that the double heterostructure had better transport characteristicat high temperatures and the electron mobility of the double heterostructure determinedto be478cm~2/Vs at573K, which is double times larger than the mobility ofconventional heterostructure (~200cm~2/Vs@T=573K).3. The AlGaN/GaN/AlGaN double heterostructure HEMTs devices were fabricated.The DC measurements shows that the maximum drain current density and thresholdvoltage|Vth|are both less than those of the conventional AlGaN/GaN HEMTs, which isbecause of the lower2DEG density of the double heterostructure. However, due to theenhanced carrier confinement, the double heterostructure HEMTs showed up numerousadvantages. The off-state leakage current of the double heterostructure HEMTs is twoorders of magnitude lower than that of the conventional one. The threshold voltage shiftand threshold slope S of the double heterostructure HEMTs at different drain voltageswere reduced as a result of the lower leakage current. Moreover, the breakdown voltageof the double heterostructure HEMTs was twice larger than that of the conventional one.4. The electrical stress measurements showed that the degradation of the doubleheterostructure HEMTs caused by the hot electron injection effect was weakened,especially the off-state leakage current and threshold voltage. The power characteristicsof AlGaN/GaN/AlGaN double heterostructure HEMTs were presented with themaximum power density of7.78W/mm and the maximum power added efficiency (PAE)of62.3%. Also, the gain at high frequencies of the double heterostructure HEMTs wasmuch higher than the conventional HEMTs.In a word, because of the better carrier confinement, the AlGaN/GaN/AlGaNdouble heterostructure has shown excellent performances both in material and devicecharacteristics and is more suitable for the applications of high voltage, high frequencyand high power devices.