Study On Mechanisms Of GaN HEMT Structure Schottky Diodes And E/D Mode Integrated Circuits

Due to its wide gap, high electron saturation velocity, high breakdown electric filed, high temperature characteristics and high-radiation resistant, the third generation semiconductor GaN has become the hotspot in microelectronics area. GaN-based Enhancement/depletion mode digital integrated circuit has been studied widely, and the basic digital gate circuits have been designed and manufactured successfully with the development of enhancement mode HEMTs. As another important component of the integrated circuit, Schottky diodes can realize functions of current-limit, voltage-limit, block, voltage shifter etc. Also, Schottky diodes are the key section of HEMTs. The Schottky diodes has a great influence on the characteristics and reliability of HEMT. So it is important to study the HEMT Schottky diodes in depth. Based on the reason mentioned above, in this thesis, the author focuses on the characteristics and mechanism of Ni-AlGaN/GaN contacts, and design the voltage shifter and SRAM unit circuits using HEMT Schottky diodes as the key components.In chapter 2, a new method of manufacturing enhancement mode HEMT has been demonstrated. AlGaN/GaN enhancement mode HEMT is realized by using 8 nm depth AlGaN barrier layer and N2 O plasma treatment. The enhancement mode HEMT has a threshold voltage as high as 0.86 V and a peak transconductance of 334 m S/mm. The pulse tests indicate that N2 O plasma treatments does not introduce serious damages into the region under gate. It shows some kind of self-termination effects during the high temperature N2 O plasma treatment in PECVD system. The conversion of depletion mode to enhancement mode is suggested because of partially oxidizing the barrier layer and the ultra-thin barrier layer.In chapter 3, the author studies the characteristics of the Ni-AlGaN/GaN Schottky diodes in forward biased, negative biased and the electron transportation mechanisms. The revers current of HEMT structure Schottky diodes consists of two parts of current, one is area-related current, and another is perimeter-related current. The difference of the I-V characteristic of the two kinds of current originates from the different electric filed distribution between the gate edge drain side and main region of the gate electrode. Dependence of electric field on voltage is also different in the two current. Gate edge leakage current is extracted from the total measured current by comparing the current-voltage curve of different area and perimeters of Schottky diodes. The area-related current dominates the total current when the Schottky contacts area is large and the voltage bias is small. While in condition of practical HEMTs and at high voltage bias, due to the small ratio of area/perimeter, gate edge leakage current could be larger the area-related current. In order to find out the mechanism of the two current conductions, an analytical model of electric field calculation is proposed to obtain the average electric field around the gate edge at high revers bias and estimate the effective range of edge leakage current. The model reveals that the increase of the electric field near the gate edge of drain side is due to the extended depletion region towards to drain. The effective range Δ of electric field induced by the extended depletion is about hundred to several hundred nano-meters. By fitting the calculated electric field, the calculated current and the measured current, when the electric filed is higher than 1.80 MV/cm, the mechanism of gate edge current conduction is Fowler-Nordheim tunneling.The forward current characteristic and transport mechanism of Ni-AlGaN/GaN Schottky diodes and F-treated Ni-AlGaN/GaN Schottky diodes are studied. By using precise model calculation, measured current-voltage curves are fitted with model calculated data. Current-voltage curve of conventional HEMT structure Schottky diodes is divided into two parts: low bias and high bias. In low bias region, the trap-assistant tunneling model calculation data is perfectly fitted with the measure current curve. The larger current at low bias than the current thermal emission predicted is caused by great amounts of trap-assistant tunneling. At high bias, the measured current-voltage curve is in accord with thermal emission theory. The current of F-treated HEMTs structure Schottkydiodes at low bias is larger than that of conventional HEMT structure Schottky diodes. It is also caused by trapassistant tunneling. The traps introduced by F-treatment process enhance the trap-assistant tunneling current.By utilizing process-optimized double gate structures, the surface leakage current of HEMTs has been separated from the total current. SiN passivation can suppress the surface leakage current. The surface leakage current is closely related to surface trap states. At the low electric field biased, the mechanism of surface conductance is considered to be twodimensional variable range hopping. At the elevated electric filed, the Frenkel–Poole trap assisted emission governs the main surface electrons transportation. A new process has been introduced to further suppress the surface leakage current at high temperature and high field.In chapter 4, the characteristics and reliability of conventional HEMT structure Schottky diodes and F-treated HEMT structure Schottky diodes under forward and negative bias is studied. The Schottky barrier height of conventional and F-treated HEMT structure Schottky diodes are determined to be 1.29 eV and 1.90 eV, respectively, by using a new testing method of connecting the HEMT structure Schottky diodes with HEMT active resistance. It is difficult to extract the Schottky barrier height of the F-treated HEMT structure Schottky diodes by thermal emission model, but the Schottky barrier height is revealed by the new test method. Reliability of conventional HEMT structure Schottky diodes and F-treated HEMT structure Schottky diodes under large forward and reverse electric stress are studied. The degeneration in high reverse stress originates from inverse piezoelectric polarization effects. The critical voltage of F-treated HEMT structure Schottky diodes is higher than conventional HEMT structure Schottky diodes, because the F ions introduced into the barrier can reduce the saturation electric field. In condition of forward stress, the Schottky barrier height of F-treated HEMT structure Schottky diodes decreases in different degrees. The main stress causing the degeneration is not voltage stress, but current stress. When the current density is larger than 530 A/cm2, the schottky barrier height of Ftreated HEMT structure Schottky diode decreases obviously, while Schottky barrier height of the conventional HEMT structure Schottky diodes is stable during the current stress. The mechanism of degeneration of Schottky barrier height of F-treated HEMT structure Schottky diodes is caused by the F ion drifting. In practical applications, the conventional HEMT and F-tread HEMT structure Schottky diodes have their advantages and disadvantages. In revers bias, F-treated HEMT structure Schottky diodes have a higher critical voltage than conventional HEMT structure Schottky diodes, but in forward bias, the conventional HEMT structure Schottky diodes are more stable.In chapter 5 and 6, six transistors SRAM unit and voltage shifter circuits are designed and demonstrated in Ga N thin barrier system for the first time. At a supply voltage 1V, the SRAM unit circuit has an output high voltage of 0.97 V, and output low voltage of 0.07 V. The writing and reading operations of SRAM are all correct. The voltage shifter uses 4 HEMT diodes in series to shift the voltage, and output two voltage. At a supply voltage 6V and-6V, the two output channels outputs voltage of-0.42 V,-5.31 V and-5.14 V, 0.63 V, respectively. It can control the on and off state of depletion-mode HEMTs. At temperature of 200°C, the outputs of voltage shifter shift to positive direction by 0.42 V. By studying the characteristics of F-treated enhancement-mode HEMT and depletion-mode HEMT in high temperature, we suggest that the voltage shifting in high temperature is because of the inconsistent degeneration of enhancement mode and depletion-mode HEMT in voltage dividing circuit and degradation of F-treated enhancment-mode HEMT according to the increasing temperature.