The Study Of P-GaN Gate GaN-based HEMT And Its Reliability

Group of ? nitride semiconductor materials are third-generation semiconductor materials with large bandgap energy,high electron saturated velocity,high temperature resistance and radiation resistance.Due to these excellent characteristics,they are ideal materials for the fabrication of power electronic devices.GaN-based high mobility transistor(HEMT)has wider application prospects in the fields of high temperature,high frequency and high power than electronic devices based on Si and GaAs materials.AlGaN/GaN heterojunction is the main structure in GaN-based HEMT.Due to the spontaneous polarization and piezoelectric polarization effects of GaN materials,a two-dimensional electron gas(2DEG)with high concentration exists in the channel.Therefore,conventional GaN-based HEMT is a normally-on(depletion mode)device with a negative threshold voltage.However,in order to simplify the complexity of circuit design and reduce the cost,normally-off(enhancement mode)devices are more preferable in the industry.Several methods have been recently proposed to deplete the 2DEG in the channel for fabrication of normally-off GaN-based HMETs.Among the approaches,the AlGaN/GaN HEMT with p-GaN gate has drawn increasing attention in the industry thanks to its strong controllability and repeatable fabrication.The reliability of devices is a big issue which limits the development of industrial applications,and needs further study.In this paper,we study the p-GaN AlGaN/GaN HEMT and its reliability by theoretical and experimental,the main contents and results are as follows:1.We study the effects of the thickness of AlGaN barrier layer and Al mole fraction on p-GaN AlGaN/GaN HEMT.By Silvaco TCAD,we change the thickness of AlGaN barrier layer and the Al mole fraction and simulate the transfer and output curves of the device.The relationships between threshold voltage,output current,thickness of AlGaN barrier and the Al mole fraction are provided.The AlGaN barrier layer thickness of 15 nm and the Al mole fraction of 0.23 are chosen to obtain larger threshold voltage and output current.2.We study the effects of traps in UID(unintentionally-doped)GaN buffer layer on the performance of p-GaN gate AlGaN/GaN HEMTs.We find that the device has larger off-state current and output current with lower trap level.This is because with the lower trap level,the conduction band energy is lower and more electrons are existed in the channel at off-state,which result in larger off-state current of the device.Simultaneously,the capture rate of ionized traps is smaller with the lower trap level,which results in larger output current.In addition,when the trap concentration reaches 5e17cm-3,and the trap cross section reaches 1e-14cm2.a negative transconductance occurred.By simulating band energy diagram and electric field,we find that larger trap concentration and trap cross section can promote tunnel effect of electrons,and enhance the electric field at the gate side,which result in a negative transconductance in the device.3.We study the gate reliability of p-GaN gate AlGaN/GaN HEMT by step-stress experiments combined with electroluminescence imaging and pulsed measurements.The results show that for gate stress(VGstress)from 0 V to 13 V,drain current is found to be stable and decreased at 13 V at off-state and increased at on-state.For VGstress from 13 V to 31 V,drain current increases at off-state and decreases at on-state,whereas VTH is increases slightly.The changes in drain current characteristic and VTH of the device after applying various VGstress are associated with the injection and trapping of holes from the p-GaN layer to the AlGaN layer,supported by the results of pulsed measurements and by the simulation of energy band diagrams.When VGsrtress is further increased to a high voltage of 31 V,VTH becomes noisy and a strong luminescence signal occurs with a sudden reduction in drain current,and finally,a catastrophic failure happens in the gate-channel region.4.We study the negative transconductance in p-GaN gate AlGaN/GaN HEMT.We find that the threshold voltage of the device shifts to negative values with increasing pulse positive gate stress.Meanwhile,the drain current after pulse measurements increases slightly when measuring the ID-VD curve.This can be attributed to ionization of donor traps in the AlGaN layer.Donor traps can be ionized when applying gate voltage,which increase the concentration of electrons in the channel,and the current of the device increases.Meanwhile,ionized donor traps become positive charges,which result in the decrease of threshold voltage.When the gate voltage further increases,the energy of fermi level increases whereas the the barrier height of AlGaN decreases with ionization of donor traps.And the fermi level can be higher than the conduction band of AlGaN in this condition.Therefore,electrons can overflow to AlGaN and transfer to p-GaN,which result in the the decrease of current and the occurrence of negative transconductance.5.This research investigates negative transconductance effect in p-GaN gate AlGaN/GaN HEMTs associated with traps in the unintentionally doped GaN buffer layer.We find that a negative transconductance effect occurs with increasing trap concentration and capture cross section when calculating transfer characteristics.A reasonable explanation related to electron tunneling through AlGaN barrier and the reduced electric field discrepancy between drain side and gate side induced by traps is provided by analyzing the band diagrams,output characteristics,and the electric field strength of the channel of the devices under different trap concentrations and capture cross sections.