GaN HFET加应力后的虚栅和亚稳态能带

Virtual Gate and Metastable Bands in GaN HFET After Applying Bias Stress

使用双曲函数拟合描绘出从应力偏置转换到测试偏置后,不同时刻的表面电势、电场强度和电场梯度的动态弛豫过程。计算出这一偏置转换引发的亚稳态能带。亚稳态能带的弛豫过程描绘出沟道夹断后的异质结充电过程。亚稳态能带计算证明外沟道中的强场峰、电场梯度峰、能带谷和能带峰都由局域电子气电荷引起,局域电子气的慢输运行为延缓了异质结充电过程,拉长了偏置转换中的亚稳态能带转换弛豫。当负应力栅压向空间电荷区注入电子给陷阱充电时,陷阱电荷叠加在局域电子气电荷上,强化了能带畸变和电流崩塌。由此提出涉及异质结能带转换的新虚栅模型。在新虚栅模型下异质结能带变化引发的电子气状态变化比旧虚栅模型中的电子耗尽作用强得多,据此能够解释从漏控DLTS测得的外沟道陷阱密度比栅控DLTS测得的内沟道陷阱密度大1~2个数量级的实验结果。使用涉及能带转换的新虚栅模型讨论了GaN HFET研究中的电流崩塌、3 mm场效应管及可靠性难题。提出二维异质结构用异质结鳍来研制场效应管的新课题。

The dynamic relaxation process of the surface potential,electric field strength and electric field gradient at different times after the transition from stress bias to test bias is plotted using hyperbolic function fitting.The metastable energy bands induced by this bias transition are calculated.The relaxation process of the metastable energy bands depicts the charging process of the heterojunction after channel pinch off.The metastable energy bands calculation proves that the strong field peaks,electric field gradient peaks,energy band valleys and energy band peaks in the outer channel are all caused by the charge of the localized electron gases,and the slow transport of the localized electron gases slows down the heterojunction charging and elongates the metastable energy band transition relaxation process during the bias transition.When the negative stress gate voltage is applied,the electrons are injected into the space charge region to charge the traps.The trapped charge is superimposed on the localized electron gas charge,enhancing the band distortion and current collapse.From this,a new virtual gate model involving heterojunction band transitions is proposed.Under the new virtual gate model,the electron gas state change induced by the energy band variation of the heterojunction is much stronger than the electron depletion effect in the existing virtual gate,which explains the experimental result of the trap density in the outer channel measured from the drain-controlled DLTS is 1-2 orders of magnitude larger than the inner channel trap density measured by gate-controlled DLTS.Current collapse,3 mm HFET development and its reliability challenges in GaN HFET research are discussed using the new virtual gate model involving band switching.A new topic of researching twodimensional heterostructures using heterojunction fins to develop field effect transistors is proposed.