Donor-like traps mainly originate from shallow impurities, interface states, and surface states. They selleckbio may interact with the negative charges in the surface which have a decisive influence on the 2DEG in the HEMT channels [12�C16]. The effects of donor-like surface traps with different energy levels and density have been ignored in previous work.In this paper, we analyze these ignored effects in AlGaN/GaN HEMTs with a two-dimensional device simulation method. We adopt the density of the surface charge as a measurement of the activities of surface traps, since filling or emptying charges on the surface from the traps can change the charge density on the surface directly and influence the response of the channel electrons to the voltage consequently.
The sensitivity of current collapse to the variation of negative charges on the surface is firstly investigated for a better understanding of behavior of donor-like surface traps.2. Method and Device StructureTwo-dimensional drift-diffusion simulations of the AlGaN/GaN HEMTs are performed. Device structure of the AlGaN/GaN HEMT is shown in Figure 1. The gate length is 0.7��m and the opening spaces between the contacts are LGD = 0.7��m and LSG = 2��m. The thickness of AlGaN layer with composition of 35% aluminum is 29nm. The mobilities of electrons and holes in GaN layer are 100cm2V?1s?1and 30cm2V?1s?1, respectively. The mobilities of electrons and holes in AlGaN layer are 100cm2 V?1 s?1 and 5cm2V?1s?1, respectively [9, 17�C21].Figure 1Cross-sectional structure of AlGaN/GaN HEMT.
Positive sheet charge +��pol is caused by spontaneous polarization and piezoelectric effect. Equivalent negative sheet charge ?��pol is fixed on the AlGaN surface. Surface trap states …A positive sheet charge +��pol caused by spontaneous polarization and piezoelectric effect [22�C24] is fixed at the interface and the equivalent negative sheet charge ?��pol on the AlGaN surface. The fixed sheet charge density is assumed to be ?1.15 �� 1013cm?2 on the AlGaN surface and 1.15 �� 1013cm?2 at AlGaN/GaN interface [25�C27], respectively. Surface states ��T are considered uniformly distributed on the regions between source and gate. When the surface states are taken into calculation, the initial charge density is modified by adding or removing static charges at the surface. The net charge density between the contacts is expressed as ��net = ?��pol + ��T.
A default temperature of 300K is employed in simulations [28]. A transient voltage Vdd = 6V is applied to the drain with maintaining gate bias VG = 0V. The drain voltage is pulsed from 0.1V to Vdd and the pulse time is adjusted in each case. 3. Results and DiscussionIn Figure 2, the reduction of average electron density in the channel is shown Entinostat as the surface charges, ��T,change from 0 to ?5 �� 1012cm?2. The increase of negative charge density (NCD) on the surface leads to the depletion of 2DEG.