7.2 Sources of gate-lag in GaN HEMTs
7.2.2 Surface trap mechanism
The surface trap mechanism involves a change in the net charge density at the AlGaN/passivation interface. The mechanism can be described using a chart of the various space-charge components in the device, as shown in Figure 70. The diagram corresponds to a vertical cut line along the gate-drain access regions. The various space charge components are described along the cut line. The Nitride, AlGaN, AlN and GaN regions are indicated. The solid arrows pointing downwards imply negative space-charge components. The angled arrows indicate free electron and hole densities near the
interfaces. The dashed arrows pointing upwards imply positive space charge components.
The blue line indicates the AlGaN/GaN interface.
Fig. 70. Qualitative diagram of space charge components in the GaN HEMT device studied.
Here ±σpol is the fixed charge density due to the polarization charge at the interfaces. σ2DEG is the 2D electron gas density, σHoles is the free hole accumulation charge density at the surface, and σDTI is the ionized donor trap density at the surface. The relationship between the surface components is described below in equation 35 [66]. The surface described here is the AlGaN surface.
!
"
net= # "
pol+ "
Holes+ "
DTI (35)Fig. 71. Gate-lag transient simulated using the surface trap mechanism.
+σDTI
Nitride
-σpol
+σpol
+σHoles
-σ2DEG
AlGaN
AlN GaN
Fig. 72. Horizontal cross-section plot at the surface comparing at 0 s and 1.1 µs.
Fig. 73. Horizontal cross-section plot at the surface comparing potential at the surface at 0 s and 1.1 µs.
Gate
X (µm)
Fig. 74. 2DEG density as a function of position at 0 s and 1.1 µs.
Figure 71 is a plot comparing the experiment and the simulated gate-lag transient.
The current has been normalized to the drain current measured in the DC characterization. The gate-pulse in the experiments and simulations in this section is from VG= -8 V to -1 V. There is good qualitative agreement between experiment and simulation between the 0 and 10 ms times. The experiment data ends at 10 ms. The signal generator used in the experiment cannot generate a pulse longer than 10 ms. The simulation included a surface fixed charge density of -1 × 1013 cm-2, and donor trap densities of 2 × 1013 cm-2 and 10 13 cm-2 for traps at 0.3 eV and 0.5 eV, respectively, were chosen. The donor trap energies are specified with respect to the valence band edge.
X (µm)
These parameters were chosen to get qualitative agreement with the measured gate-lag transients.
A net change in the space charge at the surface will affect the 2DEG density in the channel. Figure 72 is a plot of the two components described above at 0 and 1.1 µs, σpol
and σdti. These are the major space charge components at the surface; the hole concentration is negligible at the surface. At 0 s, there is a net negative charge on the surface. The two main components of this negative charge are the fixed -σpol that does not change with time and the σDTI, which is the number of ionized donor traps at the surface.
These donor traps change their state from neutral to positive once they are ionized. At 1.1 µs the σDTI decreases, which causes the surface to become more negative. This leads to the formation of a net negative charge on the surface that depletes the 2DEG. Figure 73 is a plot of the potential at the surface at these times. Finally, figure 74 is a plot of the 2DEG density. These two plots show the formation of the virtual gate and the impact it has on the 2DEG. This sequence of results suggests that the change in σDTI is the major mechanism responsible for the “virtual gate” modeled in these simulations [66]. The virtual gate refers to the temporary surface potential as described in figure 73.
Fig. 75. Hole density as a function of position at the AlGaN surface at 0 s and 1.1 µs.
The free hole-density at the surface also provides positive charge at the surface, but the magnitude of holes at the surface is much smaller than σDTI. Figure 75 plots the free-hole density at the surface at 0 and 1.1 µs. The most likely reason for the variation in σDTI is hole capture or electron emission [67, 69]. Finally, the bias condition will have a large role to play in how many of these traps get ionized. The bias condition will determine the quasi-Fermi level at the surface for the holes. This will eventually affect the amount of σDTI.
7.2.2.1 Trap energy and gate-lag
The energy of the trap relative to the valence band also affects the amount of gate- lag in the device. Figure 76 is a plot of the simulated gate-lag in the device for three different trap energies. Figure 77 is a plot of the simulated band diagram for various donor trap energies.
X (µm)
Fig. 76. Gate-lag for surface donor traps at different energies.
Fig. 77. Band diagrams for three trap energies at (VG = VD = 0 V) along a vertical cut in the middle of the gate-drain access region; the energy zero is the Fermi level position. Solid lines are the conduction band and the dashed lines are the valence band energies.
The placement of the trap in the band has the following effects. For traps close to the valence band edge, most of the donors at the surface are neutral, leading to a larger net negative surface charge (σpol - σDTI). This net negative surface charge causes the bands to bend. The bending of the bands causes the accumulation of holes at the surface.
The increase in the net negative charge also causes a reduction in the 2DEG density. The
formation of a net negative charge on the un-gated surface in the above-mentioned fashion is called the “virtual gate” model. As donor traps get farther away from the valence band, σDTI increases and the net negative charge at the surface decreases. This reduces the hole accumulation at the surface. The band bending is less severe and consequently the 2DEG is not depleted as much.
7.2.2.2 Effects of surface traps
Figure 72 compares measured and simulated gate-lag. The data match well; these results are consistent with studies suggesting that surface traps are the dominant mechanism responsible for gate-lag in these devices [28, 35, 66, 67, 69]. These studies suggest donor trap densities of the order of ~1013 cm-2 and a fixed charge density of about -1013 cm-2 at the AlGaN surface [35, 66, 67, 69]. Also, the trap energy ranges and the mechanism suggested in these studies are the same as the results in this dissertation [28, 66, 67, 69]. In summary our results suggest donor traps very close to the valence band (within 0.3 eV) at the surface cause the most dramatic gate-lag.