The investigated InGaN/AlGaN/GaN device structure as described in
[394] is shown in Fig. 5.56. A 3 m thick GaN
layer is grown on a sapphire substrate. A 20 nm thick
Al
Ga
N layer is deposited next: the first 5 nm undoped,
10 nm highly-doped (2
10
cm
) supply layer, and 5 nm
undoped material. On-top a 5 nm thick In
Ga
N layer is
deposited. All layers are non-intentionally doped, except the supply
layer. The gate length
, source gate distance is 1.5
m, and
gate drain distance is 2.4
m. Three different HEMT structures are
studied: the proposed novel normally-off device
(Fig. 5.56), a device with the InGaN layer removed in the
access regions (only the InGaN film under the gate is left), and a
conventional normally-on device (as in Fig. 5.56, but
without the InGaN layer) [395]. A diffusion of the metal source
and drain contacts reaching the highly-doped layer is assumed.
The simulation results for the transfer characteristics of the three
devices are compared to the measurements of Mizutani et
al. [394] in Fig. 5.57 for
=5 V. Good
overall agreement is achieved.
All simulations were conducted using the same parameter setup, except for the work-function energy difference of the gate Schottky contact (depending on the underlying material). The values for the interface charge density are summarized in Table 5.3. A positive charge at the channel/supply layer interface is used, a negative charge between the supply layer and the passivation (in the case of D-mode and recessed E-mode), a negative charge between the InGaN cap layer and the AlGaN supply layer (both E-mode devices), and a positive between the InGaN cap layer and the passivation (E-mode non-recessed).
Fig. 5.58 shows the effective conduction band energies of
D-mode and E-mode HEMTs at
=0 V,
=5 V in a vertical cut
under the gate metal, as computed by the simulator. The band diagrams
are shifted so that both Fermi levels are at 0 eV. Indeed, as
suggested by Mizutani et al., a 2DEG channel is present in the
D-mode device, while the negative piezoelectric charge at the
InGaN/AlGaN interface raises the conduction band in the E-mode
structure. Thus, the channel is depleted even at
=0 V and the
threshold voltage increases to positive values.
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Fig. 5.59 compares the simulated DC
for the three
structures. The drop in the measured
at higher gate bias might be
caused by non-idealities in the source and drain ohmic contacts, which are
not considered in the simulation. A relatively good agreement between
the simulated and measured output characteristics for a device with
InGaN layer is achieved (Fig. 5.60).
Small signal AC analysis using the calibrated setup delivers cut-off
frequencies of
=7 GHz for the device featuring a complete InGaN
layer and
=10 GHz for the recessed structure, respectively. The
simulations suggest that reasonably higher values can be achieved by
shorter gate lengths: e.g. peak
=30 GHz for
.