5.2.2 InGaN Cap

5.2.2.1 Device Structure

The investigated InGaN/AlGaN/GaN device structure as described in [394] is shown in Fig. 5.56. A 3 $\mu$m thick GaN layer is grown on a sapphire substrate. A 20 nm thick Al$_{0.25}$Ga$_{0.75}$N layer is deposited next: the first 5 nm undoped, 10 nm highly-doped (2$\times$10$^{18}$cm$^{-3}$) supply layer, and 5 nm undoped material. On-top a 5 nm thick In$_{0.2}$Ga$_{0.8}$N layer is deposited. All layers are non-intentionally doped, except the supply layer. The gate length $L_{\mathrm{g}} =1.9 \mu\ensuremath{\mathrm{m}}$, source gate distance is 1.5 $\mu$m, and gate drain distance is 2.4 $\mu$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.

Figure 5.56: Schematic layer structure of the three HEMTs under investigation.

5.2.2.2 Simulation Results

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 $V_\ensuremath {\mathrm {DS}}$=5 V. Good overall agreement is achieved.

Figure 5.57: Comparison of simulated and measured transfer characteristics for the three devices.

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).

Table 5.3: Summary of values of interface charge density [cm$^{-2}$].

Interface	D-mode	E-mode	Recessed E-mode
GaN/AlGaN	1$\times$10$^{13}$	1$\times$10$^{13}$	1$\times$10$^{13}$
AlGaN/InGaN	$-$	-2.2$\times$10$^{13}$	-2.2$\times$10$^{13}$
InGaN/Passivation	$-$	1.4$\times$10$^{13}$	$-$
AlGaN/Passivation	-0.6$\times$10$^{13}$	$-$	-0.6$\times$10$^{13}$

Fig. 5.58 shows the effective conduction band energies of D-mode and E-mode HEMTs at $V_\ensuremath {\mathrm {GS}}$=0 V, $V_\ensuremath {\mathrm {DS}}$=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 $V_\ensuremath {\mathrm {GS}}$=0 V and the threshold voltage increases to positive values.

Figure 5.58: Energy band diagrams of a HEMT with (dot-dashed line) and without (solid line) InGaN layer.

Fig. 5.59 compares the simulated DC $g_\ensuremath {\mathrm {m}}$ for the three structures. The drop in the measured $g_\ensuremath {\mathrm {m}}$ 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).

Figure 5.59: Simulated DC $g_\ensuremath {\mathrm {m}}$ for the three devices.

Figure 5.60: Output characteristics of a HEMT with non-recessed InGaN cap layer.

Small signal AC analysis using the calibrated setup delivers cut-off frequencies of $f_\ensuremath {\mathrm {t}}$=7 GHz for the device featuring a complete InGaN layer and $f_\ensuremath {\mathrm {t}}$=10 GHz for the recessed structure, respectively. The simulations suggest that reasonably higher values can be achieved by shorter gate lengths: e.g. peak $f_\ensuremath {\mathrm {t}}$=30 GHz for $L_{\mathrm{g}} =0.8 \mu\ensuremath{\mathrm{m}}$.