Demonstration of lateral field-effect transistors using Sn-doped β-(AlGa)₂O₃ (010)

Transmission line measurement (TLM) structures were made with Ti/Au contacts separated by various source-drain spacings (L_sd) on 100 nm thick heavily Sn-doped (Al_0.15Ga_0.85)₂O₃/100 nm thick UID (Al_0.15Ga_0.85)₂O₃ layers. The net donor concentration of the Sn-doped (AlGa)₂O₃ layer is estimated to be 2 × 10¹⁸ cm⁻³ at room temperature from capacitance–voltage (C–V) measurements. The current–voltage characteristics of the Sn-doped (AlGa)₂O₃ layer for various L_sd at room temperature are shown in Fig. 1(a). The Sn-doped (AlGa)₂O₃ layers were conductive and were electrically isolated by the bottom UID (AlGa)₂O₃ layers. The current showed a fairly ohmic behavior and increased with decreasing L_sd. In TLM, the specific contact resistance R_c and sheet resistance R_sh can be estimated by fitting the resistance values against L_sd and extrapolating the fitted line to a resistor of zero length. R_c of the (AlGa)₂O₃ layer is 9 × 10⁻⁵ Ω cm², which is close to that of the reported Ga₂O₃ layers with donor concentrations of ~10¹⁸ cm⁻³.^6,22,23) R_sh of the (AlGa)₂O₃ layer is 75 kΩ/, which is much higher in comparison with the reported Ga₂O₃ layers with 4.9 kΩ/.²³⁾ The electron mobility μ_e in the 100 nm thick (Al_0.15Ga_0.85)₂O₃ layer is calculated to be 4 cm² V⁻¹ s⁻¹ by assuming that the donor concentration corresponds to electron concentration. Further growth optimization is necessary to increase the mobility.

Zoom In Zoom Out Reset image size

The lateral V_br of the UID (Al_0.15Ga_0.85)₂O₃ and Ga₂O₃ layers was measured between source and drain contacts isolated by a 150 nm deep mesa structure. The two-terminal V_br for various L_sd at room temperature is shown in Fig. 1(b). V_br linearly increased with increasing L_sd. The lateral two-terminal V_br measurements likely have the high electric field applied at the mesa edge.²⁴⁾ The effective E_c of the UID (Al_0.15Ga_0.85)₂O₃ and Ga₂O₃ layers are 1.8 and 0.5 MV cm⁻¹, respectively. These values are higher than the one typically seen in III-nitride semiconductors,^25,26) showing that (AlGa)₂O₃ materials have high potential for high-power applications.

We fabricated a Sn-doped (AlGa)₂O₃/Ga₂O₃ MODFET, which consists of the 26 nm thick heavily Sn-doped (Al_0.08Ga_0.92)₂O₃/180 nm thick UID Ga₂O₃ structure, as schematically shown in Fig. 2(a). The net donor concentration of the (AlGa)₂O₃ barrier layer is estimated to be ~10¹⁸ cm⁻³. The fabricated MODFET had a gate length (L_g) of 2 μm, gate-source spacing (L_gs) of 2 μm, and gate-drain spacing (L_gd) of 8 μm. Figure 2(b) shows the C–V characterization at 1 MHz. The capacitance increased with increasing the voltage and exhibited the characteristic accumulation behavior near −0.5 V, indicating the presence of 2DEG. The sheet charge density n at the (AlGa)₂O₃/Ga₂O₃ interface is estimated to be 2 × 10¹³ cm⁻² by assuming n ≈ −C³(dV/dC)/(qε) under a homogeneously doped (AlGa)₂O₃ layer without compensation, where ε permittivity (10ε₀ for Ga₂O₃) and q elementary charge.²⁷⁾ This n is close to the reported values of Ge-doped (Al_0.08Ga_0.92)₂O₃/Ga₂O₃ and Si-doped (Al_0.2Ga_0.8)₂O₃/Ga₂O₃ MODFETs.^14,15) The depth profile of the charge concentrations in the (AlGa)₂O₃/Ga₂O layer is inserted in Fig. 2(b). The depth w (=ε/C) is estimated from C–V measurements. The charge-peak density is located around 20 nm away from the surface. This depth is close to the (AlGa)₂O₃ barrier thickness, indicating the charge confinement at the (AlGa)₂O₃/Ga₂O₃ interface.

Zoom In Zoom Out Reset image size

Figure 2(c) shows the output characteristics at room temperature for the (AlGa)₂O₃/Ga₂O₃ MODFET. The MODFET has a normally-on operation and pinch-off characteristics for gate voltage (V_gs) < −2.8 V. We achieved a fairly ohmic behavior of the source/drain contact due to the uniformly-Sn-doped (AlGa)₂O₃ barrier layer. The normalized R_on was measured to be 1.1 kΩ mm from a linear fit to the V_gs = 0 V curve. μ_e is estimated to be 8 cm² V⁻¹ s⁻¹, which is slightly higher than that in the (AlGa)₂O₃ layer perhaps due to 2DEG formed in the UID-Ga₂O₃ layer. However, the estimated μ_e of the (AlGa)₂O₃/Ga₂O₃ MODFET is one order of magnitude lower than that of the reported Ga₂O₃ layers.^9,23,28) The small μ_e may be attributed to the parallel conduction in 2DEG and the UID Ga₂O₃ layer. Further investigation on electrical characteristics of the (AlGa)₂O₃/Ga₂O₃ interface is necessary. Drain current (I_d) is effectively modulated by V_gs and shows good saturation. The maximum I_d, off-state I_d, and subthreshold swing were 12 mA mm⁻¹, 1.4 μA mm⁻¹ and 0.4 V decade⁻¹, respectively, as shown in Fig. 2(d). The high off-state I_d may result from the leakage current through the Ga₂O₃ buffer/substrate interface with the unintentional Si incorporation.²⁰⁾ The maximum transconductance (g_m) and I_d on/off ratio were 3.8 mS mm⁻¹ and 8.7 × 10³, respectively. The three-terminal V_br for V_gs = −10 V at room temperature was 610 V, which is higher than that of 415 V for the reported lateral Ga₂O₃ MOSFET.²¹⁾

To investigate the lateral E_c of (AlGa)₂O₃ layers, we prepared Sn-doped (AlGa)₂O₃-channel MESFETs made of a 120 nm thick Sn-doped (Al_0.16Ga_0.84)₂O₃ channel with the donor concentration of ~10¹⁷ cm⁻³ on a 120 nm thick UID (Al_0.16Ga_0.84)₂O₃ buffer, as schematically shown in Fig. 3(a). The transistors featured a 70 nm deep gate recess. Output characteristics at room temperature for the MESFET are shown in Fig. 3(b). The MESFETs have a nearly normally-off operation with pinch-off characteristics for V_gs < −0.4 V due to the light Sn-doping and gate recess structure. The subthreshold swing was 0.3 V decade⁻¹. In these devices, I_d is effectively modulated by V_gs and shows good saturation. The transfer characteristics of the MESFET with L_gd of 4 μm at a drain voltage (V_ds) = +4 V at room temperature is shown in Fig. 3(c). At room temperature, the off-state I_d was 0.6 nA mm⁻¹, which is comparable with the gate current, indicating a negligibly-small leakage current through the UID (AlGa)₂O₃ layers. The maximum transconductance was 1.6 μS mm⁻¹. The maximum I_d was 3.3 μA mm⁻¹ for V_gs = +1.0 V. The low I_d is attributed to the deep gate recess and high contact resistance. The I_d on/off ratio was around 10³. Regrowth of a n⁺⁺-(AlGa)₂O₃ contact layer or Si-ion implantation under source/drain electrodes would further increase the I_d on/off ratio.

Zoom In Zoom Out Reset image size

The three-terminal V_br characteristics for the (AlGa)₂O₃ MESFET with L_gd of 20 μm at room temperature is inserted in Fig. 3(d). The leakage current of off-state I_d was below 1 nA mm⁻¹ at −600 V. V_br for V_gs = −10 V at room temperature was 940 V, indicating the high potential of (AlGa)₂O₃ for high-power applications. The three-terminal V_br for various L_gd at room temperature are shown in Fig. 3(d). V_br increases linearly with increasing L_gd. This is because the device breakdown is mainly caused by the high electric field at the drain edge of the gate electrode, resulting in the lower effective E_c in comparison with the two-terminal V_br measurement for (AlGa)₂O₃. The effective lateral E_c of the three-terminal V_br for (AlGa)₂O₃ is 0.6 MV cm⁻¹, which is comparable to GaN HEMTs.^29,30) Further high breakdown voltages of the MODFETs and MESFETs are expected in devices with a field-plate structure.