To examine RF performance of the devices, the FOMs such as transconductance (gm) gate dependent intrinsic-capacitances (Cgd and Cgs), cutoff frequency (fT), gain bandwidth (GBW) product and output-conductance (gd) are used. All of the analysis is done by using ATLAS 2D simulator.
Fig. 4 shows the DC transfer characteristics of simulated devices, to consider the effect of NiO pocket on β-Ga2O3 MOSFET with the BP layer. The transfer and the output characteristics of the devices are shown in Figs. 4 and 5, respectively, in which the conventional device is taken from the reported literature and is very much in agreement with the experimental device demonstrated by Higashiwaki et al..
In Fig. 4, the increased carrier to carrier scattering at the channel interfaces results in a small increase in the value of drain current (ID) in p-type NiO pocket based Ga2O3 (NiO-GO) MOSFET in comparison to the Conventional Ga2O3 MOSFET (Conv. GO), experimental Ga2O3 MOSFET (Exp.) and β-Ga2O3/BP (GO/BP) heterostructure MOSFET. For the devices, the value of the Ion/Ioff ratio of about 1 × 1010 is observed with off-state leakage in the range of a few pA.
Fig. 6 shows the output conductance characteristics of the device plotted between gd and VDS with VGS = –4 V. For high gain applications, a device should possess a lower value of gd which can be seen in p-type NiO β-Ga2O3/BP (NiO-GO/BP) heterostructure MOSFET. For a device to be used for RF applications, higher output gain requires lower output conductance as depicted in Eq. (3).
By performing small-signal AC analysis after the post-processing operation of DC results, one can obtain the device Cgd and Cgs values at the constant frequency of 0.8 GHz using DC gate voltage swept from –25 to 4 V. Figs. 7 and 8 represent the variation in Cgd and Cgs for different VGS values, respectively. The increased capacitive coupling between the gate and drain terminals of the NiO-GO and NiO-GO/BP MOSFETs results in larger Cgd value in comparison with Conv. GO MOSFET. However, the Cgs value which straightly contributes to the leakage is found to be 0.7 times lesser for NiO-GO/BP MOSFET on comparing to NiO-GO MOSFET.
The fT (see Eq. (4)) is the key FOMs for the depiction of the high-frequency performance of RF devices. The cutoff frequency also stated as GBW (see Eq. (5)) is simultaneously related to H21 (short-circuit current gain). Regarding the two-port network, H21 is known as the ratio of output terminal current to the input terminal current of small-signal with output short-circuited. This parameter is frequency-dependent and its magnitude sweeps at high frequencies with a slope of –20 dB/dec. Keeping this justification, fT can be termed as the frequency value at which the magnitude of H21 decreases to unity. Using Eq. (6) we can evaluate the fT and inset of Fig. 7 shows the fT curve of the proposed device. A smaller value of fT is observed in the case of NiO-GO and NiO-GO/BP MOSFETs in comparison to the Con. GO. However, a 1.09 times improvement in the value of fT is found in NiO-GO/BP MOSFET in comparison to NiO-GO MOSFET.
In Fig. 8, the inset graph indicates that the GBW product decreases with the use of the NiO pocket region of p-type in the device. This is due to the decreased value of gm. However, to compensate for this, a thin BP layer is used below the channel region which improves GBW product in comparison to NiO-GO MOSFET.
In large-signal RF applications, POUT, PAE, and GP are analyzed. Fig. 9 (see Figs. 9(a) and 9(b)) represent the plot of POUT, GP, and PAE versus PIN at VDS and VGS of 25 and –4 V, respectively. For any MOSFET based power amplifiers, efficiency (η) can be estimated out as (see Eq. (7)):
where PLoad is the average power delivered to load terminal and PSupply is average supply power. In the power amplifiers, where heat dissipation is of major concern, PAE (see Eq. (8)) is used as an important FOM.
where PInput is the average input power.
In the NiO-GO/BP MOSFET based PA circuit, due to the increased feedback (which increases the value of Cgd as stated above), the value of both PAE & GP decreases (see Fig. 9(b)). However, these large-signal RF FOMs are found to be improved on comparing it with NiO-GO MOSFET, which is depicted because there is an introduction of the BP layer underneath the channel region. For GO/BP MOSFET, the measured GP is 13.02 dB and is 3.27 dB superior to NiO-GO MOSFET. Nevertheless, the value of PAE measured is 2.03% less efficient to NiO-GO MOSFET. Also, we can see from Table 2 that NiO-GO/BP MOSFET shows better RF performance with improved power gain and low leakages.