Ga 2 O 3 Based Heterostructure FETs (HFETs) for Microwave …
219
Fig. 7 SIMS depth profile of Si and Sn concentration, most of the Si was auto-doped into the
AGO film and diffused toward the surface as it grows, other dopant (Sn) doping was ruled-out as it
remains below the background concentration (blue arrow), redrawn from Ref. [65]
I–V characteristics of β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 heterostructure with maximum drain
current (I DMAX ) ~20 mA/mm, peak transconductance (g m ) ~4 mS/mm, pinch-off
voltage of –6 V, and sheet charge density of ~1.2 × 10
13 cm
−2 . Krishnamoorthy
et al. [66] reported vertically scaled modulation-doped β-(Al 0.2 Ga 0.8 ) 2 O 3 /Ga 2 O 3 FET
using Si delta-doping in the AGO barrier. A total charge density of ~5 × 10
12 /cm
2
in the form of 2DEG channel and electron mobility of ~74 cm
2 /Vs were estimated;
however, electrical performance was limited by large contact resistance.
These issues of lower mobility, parallel conduction, and high contact resistance, were effectively addressed by Zhang et al. [20] in β-(Al X Ga 1-X ) 2 O 3 /Ga 2 O 3
heterostructure. The β-(Al 0.18 Ga 0.82 ) 2 O 3 /Ga 2 O 3 modulation-doped structure
displayed a quantum confined 2DEG channel with room temperature mobility
of 180 and 2790 cm
2 /Vs at 50 K. In addition to DC performance (I DMAX =
46 mA/mm, and maximal g m of 39 mS/mm), small-signal high-frequency analysis has demonstrated unity current gain cut-off frequency(f T ) of 3.1 GHz, and
maximum frequency of oscillation (f MAX ) of 13.1 GHz, as shown in Fig. 8. In a bid
to increase 2DEG charge density, Zhang et al. [67] demonstrated modulation-doped
β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 double heterostructure comprised of a 3 nm β-Ga 2 O 3 layer
sandwiched between a Si delta-doped 30 nm AGO barrier and a 10 nm AGO spacer
layer, both with Al composition of 0.17. The output characteristics shows I DMAX =
257 mA/mm, maximal g m of 39 mS/mm, and off-state breakdown voltage of 458 V
with gate to drain gaps (L GD ) of 1.55 μm. The 2DEG carrier density of 3.85 × 10
12
cm
−2 and room temperature electron mobility of 123 cm
2 /Vs were estimated by
Hall measurements. However, the parallel conduction in AGO barrier has limited
room temperature carrier mobility and overestimated carrier density. The schematic
of the β-Ga 2 O 3 double heterostructure and energy band diagram is shown in Fig. 9.
In a nutshell, the modulation-doped AGO/GO heterostructure achieved maximum
carrier mobility of 180 cm
2 /Vs, and 2DEG density ~10
12 cm
−2 , still these are
almost one order less than the values in GaN HEMT [69] (~1500 cm
2 /Vs, and
219
Fig. 7 SIMS depth profile of Si and Sn concentration, most of the Si was auto-doped into the
AGO film and diffused toward the surface as it grows, other dopant (Sn) doping was ruled-out as it
remains below the background concentration (blue arrow), redrawn from Ref. [65]
I–V characteristics of β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 heterostructure with maximum drain
current (I DMAX ) ~20 mA/mm, peak transconductance (g m ) ~4 mS/mm, pinch-off
voltage of –6 V, and sheet charge density of ~1.2 × 10
13 cm
−2 . Krishnamoorthy
et al. [66] reported vertically scaled modulation-doped β-(Al 0.2 Ga 0.8 ) 2 O 3 /Ga 2 O 3 FET
using Si delta-doping in the AGO barrier. A total charge density of ~5 × 10
12 /cm
2
in the form of 2DEG channel and electron mobility of ~74 cm
2 /Vs were estimated;
however, electrical performance was limited by large contact resistance.
These issues of lower mobility, parallel conduction, and high contact resistance, were effectively addressed by Zhang et al. [20] in β-(Al X Ga 1-X ) 2 O 3 /Ga 2 O 3
heterostructure. The β-(Al 0.18 Ga 0.82 ) 2 O 3 /Ga 2 O 3 modulation-doped structure
displayed a quantum confined 2DEG channel with room temperature mobility
of 180 and 2790 cm
2 /Vs at 50 K. In addition to DC performance (I DMAX =
46 mA/mm, and maximal g m of 39 mS/mm), small-signal high-frequency analysis has demonstrated unity current gain cut-off frequency(f T ) of 3.1 GHz, and
maximum frequency of oscillation (f MAX ) of 13.1 GHz, as shown in Fig. 8. In a bid
to increase 2DEG charge density, Zhang et al. [67] demonstrated modulation-doped
β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 double heterostructure comprised of a 3 nm β-Ga 2 O 3 layer
sandwiched between a Si delta-doped 30 nm AGO barrier and a 10 nm AGO spacer
layer, both with Al composition of 0.17. The output characteristics shows I DMAX =
257 mA/mm, maximal g m of 39 mS/mm, and off-state breakdown voltage of 458 V
with gate to drain gaps (L GD ) of 1.55 μm. The 2DEG carrier density of 3.85 × 10
12
cm
−2 and room temperature electron mobility of 123 cm
2 /Vs were estimated by
Hall measurements. However, the parallel conduction in AGO barrier has limited
room temperature carrier mobility and overestimated carrier density. The schematic
of the β-Ga 2 O 3 double heterostructure and energy band diagram is shown in Fig. 9.
In a nutshell, the modulation-doped AGO/GO heterostructure achieved maximum
carrier mobility of 180 cm
2 /Vs, and 2DEG density ~10
12 cm
−2 , still these are
almost one order less than the values in GaN HEMT [69] (~1500 cm
2 /Vs, and