AlGaN Microwave Power HFETs on Insulating SiC Substrates
- PDF / 1,788,250 Bytes
- 9 Pages / 417.6 x 639 pts Page_size
- 57 Downloads / 184 Views
471 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
Table 1. Selected materials properties of SiC and GaN, compared to Si and GaAs. (aelectron mobility in bulk, lightly N-type GaN; b- electron mobility at an AIGaN / GaN heterojunction) PropertylMaterial Bandgap(eV) 5
Breakdown field [xlO V/cm] 2
Electron mobility [cm /Vs]
Si
GaAs
4H-SiC
GaN
1.1
1.4
3.3
3.4
2 1400
4 8500
8 800
8 900a
0.5
0.7
1.0
1.4
1.5
0.5
4.9
1.3
2000' 7
Maximum velocity [xl0 cm/s]
(E=8 E5 V/cm) Thermal conductivity [W/cm K]
EXPERIMENT The nitrides display a large piezoelectric effect, relative to other III-V materials [2]. For the (0001) orientation typically used for epitaxial growth, the piezoelectric effect is a major source of charge at AlGaN / GaN heterojunctions. Figure 1 shows that the electron charge density densities in undoped AlGaN / GaN heterostructures increase linearly with increasing AlGaN alloy composition, and densities well in excess of I x 1013 cm"2 can be achieved. This sheet charge density is several times larger than is typically seen in AlGaAs / GaAs heterostructures. Because the mobility of electrons in AlGaN / GaN heterostructures are several times lower than the mobility of electrons in AlGaAs / GaAs, the total current for a given gate width is very similar for the two materials systems. The drain breakdown voltage for the nitride structure is many times larger than for the arsenide structure, for similarly sized transistors. The structures reported on in this paper were all grown by low pressure MOCVD using the standard metalorganic precursors for the group III material, ammonia for the column V material, and silane for the dopant. Adding intentional donors to the A1GaN layer increases the electron sheet charge density, as expected. Figure 2 shows the measured Hall mobility versus sheet charge density for a variety of structures grown on SiC substrates, some of which contain intentional doping in the AIGaN. The structure with a 300K sheet charge density of 4.9 xl012 cm2 and a mobility of 2150 cm 2/Vs, has a 77K sheet charge density of 4.9 xl 012 cm 2 and a mobility in excess of 12,000 cm 2 /Vs. This large low temperature mobility attests to the high quality of the heterointerface.
472
Z 1.5xld
EMOCVD AMBE
3
A
LU
1ixid 33 I.C
5x 02 G) U)
0
0
0.1
0.2
0.4
0.3
Aluminum Concentration [X] Figure 1. Two-dimensional electron sheet charge density at AlGaN / GaN heterostructures, as a function of the alloy composition of the AlGaN [2].
2500 U U)
U
2000
U
E U
1500
-
.00• Ulg* U
1000
mu' U U
500 P
0 0
0.5
1
1.5
2
2.5
3
2
Sheet Charge Density [cm" ] Figure 2. Room temperature Hall mobility versus sheet charge density for a variety of AlGaN/GaN structures grown by MOCVD on SiC substrates at Rockwell.
473
The epitaxial layer structure of the devices reported on in this paper consisted of a 50 Angstrom undoped A1GaN cap, a 100 Angstrom AlGaN region doped with 5 xl0' donors/cm2 , a 50 Angstrom undoped spacer layer, a one micron thick GaN layer, and
Data Loading...
