SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

402 CHAPTER 8. FIELD EFFECT TRANSISTORS

-1.60.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 1.0x10-5

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

-1.0x10^3

-5.0x10^2

0.0

5.0x10^2

1.0x10^3

(eV)

z (cm)

InAlAs \ InGaAs \ InP HEMT CB diagram

InAlAs InGaAs InP

a.u.

Comparison of AlGaN/GaN HEMT to InAlAs/In0.53Ga0.47As/InP Substrate

InGaAs

δ

doping

InP

GaN

AlGaN

Sheet charge density ->1.2E13 cm-2 Sheet charge density-> 3e12 cm-2

0.0 2.0x10-6 4.0x10-6 6.0x10-6

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-2.0x10^3

-1.5x10^3

-1.0x10^3

-5.0x10^2

0.0

5.0x10^2

1.0x10^3

CB (eV)

x(cm)

a.u.

InAlAs

50Å

-1.60.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 1.0x10-5

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

-1.0x10^3

-5.0x10^2

0.0

5.0x10^2

1.0x10^3

(eV)

z (cm)

InAlAs \ InGaAs \ InP HEMT CB diagram

InAlAs InGaAs InP

a.u.

Comparison of AlGaN/GaN HEMT to InAlAs/In0.53Ga0.47As/InP Substrate

InGaAs

δ

doping

InP

GaN

AlGaN

Sheet charge density ->1.2E13 cm-2 Sheet charge density-> 3e12 cm-2

0.0 2.0x10-6 4.0x10-6 6.0x10-6

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-2.0x10^3

-1.5x10^3

-1.0x10^3

-5.0x10^2

0.0

5.0x10^2

1.0x10^3

CB (eV)

x(cm)

a.u.

InAlAs

50Å

Figure 8.29: Schematic structure and band diagrams of AlInAs/GaInAs and AlGaN/GaN
HFETs.

CDSand an enhanced gate-source capacitanceCGS, but a reducedCGDbecause of the screening
of the gate from the drain by the source-connected field plate. One can effectively trade off the
capacitances based on the geometry of the gate-connected and source-connected field plates,
thus mapping out a design space of gain and breakdown voltage.

8.7.6 Comparison of two disparate material systems:

AlInAs/GaInAs and AlGaN/GaN

It is instructive to compare the behavior of two families of HFET devices which could in
some ways be considered to be at opposite ends of compound semiconductor space. One is the
AlInAs/GaInAs/InP system, where the In composition in the GaInAs channel can be increased
beyond the 53% value required to achieve lattice matching to come close to the 6.1A lattice ̊
constant of InAs. The bandgap of course decreases from 0.74 eV in the lattice matched case
toward the bandgap of InAs (∼0.36 eV). The mobility in the 2DEG can increase from 9,000
cm^2 /V·s to over 15,000 cm^2 /V·s. The effective mass of the electron decreases from 0.47m 0 to
0.25m 0.