SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

404 CHAPTER 8. FIELD EFFECT TRANSISTORS

Comparison of predicted fTand experimental work

GaN

InGaAS HEMT

0.00 0.05 0.10 0.15 0.20 0.25 0.30

100

150

200

250

300

350

400

450

500

InGaAs EMC VD=1.0V

InGaAs EMC VD=0.8V

InGaAs Exp. ref-1

InGaAs Exp. ref-2

dc=250nm

f(GHz)t

LG(μm)

dc=120nm

0.00 0.05 0.10 0.15 0.20 0.25

80

100

120

140

160

180

200

220

LG(μm)

EMC VD=10V

EMC Recessed gate VD=10V

EMC Recessed gate VD=7V

EMC Al GaAs/ GaAs

Ref 1 Ref 2 & 3

Ref 4

f(GHz)t

0 0.5 1 1.5 2 2.5 3

x 10-5

0

1

2

3

4

5

6

7

8

x (cm)

vx 10

7 (cm/s)

Monte Carlo Velocity Along the Channel

0 0.5 1 1.5 2 2.5 3

x 10-5

0

1

2

3

4

5

6

7

8

x (cm)

Monte Carlo Velocity Along the Channel

Only small region

has velocity

overshoot effect

Whole gate

region has

overshoot

effect

Figure 8.30:fτvs.LGand velocity field profiles along the channel for both GaN and InGaAs
HFETs.

function of gate voltage for different drain voltages in an AlGaN/GaN HEMT. At low drain
voltages, the gate can easily modulate the electrons in the channel and a good pinch-off voltage
is obtained for a gate voltage of -5 V. However, as the drain voltage increases, the pinch-off
degrades significantly, shifting to lower VGS voltages and becoming softer. These problems
are the consequence of the poor electron confinement typical of single heterojunction devices