SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

406 CHAPTER 8. FIELD EFFECT TRANSISTORS

(a) (b)

01234

0.0

0.5

1.0

1.5

2.0

2.5

With Field-plate

Without Field-plate

Channel Electric Feild (MV/cm)

GaN Distance from source (μm)

AlGaN

SOURCE GATE DRAIN

FP

Depletion region

GaN

AlGaN

SOURCE GATE DRAIN

Depletion region

Figure 8.32: (a) Schematic diagrams of HFET structures with and without gate-terminated field
plates. In the field plated device, the depletion region extends over a larger lateral distance. (b)
Electric field profiles within the depletion region along the channel of both devices.

like the AlGaN/GaN HEMT. While there is a significant potential barrier at the AlGaN/GaN
heterointerface, there is no barrier between the channel and the buffer. Therefore, it is easy for
hot electrons to get injected into the buffer, which increases the gate to channel distance and
degrades the performance.
Especially in high frequency devices with very short channels, it is important to increase the
confinement of the channel by providing a potential barrier between the channel and the buffer.
This barrier can be formed by the conduction band discontinuity between a wide bandgap semi-
conductor buffer and a narrow bandgap channel. This is the approached normally followed in
AlGaAs/GaAs/AlGaAs transistors. The channel confinement can also be increased by doping
the bufferp−type, which generates an electric field in the buffer in a direction that opposes the
injection of hot electrons from the channel. An additional option in nitride-based devices is to
use ultra thin InGaN backbarrier layers as shown in figure 8.36. In this device, the difference
in the polarization coefficients between the GaN buffer and the InGaN backbarrier induces two
sheets of fixed charge at the GaN/InGaN interfaces. These polarization induced charges gener-
ate an electric field in the InGaN layer which lowers the conduction band in the GaN channel
with respect to the GaN buffer. This creates an effective conduction band discontinuity which
provides a barrier for the flow of electrons into the buffer as shown in the band diagram in fig-
ure 8.36b. The improved confinement provided by the InGaN back barrier allows much better
gate modulation at high drain voltages as shown in the transconductance measurements of fig-
ure 8.35b. In these improved devices, there is no degradation in the quality of the pinch-off as
the drain voltage increases, although there is still a shift in the pinch-off voltage.