SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

400 CHAPTER 8. FIELD EFFECT TRANSISTORS

GaAs

AlGaAs

n+ cap

SOURCE GATE DRAIN

(a)

SOURCE DRAIN

(b)

GATE

Figure 8.27: HFET structures with (a) a single gate recess and (b) a double recess.

The wide bandgap buffer approach provides a barrier increase of

Δφb=

ΔEC

e

(8.7.3)

The latter confinement scheme is common in AlGaAs/InGaAs/GaAs pseudomorphic HFETs and
is the preferred design for GaAs-based HFET structures.

8.7.4 Gate Recess Design ...........................

Recessed gate structures are required whenn+cap layers are employed, and they can also be
designed to improve gate leakage and breakdown characteristics as well as to control the device
threshold voltage. Designing the gate recess is one of the more important issues in HFET design.
Recess structures can generally be placed into two categories: single recess (figure 8.27a) and
double recess (figure 8.27b) structures.
The single recess is designed so that the recess and the gate metal are both defined through
a single opening in the photoresist such that the recess width is approximately equal to the gate
lengthLg. The advantage of this process is that the source and drain access resistances are
minimized, so the transit delay is determined dominantly by the gate length, as the high field
region in the structure is effectively terminated by the source and drain cap layers. The major
disadvantage of this scheme is that the lack of depletion field extension beyond the gate increases
the electric field at the drain edge of the gate, thus increasing gate leakage and decreasing the
breakdown voltage.
The double recess design, shown in figure 8.27b, allows one to trade off transit delay versus
gate leakage and breakdown. By utilizing the first recess of lengthLRto etch through then+
cap layer and the second recess to simultaneously define the gate length and threshold voltage.
Single recess structures are used for small signal analog applications such as low noise am-
plifiers and in digital circuits, whereas double recess designs are used in large signal analog
applications such as power amplifiers.