In a HEMT, the objective is to make
electrons in the channel flow faster. In a
doped channel, the doping atoms in the
lattice impede the electrons. HEMTs rely
on high-mobility electrons generated at
the heterojunction. One side has a highly
doped, wide-bandgap, n-type, donor-supply
layer like aluminum gallium arsenide
(AlGaAs). The other has a non-doped narrow-
bandgap channel layer, such as GaAs.
The heterojunction between them forms
a very thin quantum well in the conduction
band on the GaAs side. The electrons can’t
escape because of the well, but they travel
very rapidly through it because there are no
dopant atoms in the GaAs lattice. The layer
they travel in is called an electron gas.
GaAs HEMTs are still made, but GaAs
pseudomorphic high electron mobility
transistor (pHEMT) and metamorphic
high electron mobility transistor
(mHEMT) devices are gaining lots of
momentum. The terms pseudomorphic
and metamorphic refer to the addition of
indium phosphide (InP). The difference
between the two lies deep in the semiconductor’s
physics, but essentially, these techniques
allow larger bandgap differences
than conventional HEMTs. Ultimately, it
gives them better performance.
Another alternative is to build indium-gallium-
arsenide (InGaAs) HEMTs on
an InP substrate. This results in very good
high-frequency performance, but the technology
hasn’t reached commercialization
because InP wafers are extremely brittle.
Those GaAs MESFETs and HEMTs
with very short gates suffer from high
output conductance due to short-channel
effects. In a silicon lateral double-diffused
MOSFET (LDMOS), the short-channel
effect isn’t present. With a positive potential
on the gate, LDMOS devices create an
inversion channel over the laterally diffused
p-well at the silicon-oxide interface. The
effective gate length may be shorter than
the physical length of the gate electrode.
Other process steps extend RF performance
and provide a high breakdown performance,
allowing for operation at high
supply voltages. LDMOS devices can also
be made on silicon carbide.
For RF amplifiers, plain old silicon still
can challenge GaAs when the right understanding
of transistor architecture is
applied (see “HVVFETs—New In Town,”
Drill Deeper 18999). In late April, a fabless
company known as HVVi Semiconductors
announced three new devices fabbed by
ON Semiconductor in pure silicon using
the company’s high-voltage vertical field-effect
transistor (HVVFET) technology
(Fig. 5).