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).
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