By Peter H Aaen
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Extra info for Modeling and characterization of RF and microwave power FETs
Sample text
In contrast, LDMOS transistors have a higher output capacitance than comparably-rated GaAs PHEMT power transistors. This parameter is important for Doherty amplifier applications. In the Doherty amplifier configuration, an auxiliary or peaking amplifier is used to modulate the load seen by the main amplifier, to improve the overall efficiency of the power amplifier [18]. The auxiliary amplifier is switched off at lower input powers, and so it should present an open circuit to the main amplifier’s output. The ability of the transistor to present an open circuit at RF is compromised by its output capacitance.
In the Doherty amplifier configuration, an auxiliary or peaking amplifier is used to modulate the load seen by the main amplifier, to improve the overall efficiency of the power amplifier [18]. The auxiliary amplifier is switched off at lower input powers, and so it should present an open circuit to the main amplifier’s output. The ability of the transistor to present an open circuit at RF is compromised by its output capacitance. Even so, LDMOS transistors have been used successfully in Doherty configuration for RF power amplifiers [19–21].
By sandwiching a layer of very narrow-gap semiconductor between layers of high band-gap material, good charge confinement was obtained, and high transconductance and transition frequency fT were observed. The narrow-gap material, indium gallium arsenide (InGaAs), has a slightly different crystal lattice constant 28 RF and Microwave Power Transistors Fig. 10. Schematic diagram of AlGaAs–GaAs HEMT structure and its electron energy-band diagram. from the GaAs substrate, but for low concentrations of indium in the alloy, typically 25% or below, the strain in the structure can be accommodated without generating the crystal defects that would destroy the device performance.