DETAILED DESCRIPTION The device and the method of making the same discussed in this application has substantial commonality with the devices, and method of making those devices, as discussed in U.
S.
Pat.
No.
5,041,393 by Ahrens et al.
, and assigned to the same assignee as this invention.
The Ahrens et al.
application is included herein by reference.
Although the compound semiconductor material referred to here is based on gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs), other compound semiconductor material systems could be substituted, such as indium aluminum arsenide (InAlAs)/indium gallium arsenide (InGaAs) or silicon/germanium.
The concentration (i.
e.
, mole fraction) of aluminum in the AlGaAs is usually identified as the percentage of aluminum in AlGaAs displacing the gallium therein, where the mole fraction can range from 0 percent (GaAs) to 100 percent (AlAs).
Generally, the more the aluminum, the higher (wider) the bandgap energy of the AlGaAs material, i.
e.
, AlGaAs is wider bandgap material than GaAs material.
The device structure shown in FIG.
1 is an exemplary embodiment, according to one aspect of the invention, of a self-aligned field-effect transistor (FET) 1 having a substantially linear input voltage-to-output current transfer characteristic over a predetermined range of input voltages.
This device is especially useful in providing large power gain at microwave frequencies, such as above 10 GHz.
To allow for such a transfer characteristic, the FET 1 is configured have a plurality of channels to confine and support charge carriers therein (here a two-dimensional electron gas or 2-DEG).
The channels are the active regions in the FET 1 and contact the source and drain diffusions 10, 11.
The channels (2-DEG) are formed by the heterojunctions of wide-bandgap donor layers 15, 16 and narrow band-gap channel support layers 17, 18, as will be explained in more detail below.
The thicknesses of the layers 15-18, the doping in layer 15, 16 and the depths of channels below the gate 22,23 of FET 1, as will be explained in more detail below, adjusted to give the desired linear transfer characteristics
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