A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al
0.5
Ga
0.5
As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 10
¹⁷
-cm
^-3
Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 10
¹⁸
-cm
^-3
Ge-doped Al
0.5
Ga
0.5
As gate layer and a p
⁺
-type 5 × 10
¹⁸
-cm
^-3
Ge-doped GaAs "contact and cap" layer on the top of the gate. The gate structure is obtained by selectively etching the p
⁺
-type GaAs and Al
0.5
Ga
0.5
As. Undercutting of the Al
0.5
Ga
0.5
As layer results in submicrometer gate lengths, and the resulting p
⁺
-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.