Modification of transconductance characteristics for ion-implanted GaAs/AlGaAs heterojunction MESFETs

Inverted GaAs/AlGaAs heterostructures grown by MOCVD have been used to fabricate conventional ion-implanted MESFETs. Two types of GaAs/AlGaAs heterojunctions are studied. One type has a compositionally graded AlGaAs layer which provides a built-in field and corresponding quantum well at the heterointerface. The other type has a constant-composition AlGaAs layer. 0.5 mu m gate devices fabricated using the ungraded AlGaAs layer show a maximum extrinsic transconductance G_m of 280 mS/mm and a small G_m variation over a gate voltage range of 1.5 V. In comparison, devices fabricated using the graded AlGaAs layer exhibit higher transconductance over all the gate voltages and an enhancement of G_m up to 420 mS/mm at low gate bias.