A 3-channel InP-HEMT with low output conductance

A triple channel (InGaAs-AlGaInAs-InP) HEMT structure has been investigated for high breakdown voltage applications. This structure takes advantage simultaneously of both the high electron mobility at low field in the InGaAs channel and the low impact ionization coefficient in the InP channel. The transistor actually shows good dynamic performances and high breakdown voltage: for a 0.8 μm gate length we obtained: gm=240 mS/mm, Idss=225 mA/mm, Vbrd=6.5 V, f_t=35 GHz, f_max>90 GHz. A specific feature of this HEMT structure is its low output conductance (gm/gd=40 at both low and high frequency) attributed in particular to the fact that the barrier layer is undoped, with the δ-doping sitting in the middle of the channel. For the 0.8 μm gate length devices investigated, the output conductance is much lower than for single or dual channel HEMTs fabricated with similar geometry. This feature is maintained for shorter gate length (at Lg=0.2 μm). In this paper an analysis of this output conductance is presented, illustrating the importance of impact ionization and residual traps