6.1.2.3 Reduction of the Recess
Length
As demonstrated in Section 6.1.1, g_{m
ext} is increased when L_{R} is reduced. The reason
is a decrease of the ohmic resistance in the current path. Therefore, g_{m}
is expected to depend roughly linearly on L_{R}. It is found
that the dependence of C_{G} on L_{R} can
be modeled by
(63)

The rapid increase in C_{GD} has even a more significant
impact on f_{max}. To determine f_{max} R_{S}
is calculated by the assumption that the intrinsic g_{mi}
remains constant and the reduction of the extrinsic g_{m}
is purely attributed to an increase of R_{S} with L_{R}.
The gate resistance is assumed to be independent of L_{R}
and is taken to be zero in the simulations. To calculate f_{max}
the measured value of R_{G} of HEMT
A is taken. In Figure
6.18 f_{T} and f_{max} are shown as a
function of L_{R} for fully passivated (e_{r}
= 7) and unpassivated devices (e_{r}
= 1). For e_{r} = 7 and small
L_{R} C_{GD} is very large due to the small
distance between the gate and drain contact metals which leads to rather
low f_{T} and f_{max}. Both f_{T}
and f_{max} increases with increasing L_{R}
and reach a local maximum. But the maximum of f_{max} at
L_{R} = 150 nm is obtained for significant larger L_{R}
than the maximum for f_{T}. The reason is that f_{max}
depends much stronger on C_{GD} than f_{T}.
Therefore, a decrease of C_{GD} is overcompensated by the
linear increase in R_{S} for larger values of L_{R}.
The investigations on low noise HEMTs show very good agreement between the calculated and measured characteristics of the devices. It is possible to trace differences in manufactured devices, for instance, the recess depth with an accuracy in the order of 1 nm. Therefore, very accurate information for improvements of device characteristics or the technology of these devices can be given.
All devices were based on a homogeneously doped single heterojunction structure, i. e. a GaAs/InGaAs/AlGaAs layer sequence. As already discussed the disadvantage of this sequence is that the energy barrier on the backside of the channel is rather low which limits the power capability. In the following sections HEMTs based on delta doped double heterojunction structures similar to the structure of HEMT_{ref} will be investigated.
Helmut Brech 19980311