22.214.171.124 Reduction of the Recess Length
As demonstrated in Section 6.1.1, gm
ext is increased when LR is reduced. The reason
is a decrease of the ohmic resistance in the current path. Therefore, gm
is expected to depend roughly linearly on LR. It is found
that the dependence of CG on LR can
be modeled by
The rapid increase in CGD has even a more significant
impact on fmax. To determine fmax RS
is calculated by the assumption that the intrinsic gmi
remains constant and the reduction of the extrinsic gm
is purely attributed to an increase of RS with LR.
The gate resistance is assumed to be independent of LR
and is taken to be zero in the simulations. To calculate fmax
the measured value of RG of HEMT
A is taken. In Figure
6.18 fT and fmax are shown as a
function of LR for fully passivated (er
= 7) and unpassivated devices (er
= 1). For er = 7 and small
LR CGD is very large due to the small
distance between the gate and drain contact metals which leads to rather
low fT and fmax. Both fT
and fmax increases with increasing LR
and reach a local maximum. But the maximum of fmax at
LR = 150 nm is obtained for significant larger LR
than the maximum for fT. The reason is that fmax
depends much stronger on CGD than fT.
Therefore, a decrease of CGD is overcompensated by the
linear increase in RS for larger values of LR.
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 HEMTref will be investigated.
Helmut Brech 1998-03-11