Millimeter wave HEMTs are optimized with respect to their RF performance, i. e. high fT and fmax. The most important parameters are large gm and small CGS and CGD. To optimize these parameters some basic considerations have to be made.
The contributions to the capacitances CGS and CGD were described in Section 6.1 and 6.2. CGS and CGD are reduced by optimizing the shape of the Tgate. Reducing LG significantly reduces CGS but increases short channel effects such as large output conductance g0. Larger lateral spacings LR result in smaller CGD but increase RS. The optimum LR depends on the shape of the Tgate and the passivation thickness. It shifts towards larger LR if the passivation thickness is increased.
As described in Section 5.3.1 the most important
parameter for gm is the gate to channel separation dGC.
If dGC is decreased gm increases significantly.
Unfortunately, this goes along with an increase of the intrinsic part of
CGS, namely the third term of (62), 
.
Thus the impact of a reduced dGC on fT
is not obvious. The lower limit for dGC is given by the
capability of the applied technology to control the recess depth. The physical
lower limit is the barrier height of the Schottky contact which is reduced
when the distance between delta doping and contact gets too small.
In the following simulations of two millimeter wave HEMTs with gate
lengths of about 120 nm are verified against measurements. Based on the
simulations of these HEMTs practical and theoretical limits of device parameters
important for the RF performance will be examined.
Helmut Brech 1998-03-11