Millimeter wave HEMTs are optimized with respect to their RF performance, i. e. high f_{T} and f_{max}. The most important parameters are large g_{m} and small C_{GS} and C_{GD}. To optimize these parameters some basic considerations have to be made.
The contributions to the capacitances C_{GS} and C_{GD} were described in Section 6.1 and 6.2. C_{GS} and C_{GD} are reduced by optimizing the shape of the Tgate. Reducing L_{G} significantly reduces C_{GS} but increases short channel effects such as large output conductance g_{0}. Larger lateral spacings L_{R} result in smaller C_{GD} but increase R_{S}. The optimum L_{R} depends on the shape of the Tgate and the passivation thickness. It shifts towards larger L_{R} if the passivation thickness is increased.
As described in Section 5.3.1 the most important parameter for g_{m} is the gate to channel separation d_{GC}. If d_{GC} is decreased g_{m} increases significantly. Unfortunately, this goes along with an increase of the intrinsic part of C_{GS}, namely the third term of (62), . Thus the impact of a reduced d_{GC} on f_{T} is not obvious. The lower limit for d_{GC} 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