6.4 Comparison between Low Noise, Power, and Millimeter Wave HEMTs

In Table 6.6 the values A₁, A₂ and A₃ of the low noise HEMT A, the power HEMT and the millimeter wave HEMTs with d_GC = 10 nm and d_GC = 13 nm are compared. A₁ accounts purely for the fringe capacitance in the semiconductor. For all HEMTs with a delta doping a value of about 120 fF/mm is obtained. Due to the homogeneously doped supply layer of the low noise HEMT this value is increased significantly to almost 180 fF/mm.
 

The low noise HEMT and both millimeter wave HEMTs have self-aligned gate contacts with similar shapes of the T­gate. This results in very similar values for A₂ of these devices. Only A₂ for the power HEMT without a self-aligned gate is strongly increased due to the T­gate shape as discussed in Section 6.2.

The different values for A₃ reflect the different electron sheet charge densities in the channels of the devices. The low noise HEMT is the only device with a GaAs barrier below the channel. This is combined with better transport properties assumed in the simulation for the channel compared to the double heterojunction HEMTs. Thus, the lowest A₃ is obtained for the low noise HEMT. The largest A₃ is found for the device with the smallest d_GC as discussed in Section 6.3.2.2.
 

• 6.5 Influence of Backside Doping on the C_G(V_GS) Characteristics