8.7. DESIGN ISSUES IN HFETS 407
14 16 18 20 22 24 26 28 3030405060708090100110120130Lg=230 nmAlGaN/GaN HEMTFrequency (GHz)
Gate-to-channel distance (nm)
fτ
fmaxFigure 8.33: Effect of the gate-to-channel distance in the frequency performance of an Al-
GaN/GaN HEMT with a gate length of 230 nm.
The higher channel confinement of double heterojunction devices is also beneficial in increas-
ing the output resistance of the transistors. Figure 8.34 compares the output resistance as a func-
tion of gate length for several standard AlGaN/GaN HEMTs and some AlGaN/GaN HEMTs
with InGaN backbarrier. Almost a 50%increase in the output resistance can be measured in
the devices with higher channel confinement. This increase in output resistance also causes an
increase in thefmaxof the devices, as predicted by equation 8.8.12, and shown in figure 8.37.
Another interesting non-ideality in the behavior of many transistors is the decrease of gm with
drain current. From equation 8.8.2, the transconductance of a HEMT operating in the saturated
mode should be independent of the drain current level. However, this is normally not the case.
As shown in figure 8.38 for an AlGaN/GaN HEMT,gmdecreases as current increases once that
the maximumgmhas been reached. This kind of behavior has been observed in many differ-
ent transistor technologies, including Si MOSFETs, AlGaAs/GaAs MODFETs and AlGaN/GaN
HEMTs. The cause of this decrease is different in each technology. In Si MOSFETs, simula-
tions have related this decrease in performance with roughness at the Si/SiO2 interface. On the
other hand, in AlGaAs/GaAs HEMTs, as the drain current increases, there is a reduction in the
modulation efficiency of the gate due to the capture of channel electrons by the ionized donors in
the AlGaAs barrier, which reducedgm. Finally, in GaN technology, the reason for this decrease
is related to the increase in small signal source access resistance due to a reduction in the elec-
tron mobility at higher electric fields in this material system. Other studies have also proposed
the emission of hot phonons and the subsequent reduction of the electron velocity as a possible
cause for this decrease in performance.