To extract the parameters for the hydrodynamic impact ionization model, a combination of experimental results and comparison with a MC model is used. Using the Keldish  impact ionization model in a one-dimensional MC simulation, the impact ionization rates can be evaluated for the same potential situation as in the simulation by MINIMOS-NT.
Additionally gate current information is available from measurements, where a separation of impact ionization and thermionic field emission effects is necessary. From gate current analysis as a function of temperature, we obtain further experimental data for the temperature dependent modeling. Fig. 3.31 shows a comparison of the measured and modeled gate currents for two different lattice temperatures .
As was consistently stated for InGaAs [75,179,193] there is a positive temperature coefficient for the impact ionization. As can clearly be seen for GaAs in Fig. 3.18 and was shown for pseudomorphic HEMTs with In contents 0.25  a negative coefficient for impact ionization is observed, as the rates decrease with rising . The positive coefficient can further be derived from the on-state breakdown voltage, as will be seen in Fig. 6.13 in Chapter 6.
To realistically model the transition from the pseudomorphic AlGaAs/InGaAs HEMT with x= 0.25, a metamorphic InAlAs/InGaAs with x= 0.3-0.6 , and lattice matched to InP (= 0.53) the transition for the impact ionization coefficient from a positive to negative value as a function of In content must be found. In  Rohdin et al. stated, that with the use of = 0.41 in a metamorphic structure no specific change of the on-state breakdown behavior was observed relative to metamorphic HEMTs with = 0.53. Consequently, the area of interest is between = 0.25 to about = 0.4, while the barrier material InAlAs becomes an indirect semiconductor at y= 0.3, which in this case deteriorates the transport properties of InAlAs significantly .
In Fig. 3.32 the midchannel hydrodynamic impact ionization generation rates are shown for a single recess device pseudomorphic HEMT for the same current . The rate is given along the middle of the channel and the gate extends between x = 0 and 0.14 m. We see an impact ionization rate at the drain side of the gate that negligible for = 1 V, while for = 5 V a significant rate cm is observed, as the device is not protected by a second recess and has a gate to drain breakdown voltage of about 5 V.