220.127.116.11 Simulation of the Output Characteristics
In Figure 5.24 the measured output characteristics of HEMTref is shown. The left dark shaded area indicates the linear region of the device were the electrons do not reach their saturation velocity yet. The light shaded area indicates the active region in which the device is usually operated. It is characterized by the output conductance g0. and ID for a certain DVGS, i. e. gm. At about VDS = 2.0 V a slight kink can be observed. According to  it is caused by impact ionization.
A strong increase in g0 in the right dark shaded area
indicates breakdown due to impact ionization. Inclusion of impact ionization
in the simulation leads to very unstable convergence. Therefore it is not
considered in the simulations of this thesis. For comparison of measurements
and simulation only data with negligible impact ionization can be used.
5.25 shows the simulated output characteristics indicated by circles
along with measured data. As depicted the characteristics agree very well
for VDS < 2.0 V and VGS < 0.6
V. The discrepancy between simulation and measurements for VDS
< 1.0 V and VGS > 0.2 V is related to the interface
model and the transport model in the channel. With the applied models and
the corresponding fitting parameters it was not possible to reproduce this
part of the output characteristics very well.
It can be observed that for low electron concentrations in the channel, i. e. small VGS the output characteristics is modeled quite well. For higher carrier concentration, i. e. large VGS, the current increases only slowly with VDS. This indicates that the interface model does not describe high currents properly for small voltage drops over heterojunction barriers and/or the hydrodynamic transport model in the channel underestimates the velocity for high carrier concentration in a certain range of electric field.
For larger VDS g0 in Figure 5.25 is underestimated because impact ionization is not included in the simulation. For high VGS g0 is overestimated which is most likely due to temperature effects as the device heats up and the transport properties are deteriorated. This in return reduces the current.
Thermal effects can be reduced in the measurements if VGS
is only applied during short duty cycles. In Figure
5.26 DC and pulsed measurements of an output characteristics according
are compared. It appears that temperature effects can account for more
than 10 % change in ID at VDS = 5.0
Next: 5.4 Model Parameters used for Simulation Up: 5.3.2 Fitting Procedure Previous: 18.104.22.168 Simulation of the Transfer Characteristics
Helmut Brech 1998-03-11