5.3.2.2 Simulation of the Output Characteristics
 
In  Figure 5.24 the measured output characteristics of HEMT_ref 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 I_D for a certain DV_GS, i. e. g_m. At about V_DS = 2.0 V a slight kink can be observed. According to [63] 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. Figure 5.25 shows the simulated output characteristics indicated by circles along with measured data. As depicted the characteristics agree very well for V_DS < 2.0 V and V_GS < 0.6 V. The discrepancy between simulation and measurements for V_DS < 1.0 V and V_GS > 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 V_GS the output characteristics is modeled quite well. For higher carrier concentration, i. e. large V_GS, the current increases only slowly with V_DS. 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 V_DS g₀ in  Figure 5.25 is underestimated because impact ionization is not included in the simulation. For high V_GS 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 V_GS is only applied during short duty cycles. In  Figure 5.26 DC and pulsed measurements of an output characteristics according to [64] are compared. It appears that temperature effects can account for more than 10 % change in I_D at V_DS = 5.0 V.
 

• 5.4 Model Parameters used for Simulation
• 5.5 Investigated Parameters

   
Next: 5.4 Model Parameters used for Simulation Up: 5.3.2 Fitting Procedure Previous: 5.3.2.1 Simulation of the Transfer Characteristics