In recent years, Heterostructure Field-Effect Transistors (HFETs) are amongst the heterostructure devices which have become an essential component of industrial electronics because of their outstanding high-frequency performance. Gate lengths below 250nm and gate-to-channel distances of about 30nm are state-of-the-art. A further reduction of the feature sizes to improve the high-frequency performance poses new problems of the device design. Besides the requirements on process technology these problems can be attributed to non-local effects such as velocity overshoot of electrons within the channel, tunneling effects at the interfaces, and the real-space transfer due to carrier heating.
Thus, the simulation of HFETs becomes increasingly important to study the influence of non-local effects on device characteristics. The device simulator MINIMOS-NT has been developed as a flexible tool for the simulation of HFETs. It is capable of analyzing the complex structures of various transistors and to select the appropriate physical models automatically. To consider non-local effects the modeling of the interfaces between different layers is crucial. Therefore, several interface models for heterojunctions have been developed and implemented.
To properly account for the real-space transfer it was necessary to implement a model for thermionic emission of the electrons. For the calculation of the carrier temperatures, which are essential for this model, the hydrodynamic model of MINIMOS-NT was used. Additionally, the modeling of the current flow across the heterojunction into the upper barrier layer at the drain-sided end of the channel must include tunneling effects. Thus, the thermionic emission model has been extended to the thermionic field emission model, which accounts for the field related tunneling of the carriers through the interface.
The exponential dependence of the thermionic field emission on the carrier temperature tends to deteriorate the efficiency of the simulator, which must be taken into account for the implementation of these models. MINIMOS-NT is built of several modules which assemble the discretized partial differential equations part by part. This functionality has been extended for the interface models and, thus, makes it possible to control the influence of the interface models on the behavior of the simulation process.
The simulations of two different state-of-the-art HFETs are presented. The results were compared to measurements showing that the modeling of the interface characteristics are of crucial importance. The achievement of simulation results with suitable accuracy to sustain the further development of HFETs is impossible without appropriate interface modeling.