The temperature model limits for high electron mobility transistors

Comparison of temperature and hydrodynamic models for high electron mobility transistors has been carried out. It is shown that the real space transfer in the transistor heterostructures and the strongly energy dependent relaxation times are mostly responsible for the spectacular difference in calculations for HEMT with gate length sufficiently greater than momentum relaxation length. The temperature model may lead to more than 20% error in current and transconductance, beginning from length as long as 0.5μ, which markedly exceeds the gate length characteristic to modem HEMT. The different models produce a big difference in the drift velocity distribution in the transistor channel, especially when the channel is open and the real space transfer is considerable. The reasons for this effect are as follows: the lateral size of the quantum well is much less than electron momentum relaxation length and electron current flowing transverse to the heterostructure border is really high, even in comparatively low fields. The electron density in the quantum well strongly depends on transverse electron current, which is different for the temperature and hydrodynamic models. For example, in In_0.52Al_0.48As-In_0.53Ga_0.47As HEMT with 1μ gate length, the drift velocity under the gate is 30% more in the temperature model than in the hydrodynamic model.