In Fig. 7.2 the output characteristics of a pseudomorphic AlGaAs/ InGaAs/GaAs HEMT is shown in comparison with measurements. Generation/recombination and self-heating are included in these simulations. These two effects allow to match the output conductance correctly for a wider bias range than previously reported in . Self-heating reduces the drain current and the transconductance. The generation/recombination mechanisms cause an effective modification of the gate potential to more positive values. This modification depends on , so that the output conductance is modified even for low voltage. More details are given with the power HEMT in the next section.
For = 300 K a second peak of the transconductance is visible, which is due to the parasitic MESFET, i.e., additional current flow in the AlGaAs barrier layer. For higher temperatures the second peak reduces. The occurrence of a pronounced second peak, i.e., an increase of with for for is due to a number of conditions. In principal the can be understood to be composed of two curves. The first is the regular HEMT channel curve with current flow exclusively in the channel. This produces the peak and has a sharp drop for for . As was seen in Chapter 3, Real Space Transfer (RST) is also responsible for the reduction of this first peak relative to an ideal situation, where no real space transfer at all is possible.
The second peak is composed of contributions from current flow in the barrier material AlGaAs, which results in a second curve with a for . The distance between these two peaks is now related to various physical quantities, but most important to:
In the investigated device in Fig. 7.4 the ohmic contact situation is similar to Case II in Fig. 3.25, i.e., the channel is not directly contacted. Thus, is reached for some bias which is determined by the doping and the gate-to-channel separation . The current transfer into the spacer and barrier is dominated by RST. In comparison with a directly contacted channel (Case I in Fig. 3.25), for is relatively more positive due to the additional line resistance of the caps. At the same time, the utmost right position for the second peak is limited by the opening gate-diode at = (barrier height), in this case about = 0.7 V. Due to the relatively positive , the parasitic MESFET behavior, i.e., current flow in the barrier, occurs at bias close to the , where the gate-diode opens, which is not necessarily the case for a HEMT with a more negative threshold voltage . To the right the peak is limited by the opening diode, since the opening diode leads to a drastic reduction of the current gain. As a consequence, a peak position is visible for this particular case.
This argumentation can also explain the temperature dependence. Effectively three temperature effects occur for rising temperatures: the effective Schottky barrier height decreases, the band edge discontinuity decreases, and the overall effective carrier velocity drops in both, the channel and the barrier.
The effective barrier height reduction shifts the peak of the parasitic MESFET to more negative values, while the sum of the two contributions drops. Thus, the curve looks more homogeneous, although it still consists of the two contributions.