Simulating Pseudomorphic HEMTs: Optimizing Performance to Achieve Multi-terahertz Operating Frequencies

Abstract

High-electron mobility transistors (HEMTs) have become an important device for high frequency and low noise applications. The maximum cut-off frequency, fT, that has been achieved thus far in a real device is 610 GHz [1]. Through simulation, we have been investigating how these frequencies may be pushed even higher. For this, we have used a full-band, cellular Monte Carlo transport program, coupled to a full Poisson solver to study a variety of InAs-rich, InGaAs pseudomorphic HEMTs and their response at high frequency, concentrating on devices with a structure (from the substrate) InP/InAlAs/InGaAs/InAlAs/InGaAs, with the quantum well composed of Ino.75Gao.25As [2,3,4]. By studying the frequency response behavior as the gate length was shortened in a scaled structure, we were able to establish in previous work a theoretical maximum forfT given a device that was 300 nm in total source-drain length. This upper limit was found to 2.9 THz for a device with an 18 nm channel thickness [3], and 3.1 THz for a 10 nm channel thickness [4]. These limits were achieved by, first relating the physical gate length to an effective length which included depletion extensions, and then extrapolating to zero physical gate length, while at the same time reducing the gate to channel distance. A key element of these calculations was the establishment of an accurate definition for the effective gate length. Importantly, we found that this quantity was considerably shorter than the depletion length when one used the results of full RF simulations in conjunction with an integration over the velocity in the gate region.In the present work, we consider other physical factors beyond the physical gate length, which may enhance or inhibit fT, to see if the limits discussed above are indeed achievable in real devices. For example, it has been traditional microwave transistor design to use an Lsg that is about a third the total device length (100 nm in this case), which is what was used in the previous simulations to extract the upper limits [3,4]. However, we have found that when Lsg is reduced below this value, fT is increased, by more than 10%. We believe this to be a result of a reduction in the effective gate length of the device, a quantity that our previous limit studies showed to be crucial.The lower the effective gate length is for a given physical gate length, the higher fT can be. We shall also discuss other means by which this may be achieved, and we will present a thorough investigation of device operation as a function of gate and drain voltage to examine where a "sweet spot" for fT may be found. An important physical limitation for device operation is gate leakage. Of course, this can be reduced by increasing the gate to channel separation, and we study how far this separation can be made without sacrificing too much in terms of a highfT. As we shall show, even increasing the gate to channel separation by a more than a factor of three, one can still achieve an fT greater than 2 THz in a device with a short physical gate length, an...

Some Considerations on Nanowires in Nanoelectronics

“…In the quantum simulations of InAs nanowire MOS transistors with 30-nm gate length, this triodelike drain-current behavior has been observed to occur [13]. This behavior was also seen in [10], and we have also seen it in the semiclassical Monte Carlo simulations of InGaAs HEMTs with 10-nm gates [14]. Thus, it is clear that the tendency toward triodelike behavior as…”

“…Signatures of ballistic transport are then quite subtle, such as the change in the dependence of the saturation current on gate voltage in (6). However, based upon the observations found in the quantum and semiclassical simulations of ultrasmall FETs [10], [13], [14], this saturation may not be apparent in the characteristic curves, at least not to the extent seen in the longchannel devices. The superlinear triodelike behavior discussed earlier describes at least the "linear" part of the curves below the saturation current.…”