Ballistic transport in nanostructures at room temperature

Ballistic transport appears when the size of electronic devices is reduced below the electron mean free path. By using latest fabrication techniques and proper material system, the ballistic behavior can be achieved in nano-scale devices even at room temperature. In [1], Song has presented a ballistic rectifier which demonstrates the nonlinear transport at room temperature. However, the functionality of this device was constraint to rectification only. Using this well established theory, and to extend the functionality beyond rectification, our group proposed a novel device in which we added two in-plane strategically placed gates as shown in SEM image in Fig. 1. This led to formation of ballistic deflector transistor (BDT) [2]. In BDT, without biasing the lateral gates, we replicated the rectifying behavior shown in Fig. 2, certifying the presence of non-linear effect at room temperature. BDT is a unique planar device that possesses both a positive and a negative transconductance region and is capable of operating into the THz frequency regime at the room temperature. BDT is based upon an electron steering and a ballistic deflection effect. Modeling and experimental measurements have indicated that the transconductance of the device increases with applied drain-source voltage. The differential mode of operation provides two drain outputs, which depending on the gate bias, are either complementary or non-complementary. The latter facilitates a wide variety of circuit design techniques. The extremely low gate capacitance of the BDT planar structure contributes to THz performance. To investigate the gate control along the channel we fabricated only the channel of BDT where we removed left drain, right drain and deflector, and bias two gates in push-pull fashion as shown in Fig. 3(a). It can be noticed in Fig. 3(b) that drain current (ID) first increases as a function of gate voltage (VLG) then decreases. This behavior is due to the fact that the channel first is being pinched off, then as the VLG is increased, the channel opens and then eventually it pinches off again. As the channel width (WC) increases from 200 nm to 400 nm for a trench width (Wt) of 150 nm, the difference in the maximum to minimum current (ΔI) (Fig. 3(c)) is reduced from 20.15 μA to 15.26 μA, which indicates that early pinch off can be achieved with narrower channel. Increase in current with WC is attributed to the increase in carrier concentration. It is also observed that for WC=200 nm, the current rises exponentially as we narrow down the trenches. In this case, the field structure is modified at the heterostructure interface as a result of the ability to break thermionic barrier becomes easier and the gate current starts flowing into the channel. Another explanation can be given on the basis of fabrication because we applied same dose while etching Wt of 100, 150 and 200 nm, which made the wider trenches shallower resulting into higher leakages and low currents. The leakage could be reduced by p-doping in the etched region or deeper trenches to avoid possible parasitic currents. Measured results from the fabrication of a BDT NAND gate and other BDT gate structures are presented. The 2-input NAND gate logic function created with BDTs is shown in Fig. 4 and the measured results are shown in Fig. 5 [3]. The results in the quasi-ballistic regime are further supported by an empirical model generated from a fabricated BDT output response. Furthermore, our Monte Carlo analysis reports on the effect of different geometry parameters on the transfer characteristics [4]. The strength of the gate control in the InGaAs channel is analyzed. We propose different models for the surface charge density to explain the observed experimental measurements. Finally, we present our time-domain spectroscopy studies used to successfully demonstrate a THz response of ballistic nanodevices at room temperature, excited by picosecond electrical pulses.