Intrinsic feedback and bistable switching in Y-branched nanojunctions

We present intrinsic feedback and nonlinear switching in Y-branched nanotransistors. For this type of mesoscopic transistor the gate efficiency depends on the density of states in the gate electrode which is altered by voltage shifts at the drain. This nonclassical property of a Y junction introduces positive voltage feedback which leads to pronounced nonlinearities and is exploited to demonstrate highly efficient gating and bistable switching. Our results demonstrate that the efficiency of nanoscaled transistors can be enhanced dramatically by exploiting quantum effects.

Figure 1

(a) Transfer characteristics of a symmetric Y junction. (b) SEM image of a symmetric Y junction and a schematic view of the measurement setup.

Figure 2

(Color online) (a) Voltage $V_p$ at which the maximum $\left(V_{bl,max}\right)$ in the $V_{bl}\left(V_{bias,r}\right)$ trace occurs as a function of $V_{bl,max}$. (b) Qualitative illustration of the electrostatic potential along the right branch-stem (thick lines) and left branch-stem (thin lines) sections of the Y junction for two voltages, $V_{bl,1}$, $V_{bl,2}$ with $V_{bl,1}<V_{bl,2}$, applied to the left branch. An increase in $V_{bl}$ leads to a reduction in $E_{br,max}$ which in turn lowers $V_p$.

Figure 3

Diagrams illustrating schematically the electrical characteristics of a symmetrical Y junction for three different values of $V_{br}$: (a) injection regime, (b) gating regime near the transition to the injection regime, and (c) gating regime. Black arrows illustrate the motion of electrons with their width being a measure of the magnitude of the attributed current.

Figure 4

(Color online) (a) Transfer characteristics of a symmetric and (b) an asymmetric Y junction, respectively. The dashed vertical lines indicate the input voltage for which the transition from the gating regime to the injection regime occurs. An absence of gate leakage currents would mean $V_{br}=V_{bias,r}$ which is indicated by the dashed lines in the lower panels. (a) The $V_{bl}\left(V_{bias,r}\right)$ trace of the symmetric Y junction shows a pronounced nonlinearity associated with a maximum voltage gain of $-30$. The onset of leakage current is reflected in the increase in ratio $I_r/I_l$ and the deviation between $V_{br}$ and $V_{bias,r}$. Calculated values according to Eqs. (3, 4, 5) are plotted for three values of the internal switching efficiency ${\eta }^{\ast }/V_s$. Best agreement between experiment and theory is obtained for ${\eta }^{\ast }/V_s=4.5\times {10}^{-2}$. (b) The input-output characteristic of the asymmetric Y junctions exhibits a significantly higher voltage gain $g_{max}=-580$. Due to the constriction close to the branching section and the associated tunnel barrier (cf. Fig. 5, inset), the leakage current from the right branch into the branching section is significantly reduced which is reflected in a lower $I_r/I_l$ ratio and $V_{br}\approx V_{bias,r}$.

Figure 5

Bistable switching of an asymmetric Y junction. The $V_{bl}$ vs $V_{bias,r}$ characteristic shows a clear bistable switching behavior associated with an hysteresis of 2 mV between the up and down sweep. Inset: SEM image of an asymmetric Y junction.