Hot Carrier Nanowire Transistors at the Ballistic Limit

Abstract

We demonstrate experimentally nonequilibrium transport in unipolar quasi-1D hot electron devices reaching the ballistic limit at room temperature. The devices are realized with heterostructure engineering in nanowires to obtain dopant- and dislocation-free 1D-epitaxy and flexible bandgap engineering. We show experimentally the control of hot electron injection with a graded conduction band profile and the subsequent filtering of hot and relaxed electrons with rectangular energy barriers. The number of electrons passing the barrier depends exponentially on the transport length with a mean-free path of 200-260 nm, and the electrons reach the ballistic transport regime for the shortest devices with 70% of the electrons flying freely through the base electrode and the barrier reflections limiting the transport to the collector.

Keywords: ballistic electrons; bandgap engineering; hot carrier transistors; quantum mechanical transmission.

Figure 1

(a) The conduction band diagram of the ballistic hot electron system consisting of a graded potential barrier as a hot electron injector and a rectangular barrier for filtering the electrons. (b) Scanning transmission electron microscope image of a nanowire heterostructure obtained through growth engineering showing an InAs_1–xP_x-based quasi-1D hot electron injector and InP-based energy filter. (c) Compositional map of In, As, and P along the nanowire.

Figure 2

(a) A scanning electron micrograph showing one of the measured unipolar 3-terminal hot electron nanowire devices. The middle base electrode needs to be positioned and defined sharply between the injector and the energy filter, spaced by a distance l. Voltage V_eb biases the injector and V_bc the filter. (b) Band diagram under typical operation. The voltage V_eb lifts the energy of the emitter electrons and flattens the graded barrier, giving rise to electron injection. Ballistic electrons fly over the filter to an unbiased (V_bc = 0) collector.

Figure 3

Measured current–voltage curves for a device with a base length l = 150 nm. The emitter current I_e and collector current I_c were measured simultaneously as a function of the injector voltage V_eb. The energy filter was kept unbiased at V_bc = 0. The inset shows the transfer ratio I_c/I_e at the injection regime. All measurements took place at room temperature.

Figure 4

Collector current I_c as a function of the base-collector voltage V_bc for emitter-base voltages V_eb = −0.8, ..., 0.8 V. The top panel shows the current on a logarithmic scale, and the bottom panel repeats the data with a linear scale. The device has a base length of l = 160 nm.

Figure 5

Base length l dependence of transfer ratio I_c/I_e. Dots are experimental data from several devices, and the solid lines are fits to exponential dependence I_c/I_e = T exp(−l/l_r). The red and blue data show the results with nanowires sorted to two diameters: d = 60–65 nm and 76–82 nm, respectively.