5.2 Resonant Tunneling Diode Structure

A resonant-tunneling diode requires a band-edge discontinuity at the conduction band or valence band to form a quantum well and, thus, necessitates heteroepitaxy. The most common combination used is GaAs-AlGaAs. The middle quantum-well thickness is typically around $5 \mathrm{nm}$, and the barrier layers range from 1.5 to $5 \mathrm{nm}$. Symmetry of the barrier layers is not required so their thickness can be different. A typical resonant tunneling diode structure with analytical band edge model as used in simulations is depicted in Figure 5.1.

Figure 5.1: Conduction band edge of the RTD for different voltages. A linear voltage drop is assumed over a distance of 40 nm.

The well layer (GaAs) and the barrier layers (AlGaAs) are all undoped, and they are sandwiched between heavily doped, narrow energy-gap materials, which usually are the same as the well layer. Adjacent to the barrier layers are thin layers of undoped spacers to ensure that dopants do not diffuse to the barrier layers.

The region between the two barriers defines a virtual quantum well since the electrons can escape the well confinement by tunneling. The resonant tunneling diode (RTD) is thus an open quantum system in which the electronic states are scattering states with a continuous distribution in energy space, rather than bound states with a discrete energy spectrum. Under these circumstances quasi-bound states (resonant states) are formed in the quantum well which accommodate electrons for a time that is characteristic for the double-barrier structure. So-called resonant tunneling through the double-barrier structure occurs when the energy of the electrons flowing from the emitter coincides with the energy of the quasi-bound state, $E_0$, in the quantum well. The effect of the external bias $V$ is to sweep the alignment of the emitter and quasi-bound states.

For many applications, negative differential resistance (NDR) devices should have a large peak current and a small valley current, where the latter is the minimum current following the peak current as the magnitude of the voltage increases. Therefore an important figure of merit for an NDR device such as the RTD is the peak to valley ratio (PVR). For good devices with thin AlAs barriers, PVRs close to 4:1 and peak current densities in excess of $\mathrm{10^5 A/cm^2}$ may be obtained at $300 \mathrm{K}$ (see [FG01], page 96), although not in the same structures, since there is usually a trade-off between these two parameters in terms of device design.

Because tunneling is inherently a very fast phenomenon that is not transit-time limited, the resonant-tunneling diode is considered among the fastest devices ever made. On the other hand, using resonant-tunneling diodes it is more difficult to supply high current and the output power of an oscillator is limited.

Integration of RTDs with MOSFETs provides high speed operation due to the inherently fast tunneling process, and negative differential resistance regime (NDR) that provides at least two stable operating points (i.e., multiple valued logic) when combined with MOSFETs [RO001].

The resonant tunnel devices for logic applications include resonant tunnel transistors (RTT) and hybrid devices incorporating resonant tunneling diodes and one or more FETs (RTD-FET). RTD designs can offer a reduction in circuit component count by up to 40% when compared with the equivalent CMOS logic family.

The major problem is the extreme sensitivity of device characteristics to the thickness of the tunneling well as the tunneling current depends exponentially on the thickness of the tunnel barrier. The challenge to the process engineer is to match device properties across the wafer.

Overall, the resonant tunneling devices may be useful for certain niche applications requiring high speed and low dynamic range, provided the manufacturing issues associated with uniformity of the tunneling barrier can be resolved. Thus, RTDs are on the verge of commercialisation. Potential practical applications are high-speed microwave systems and novel digital logic circuits.