Gated Resonant Tunneling Devices May Become Practical

Quantum tunneling transistor may become practical for general use.

John Baliga, Associate Editor -- Semiconductor International, 5/1/1998

Researchers at Sandia National Laboratories (Albuquerque, N.M.) have developed a double quantum well (DQW) tunneling transistor designed for reliable gating, called the double electron layer tunneling transistor (DELTT). They presented their work in December at the International Electron Devices Meeting (IEDM) in Washington, D.C., and the International Semiconductor Device Re-search Symposium (ISDRS) in Charlottesville, Va.

Quantum well devices are typically made in the AlGaAs material set. A thin layer of GaAs is sandwiched by two wider bandgap layers of Al_x Ga_1­xAs, and techniques like molecular beam epitaxy (MBE) are used to make the borders between the layers well defined. A number of devices have been investigated that use tunneling through a quantum well. Using the resonance between a Fermi level   outside the well with a level in the well, switching speeds up to 712 GHz have been demonstrated. This class of devices also has a negative differential resistance (NDR) regime, making it useful for various novel applications like tunable oscillators (Fig. 1).

Many resonant tunneling devices that have been studied depended on tunneling from a bulk emitter region, with a three-dimensional momentum state space, through a quantum well that has a two-dimensional momentum state space. The resonance necessary for tunneling requires matching both the energy and the momentum of states, which can be difficult when the two state spaces are that different. This is one of the reasons the re-searchers chose a DQW structure. Tunneling from one well to the other involves two regions that have two-dimensional state spaces.

Another reason for choosing a double well structure was to make a reliable and repeatable three-terminal device. Most resonant tunneling devices are essentially two-terminal devices, and are therefore called resonant tunneling diodes (RTDs). Adding a gate to them requires etching them into a post shape, then adding a contact. The difficulty with that approach is sensitivity to lateral features. A device with a post diameter of 1000 Å would have much different circuit characteristics than one with a 900 Å diameter. By using tunneling between quantum wells, the device characteristics are determined by vertical features only, which are determined by the epitaxial growth process.

The wells in the DELTT device are the same thic  kness (~150 Å). The wells are given different electron densities to make their Fermi energies different and therefore make resonance possible at a nonzero bias. The problems of gating and making contacts directly to the quantum wells are solved simultaneously using the contact scheme of Figure 2. Two ohmic (Ge/Ni/Au) contacts are made that each contact both wells. Depletion gates (Ti/Au) are used to "disconnect" each contact from a well. Additional Ti/Au contacts act as control gates.

Gaining access to contacts on both sides of the device is done using a technique the researchers call epoxy bond and stop etch (EBASE). The first step in fabricating the device is to grow an aluminum-rich AlGaAs layer. After all the layers are grown and the top side contacts are made, the device is epoxied epi-side down onto a host substrate (Fig. 3). The original substrate is lapped, and is then etched using the AlGaAs layer as an etch stop.

The basic figures of merit for resonant tunneling devices are the peak-to-background and peak-to-valley ratios (Fig. 1). The background current is sensitive to temperature, and it essentially determines the valley current. The DELTT has exhibited a peak-to-valley ratio of ~20:1 at 1.6 K and an ~4:1 ratio at 77 K. Narrower wells and larger Fermi energy differences are expected to make room temperature operation possible. The group is investigating the InGaAs material system to this end.