L. F. Luo

Interband tunneling in polytype GaSb/AlSb/InAs heterostructures

Polytype heterostructures of GaSb/AlSb/InAs show interband tunneling due to the 0.1 eV overlap of the InAs conduction band and the GaSb valence band. This broken‐gap configuration results in a novel mechanism for negative differential resistance that has potential applications in high‐speed devices. We have demonstrated for the first time interband tunneling in single‐barrier and double‐barrier polytype heterostructures. Single‐barrier structures show negative differential resistance due to the change in interband tunneling with applied bias. A peak‐to‐valley ratio of 2.7:1 at 77 K was observed in this case. Double‐barrier structures using an InAs quantum well exhibit resonant interband tunneling with a peak‐to‐valley current ratio of more than 60:1 at 77 K. This structure is promising for applications to three‐terminal devices because of the very wide quantum well that can be achieved.

Resonant tunneling in AlSb/InAs/AlSb double-barrier heterostructures

We report the first observations of resonant tunneling in the AlSb/InAs material system, with a maximum peak‐to‐valley current ratio of 1.8:1 at room temperature and 9:1 at 77 K. The large AlSb/InAs barrier height of 1.8 eV for electrons and high‐mobility InAs will be advantageous in device applications. In particular, the small electron effective mass in InAs makes it possible to demonstrate quantum effects in a 24 nm well, the longest coherence distance reported for double‐barrier tunneling structures. We estimate that an AlSb/InAs resonant tunneling transistor can significantly outperform similar devices based on AlGaAs/GaAs.

Heterojunction field‐effect transistors based on AlGaSb/InAs

We have fabricated the first InAs‐channel field‐effect transistor, which shows a transconductance of 180 mS/mm at 1 V drain‐source bias (77 K). An improved buffer layer could significantly improve the device performance. In addition, we propose a new broken‐gap heterojunction field‐effect transistor based on these materials that could provide an order of magnitude higher transconductance compared to existing device configurations based on AlGaAs/GaAs.

Resonant interband tunneling in InAs/GaSb/AlSb/InAs and GaSb/InAs/AlSb/GaSb heterostructures

Tunneling in InAs/GaSb/AlSb/InAs and GaSb/InAs/AlSb/GaSb structures has been observed for the first time and shown to offer large peak‐to‐valley ratios at higher peak current densities than previous double‐barrier and single‐barrier polytype interband tunneling results. Room‐temperature peak‐to‐valley ratios as high as 20:1 were observed at peak current densities of 28 kA/cm2. The highest peak‐to‐valley ratio observed at 80 K was 80:1 at 1.2 kA/cm2. The large peak‐to‐valley ratios are attributed to resonant interband tunneling with a confined state and band‐gap blocking of nonresonant currents. The operation of these devices is similar to asymmetric double‐barrier structures.

Resonant interband coupling in single-barrier heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb

A new mechanism for negative differential resistance due to electron/light hole coupling has been observed using broken gap heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb. The best peak‐to‐valley ratio is about 2:1 (3.7:1 at 77 K) for a GaSb layer width of 10 nm. The peak current density of 4.2 kA cm−2 and the peak voltage of 300 mV are consistent with the interpretation of these experiments as interband coupling between the InAs conduction band and the GaSb valence (light hole) band.

A heterojunction bipolar transistor with separate carrier injection and confinement

A heterojunction bipolar transistor (HBT) structure in which the wide-gap material serves to confine minority carriers only while the injection of carriers into the base is controlled by a homojunction is discussed. This structure offers several advantages over conventional HBTs, including improved electron injection efficiency without bandgap grading. The thickness of the narrow-gap emitter has to be optimized in order to achieve a good confinement effect. The concept can be applied to other HBTs. >

Negative differential resistance in AlGaSb/InAs single‐barrier heterostructures at room temperature

We have observed for the first time negative differential resistance at room temperature in a single‐barrier tunneling heterostructure. A typical InAs/AlGaSb/InAs structure exhibits a current peak of 2.1×103 A cm−2 at 0.28 V and a peak to valley ratio of 1.6:1. We attribute the observation of room‐temperature negative differential resistance to the favorable band alignment in the AlGaSb/InAs material system, which appears promising for device applications of single‐barrier tunneling.

An AlGaAs/GaAs heterostructure-emitter bipolar transistor

An AlGaAs/GaAs heterostructure-emitter bipolar transistor using separate carrier injection and confinement is discussed. A common-emitter current gain of 28 with BV/sub CEO/=15 V was obtained at a base doping level of 1*10/sup 19//cm/sup 3/. No spacer layer was inserted in the structure. This transistor combines the merits of homojunction transistors and regular heterostructure bipolar transistors (HBTs) and is simple to fabricate.<<ETX>>

Resonant interband tunneling through a 110 nm InAs quantum well

The mechanism of resonant interband tunneling in polytype heterostructures of GaSb/AlSb/InAs gives excellent peak‐to‐valley current ratios due to the band‐gap blocking of the nonresonant current components. Using InAs as the base in a double‐barrier polytype heterostructure, it is possible to demonstrate resonant tunneling at room temperature through a quantum well as wide as 110 nm. At this width, which is about 20 times larger than that typically used in resonant tunneling diodes in the GaAs/AlGaAs system, the peak‐to‐valley ratio is 44:1 (77 K). Significant negative differential resistance is observed even for 240 nm wells. The projected device response time for a resonant tunneling transistor with a wide InAs quantum base is more than five times faster than for a GaAs device, due to the reduced base resistance.

Interband tunneling in single‐barrier InAs/AlSb/GaSb heterostructures

Negative differential resistance due to interband tunneling has been observed at room temperatures for the first time in polytype heterostructures of InAs/AlSb/GaSb. The peak‐to‐valley ratio is about 1.7:1 (5.5:1 at 77 K) for an AlSb barrier width of 2.5 nm. The peak current density is studied as a function of barrier width and compared to calculations based on the two‐band model.