Tunneling Hot Electron Transfer Amplifier Research Articles

Hot electron spectroscopy of carrier relaxation within indirect AlAs tunnel barriers

Hot electron spectroscopy has been performed on electrons which have tunneled through indirect AlAs potential barriers in a tunneling hot electron transfer amplifier. We find that only about 1% of the electrons are collected close to their injection energy in stark contrast to an otherwise identical structure with an Al0.5Ga0.5As barrier where this fraction was 30%. Measurements under hydrostatic pressure show clear evidence for the real-space transfer of electrons from the emitter electrode into an X-point barrier subband adjacent to the emitter Fermi band. A detailed analysis of hot electron spectra reveals that the transferred electrons undergo strong inelastic scattering within the AlAs barrier and relax down through the ladder of X-point subbands there before being reemitted into the base layer.

Tunneling hot electron transistor as a high power source at terahertz frequencies

A novel device is proposed, based upon a tunneling hot electron transfer amplifier, which exhibits the characteristics of negative differential resistance (NDR) coupled with high current gain. The mechanism which produces the NDR is known to be extremely fast. The combination of these features suggests that such a device could be used as a high power source of terahertz radiation.

Improved turn-on characteristics of a hot electron transistor at 300 K

Through compositional grading of the collector, we have significantly reduced both the offset and saturation voltage of a tunneling hot-electron transfer amplifier relative to a similar transistor with a fixed composition collector structure. In the absence of a collector-base voltage, the electric field produced by the compositional grading improves the collection efficiency of ballistic electrons transported across the base. >

Integration of resonant-tunneling transistors and hot-electron transistors

We have integrated a tunneling hot-electron transfer amplifier (THETA) and a novel resonant-tunneling hot-electron transfer amplifier (RTHETA) within a single epitaxial growth. At room temperature, the THETA exhibits a common-emitter current gain of greater than six and a voltage swing of 800 mV when measured in an inverter configuration. The RTHETA exhibits similar common-emitter current gain and a four-state voltage transfer characteristic with an output voltage swing of 1 V. In contrast to the resonant-tunneling hot-electron transistor (RHET), the RTHETA exhibits current gain both before and after the resonant peak voltage. From on-wafer S-parameter measurements, the current-gain cut-off frequency (f/sub T/) and the maximum frequency of oscillation f/sub max/) for both transistors are approximately 20 GHz and 9 GHz, respectively.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Room-temperature operation of a tunneling hot-electron transfer amplifier

We demonstrate a tunneling hot-electron transfer amplifier that exhibits both dc and rf current gain at room temperature. The measured peak common-emitter current gain of the transistor approaches 7 while the dc differential gain is close to 10. S-parameter measurements, performed at a current density of 1.8×104 A cm−2, yield a current gain cutoff frequency of 6 GHz and a maximum frequency of oscillation of 12.5 GHz.

Optical suppression of ionized impurity scattering in vertical hot-electron transport

A striking effect of illumination on the vertical nonequilibrium electron transport has been observed in the GaAs-based tunneling hot electron transfer amplifier (THETA). Weak illumination can considerably increase the transparency of a THETA structure for quasiballistic electrons if the photon energy exceeds the GaAs band gap. The temperature and illumination intensity dependencies indicate that the effect is caused by photoneutralization of ionized impurities which are a major source of hot electron scattering.

Bleaching effect of illumination on the vertical hot-electron transport

A striking effect of illumination on the vertical nonequilibrium electron transport has been observed in the GaAs-based tunneling hot electron transfer amplifier (THETA). Weak illumination can considerably increase the transparency of a THETA structure for quasi-ballistic electrons if the photon energy exceeds the GaAs band gap. The temperature and illumination intensity dependences indicate that the effect is caused by photo-neutralization of ionized acceptors which are a major source of hot electron scattering.

Ballistic hot-electron transistors

We present an overview of work at the IBM Thomas J. Watson Research Center on the tunneling hot-electron transfer amplifier (THETA) device—including its use as an amplifier and as a tool for investigating ballistic hot-electron transport. In the initial, vertically configured version of the device, a quasi-monoenergetic, variable-energy, hot-electron beam is generated (via tunneling) which traverses a thin GaAs region and is then collected and energy-analyzed. As the hot electrons traverse the device, they are used to probe scattering events, band nonparabolicity, size-quantization effects, and intervalley transfer. A recent, lateral version of the device has been used to demonstrate the existence of ballistic hot-electron transport in the plane of a two-dimensional electron gas, and the associated possibility of achieving high gain.

Lateral tunneling and ballistic transport in two-dimensional electron gas devices defined by nanostructure gates

Novel, laterally configured, tunneling hot electron transfer amplifier (THETA) devices have been formed using nanostructure metal gates to induce potential barriers separating emitter, base, and collector regions in a two-dimensional electron gas. The gates were formed by a liftoff technique using PMMA resist and a high resolution electron beam writing tool. Lateral electron tunneling has been directly observed, using electron energy spectroscopy, through potential barriers under metal gates as wide as 52 nm. The energy distribution of the hot electrons was only ≂5 meV wide. Certain gate configurations were found to increase the fraction of emitted electrons which reached the collector. Current gain as high as 105 was observed. This is better than the best results previously obtained with vertical THETA devices.

Transient simulation of AlGaAs/GaAs/AlGaAs and AlGaAs/InGaAs/AlGaAs hot-electron transistors

The intrinsic switching characteristics of tunneling hot electron transfer amplifier (THETA) devices has been simulated using a time-dependent ensemble Monte Carlo method. Results which show that there is no significant difference between the switch-on and switch-off times of THETA devices and that these switching times are always larger than the average total transit time through the base and the collector barrier are presented. The choice of the base width is important to determine the base transport factor, but it is not a direct measure of the intrinsic switching speed. The importance of velocity overshoot in the collector barrier region for ultra-high-speed device operation is demonstrated. An improved device structure that leads to subpicosecond switching speeds in the THETA structures is proposed. >

First observation of ballistic holes in a p-type THETA device

Novel p-type tunneling hot electron transfer amplifier (THETA) devices have been fabricated for the first time. A tunnel injector is used to separate the light holes from the heavy ones. In the present case, in p GaAs doped to 2*10/sup 18/ cm/sup -3/, the fraction of light holes is only about 6%. After tunneling through a 10-nm-thick AlGaAs barrier, 0.2-eV high, the current due to light holes is more than 10/sup 4/ times greater than that due to the heavy holes. This method of injection has been used to launch primarily light holes into a 30-nm p-GaAs layer, doped as above, and performed energy spectroscopy with another, relatively thick, AlGaAs spectrometer barrier at the exit. It was found that 10% of the injected holes traversed the GaAs layer and the spectrometer ballistically with narrow energy distributions, 35-meV wide. The nature of the ballistic transport was also independently verified to be due to the light holes. This was done through the observation of quantum interference effects of the ballistic holes in the thin GaAs base. The results show that p-type ballistic devices with performance potential approaching that of n-type devices may be possible. >

Monte Carlo Studies of Hot Electrons

Through the most recent technological advances, it has been possible to get into the subpicosecond and submicron regimes, where hot electron effects are very important. The purpose of this paper is to show how the Monte Carlo technique can be successfully applied to the study of ultrafast phenomena in semiconductors. Two main topics will be dealt with:(1) the energy relaxation of photoexcited electrons in bulk GaAs and GaAs-AlGaAs quantum wells. The dynamical evolution of the polar optical phonon distribution is followed in the simulation. It is found that the presence of hot-phonons greatly reduces the cooling rate of the photo-excited electrons, in qualitative agreement with available experimental results.(2) The energy and momentum losses of hot electrons injected into a doped region, as found for example in the planar doped barrier (PDB) and in the tunneling hot electron transfer amplifier (THETA) devices. The interaction between the injected electrons and the background of cold carriers is shown to be a very effective channel of dissipation.

Dc performance of ballistic tunneling hot-electron transfer amplifiers

We present new experimental results of ballistic electron transport through thin n+-GaAs layers. Measurements were done on tunneling hot-electron transfer amplifier devices composed of GaAs and AlGaAs layers. In devices with GaAs active regions (bases) of 300 and 800 Å, collisionless or ballistic transport was observed. By performing hot-electron energy spectroscopy we found that the collected ballistic distributions were similar in shape but differed in magnitude. This suggests the existence of a strong scattering mechanism which randomizes the otherwise ballistic electrons. The maximum differential current transfer ratio α was 0.9 in devices for which about 75% of the injected current traversed the base ballistically. The presence of ballistic transport has also allowed the measurement of the AlGaAs barrier height through observation of the onset of current collection in the devices. Barrier heights higher than those recently reported have been measured. In addition we show the effects of grading the collector barrier. The most noted effect in these cases was a higher transfer ratio.

Tunneling hot-electron transfer amplifier: A hot-electron GaAs device with current gain

Tunneling hot-electron transfer amplifier (THETA) devices, based on GaAs-AlGaAs heterojunctions, were fabricated and tested. Hot-electron transfer (α) through a 1100-Å base in excess of 70% was found at 4.2 K. This resulted in a corresponding current gain ( β) in a common emitter configuration of about 2.3. In the temperature range of 4.2–80 K and under constant biasing conditions, α was nearly temperature independent. Electron energy distributions for motion normal to the layers and electron total energy loss while traversing the device were estimated. Typical widths of the energy distributions were less than 200 meV, and both widths and energy peak positions were only slightly dependent on temperature and initial injection energy.