T. C. L. G. Sollner

Resonant tunneling through quantum wells at frequencies up to 2.5 THz

Resonant tunneling through a single quantum well of GaAs has been observed. The current singularity and negative resistance region are dramatically improved over previous results, and detecting and mixing have been carried out at frequencies as high as 2.5 THz. Resonant tunneling features are visible in the conductance‐voltage curve at room temperature and become quite pronounced in the I‐V curves at low temperature. The high‐frequency results, measured with far IR lasers, prove that the charge transport is faster than about 10− 1 3 s. It may now be possible to construct practical nonlinear devices using quantum wells at millimeter and submillimeter wavelengths.

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

Quantum well oscillators

Oscillations have been observed for the first time from double barrier resonant tunneling structures. By eliminating impurities from the wells, we have been able to increase the tunneling current density by a factor of nearly 100. With the attendant increase in gain and improved impedance match to the resonant circuit, the devices oscillated readily in the negative resistance region. Oscillator output power of 5 μW and frequencies up to 18 GHz have been achieved with a dc to rf efficiency of 2.4% at temperatures as high as 200 K. It is shown that higher frequencies and higher powers can be expected.

Fundamental oscillations up to 200 GHz in resonant tunneling diodes and new estimates of their maximum oscillation frequency from stationary‐state tunneling theory

Fundamental oscillations have been measured up to 200 GHz in resonant‐tunneling diodes at room temperature. Oscillations in the range 102–112 GHz were achieved with diodes mounted in a WR‐6 waveguide resonator, and the peak output power in this range was approximately 5 μW. The same diodes oscillated between 192 and 201 GHz and generated about 0.2 μW when mounted in a WR‐3 resonator. The estimated maximum oscillation frequency ( fmax) for these devices is 244 GHz, assuming the average drift velocity across the depletion layer to be 4×107 cm s−1. This estimate has been obtained from a new phenomenological theory of the negative differential conductance which accounts for the frequency‐dependent spreading resistance and transit‐time delay. The theory is also used to show that diodes having fmax exceeding 600 GHz are feasible simply by modifying the doping profile in the regions on either side of the double‐barrier structure.

Observation of millimeter‐wave oscillations from resonant tunneling diodes and some theoretical considerations of ultimate frequency limits

Recent observations of oscillation frequencies up to 56 GHz in resonant tunneling structures are discussed in relation to calculations by several authors of the ultimate frequency limits of these devices. We find that calculations relying on the Wentzel–Kramers–Brillouin (WKB) approximation give limits well below the observed oscillation frequencies. Two other techniques for calculating the upper frequency limit were found to give more reasonable results. In one method we use the solution of the time‐dependent Schrodinger equation obtained by Kundrotas and Dargys [Phys. Status Solidi B 134, 267 (1986)], while in the other we use the energy width of the transmission function for electrons through the double‐barrier structure. This last technique is believed to be the most accurate since it is based on general results for the lifetime of any resonant state. It gives frequency limits on the order of 1 THz for two recently fabricated structures. It appears that the primary limitation of the oscillation freque...

Effect of quasibound‐state lifetime on the oscillation power of resonant tunneling diodes

A new equivalent circuit is derived for the double‐barrier resonant tunneling diode. An essential feature of this circuit is the addition of an inductance in series with the differential conductance G of the device. The magnitude of the inductance is τN/G where τN is the lifetime of the (Nth) quasibound state through which all of the conduction current is assumed to flow. This circuit model is used to derive values of theoretical oscillator power that are in much better agreement with experimental results than theoretical predictions made without the inductance. The conclusion is drawn that the response of the double‐barrier structure to a time varying potential is consistent with the coherent picture of resonant tunneling.

Millimeter‐band oscillations based on resonant tunneling in a double‐barrier diode at room temperature

A double‐barrier diode at room temperature has yielded oscillations with fundamental frequencies up to 56 GHz and second harmonics up to 87 GHz. The output powers at these frequencies were about 60 and 18 μW, respectively. These results are attributed to a recent improvement in the material parameters of the device and to the integration of the device into a waveguide resonator. The most successful diode to date has thin (∼1.5 nm) AlAs barriers, a 4.5‐nm‐wide GaAs quantum well, and 2×1017 cm−3 doping concentration in the n‐GaAs outside the barriers. This particular diode is expected to oscillate at frequencies higher than those achieved by any reported p‐n tunnel diode.

Picosecond switching time measurement of a resonant tunneling diode

Picosecond bistable operation has been experimentally observed for the first time in a double‐barrier resonant tunneling diode. A rise time of 2 ps was measured using the electro‐optic sampling technique; this is the fastest switching event yet observed for an electronic device. This time domain measurement adds necessary information to the understanding of the transport mechanisms in the resonant tunneling diode and is consistent with switching time limitations computed for the device. It also demonstrates that appropriately designed double‐barrier quantum well diodes have a response time comparable to that of the fastest all‐optical logic elements, and that they may be very useful in high‐speed logic applications.

Large room‐temperature effects from resonant tunneling through AlAs barriers

At room temperature, we have observed negative differential resistance in AlAs double‐barrier structures and a large hysteresis in the current‐voltage characteristic of a stack of five AlAs double‐barrier structures. The peak‐to‐valley ratio of the current was as high as 3.5:1 in a double‐barrier structure. To the best of our knowledge, this is the largest room‐temperature peak‐to‐valley ratio observed to date in a double‐barrier structure and the first report of a room‐temperature hysteresis in a stacked structure. These structures were grown by molecular beam epitaxy using thin AlAs barriers in GaAs. Both the first and second resonances were observed, and are well explained by simple tunneling theory assuming a value of 1.0±0.1 eV for the GaAs‐AlAs conduction‐band discontinuity seen by the tunneling electrons. This value is very close to the difference in conduction‐band energy at the Γ points found by using the accepted values of GaAs and AlAs band gaps with 65% of the band‐gap difference appearing in ...

Microwave and Millimeter-Wave Resonant Tunneling Diodes

Resonant tunneling through double-barrier heterostructures has attracted increasing interest recently, largely because of the fast charge transport [1] it provides. In addition, the negative differential resistance regions which exist in the current-voltage (I–V) curve (peak-to- valley ratios of 3.5:1 at room temperature [2–4] and nearly 10:1 at 77 K have been measured) suggest that high-speed devices based on the peculiarities of the I–V curve should be possible. For example, the negative differential resistance region is capable of providing the gain necessary for high-frequency oscillations [5]. In our laboratory we have been attempting to increase the frequency and power of these oscillators [6], and others have worked toward a better understanding of the equivalent circuit of the device [7] and the underlying processes responsible for the frequency response [8–10]. Three-terminal devices using resonant tunneling in various ways have also been proposed and fabricated [11–13]. In this paper we will describe our most recent results for oscillators as well as some new resonant-tunneling devices that have application in the millimeter and submillimeter-wave spectrum.