11111L5 lL 1.8 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS-1I963-A

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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.

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.

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.

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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 Schrödinger 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 frequency for double-barrier resonant tunneling diodes will be imposed by intrinsic device circuit parameters and by the transit time of the depletion layer rather than by time delays encountered in the double-barrier region.

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.