Nano-diode that does not rely on pn junction

Professor A. M. Song

 

The following novel diode concept that we recently demonstrated is perhaps not only the simplest diode to date but also the quickest electronic nanodevices ever reported.

The self-switching device (SSD) is made in only one nanolithography step, by simply creating insulating trenches in a semiconductor layer. In the following picture, (a) is a micrograph of the device showing that a narrow channel is formed between two (etched) insulating trenches. Since there is always some native depletion region (grey area) close to etched interfaces, the real channel is even narrower (b). If a positive voltage is applied to the right hand side, the positive potential on both sides of the channel will attract the (negatively charged) electrons into the channel, leading to a large current, as shown in the current-voltage characteristic in (e). In contrast, if a negative voltage is applied, the electrons will be repelled away from the channel by the negative potential on both sides of the nanochannel, making it difficult or impossible for current to flow. As such, the diode functionality is realised. This is a completely new device concept, since the SSD is not based on any doping (pn) junction or energy barrier structure as in a conventional diode. 

The SSD is made in only one nanolithography step, and is planar with both the electrodes and the active semiconductor within a two-dimensional plane. This is in great contrast to a traditional diode, which is made in many steps of lithography and is always a complex three-dimensional vertical structure. The simplicity in the device structure and lithography obviously leads to a significant reduction of the production cost.

The device shows some unique properties. For example, the threshold voltage of the self-switching device can be arbitrarily tuned, from zero to about 10V. A zero threshold allows us to detect extremely weak signals, even when no external bias circuit is used. A normal diode always has a threshold voltage, typically around 0.7V, which is fixed and determined by the semiconductor used. If the applied signal is below the threshold, the diode will not operate unless the diode is externally biased beyond the threshold.

Because of the simple planar structure, even some simple circuits can be made by just "writing" insulating lines on a semiconductor layer. The following are two examples: the left is diode logic OR gate and the right is a bridge rectifier.

The extremely simple architecture enables a very low parasitic capacitance and therefore a very high operation speed. In our experiments, the device showed very stable frequency dependence up to 110 GHz, which is the highest frequency we could measure in our laboratory. Nevertheless, this is the highest speed that has ever been demonstrated in various types of novel electronic nanodevices to date. From simulations, the device is expected to operate also in the THz (1000 GHz) frequency regime, in which a very broad range of applications have been envisioned.

For GHz or THz microwave detections, the SSD can be made into an array very easily. The following is a micrograph of an SSD array and the 110 GHz microwave measurement setup. Our most recent experiments demonstrated room-temperature operation of the SSD at 1.5 THz (1,500 GHz), which is to our knowledge the quickest novel nanoelectronic device to date. Currently, the device is being exploited for energy harvesting applications, due to its ultra-high speed and zero threshold.

The technology has led to a spin-out company working on printed electronics. For more details on Nano ePrint Ltd, you can click here Nano ePrint Ltd. 

References:  

Scientific articles:  

o        J. Mateos,a! B. G. Vasallo, D. Pardo, and T. González, Operation and high-frequency performance of nanoscale unipolar rectifying diodes, Applied Physics Letters, 86, 212103 (2005)

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