Speed of Response

The intrinsic speed of semiconductor detectors is limited by the transport of electrons through the semiconductor bands. Some materials, such as gallium arsenide and germanium, have higher electron mobilities than silicon, and are preferable for high frequency applications. Even in these high-speed materials, the response is limited by the semiconductor plasma frequency. In metal-insulator detectors, the MIIM diode operates at extremely high frequencies due to higher plasma frequencies in metals and ultra-short tunneling times through the diode.

In radio wave terminology, these frequencies are the carrier frequencies. The RC time constant (determined by device area, materials, thicknesses and lead dimensions) ultimately limits the signal to terahertz frequencies.

Using the laboratories at Harvard-Smithsonian, Phiar conducted a rigorous mixing experiment. The blue curve represents Phiar’s modeled performance based on diode measurements under DC current. Not only did the device perform well, its behavior at 200 GHz was predictable.

Phiar has tested metal-insulator detectors at 200 GHz by mixing two high frequency signals together to obtain the difference frequency (Figure 1). By detecting the difference signal we could be sure that we were not detecting a thermal response of the diode, but that the diode was in fact rectifying at the high frequency. (The measurements were carried out using two approximately 200 GHz sources in a Harvard-Smithsonian laboratory.) The resulting response agreed with what we calculated based upon the low-frequency current-voltage characteristic of the device.

Phiar has also validated its technology at 1 THz. At the University of California Santa Barbara, Phiar aimed a free-electron laser at a MIIM detector manufactured on a quartz wafer.

The graph shows the response to a laser pulse from UC Santa Barbara's free electron laser. The sharp rise in voltage at zero seconds is the pulse detection. The decay after the spike is an artifact of the laser rather than an effect from Phiar’s diode.

Figure 2 shows the Phiar detector’s response to one of the laser’s 1 THz pulses. The sharp rise seen in the graph illustrates the speed of response. (The decay seen to the right of the sharp rise is an artifact of the laser’s pulse, rather than the detector.)

Phiar's 1 THz bowtie detector shown both in a micrograph and modeled showing power distribution. Note the highest concentration of power for the 1 THz signal is at the diode at the intersection of the antenna's lobes where rectification occurs. The detector used in the UCSB experiment is shown in Figure 3.

When the diode is connected to an antenna, the antenna impedance has an effect on the frequency response (Figure 5). Because of this, detection of very high frequencies requires that the diode be very small to reduce capacitance. In some cases, the size required to achieve this low of capacitance is beyond the limits of state-of-the-art fabrication equipment. To circumvent the RC-time limit, Phiar has patent-pending traveling wave detector structures. In a traveling wave device, electromagnetic energy couples from the antenna to surface plasmon mode of the MIIM tunnel junction. This mode of propagation results in efficient rectification, even at very high frequencies.

Phiar's traveling wave structures eliminate manufacturing tolerance issues that could crop up for very high frequency detectors made in traditional 'lumped element' configurations.