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Infrared Physics & Technology 51 (2008) 470–472

Active electric near field imaging of electronic devices

A. Coppa a,*, V. Foglietti a, E. Giovine a, A. Doria b, G.P. Gallerano b, E. Giovenale b,A. Cetronio c, C. Lanzieri c, M. Peroni c, F. Evangelisti a

a Istituto di Fotonica e Nanotecnologie, Via Cineto Romano 42, 00156 Roma, Italyb ENEA – Dipartimento Tecnologie Fisiche e Nuovi Materiali, 00044 Frascati, Italy

c Selex Sistemi Integrati S.p.A., Via Tiburtina, Km 12,400; 00131 Roma, Italy

Available online 1 January 2008

Abstract

An active two dimensional near field imaging of a High Electron Mobility Transistor (HEMT) used as THz detector has been per-formed. The reflective imaging system developed at the ENEA FEL Facility in Frascati has been used to this purpose. This imagingtechnique has shown to be particularly powerful in resolving various coupling mechanisms of the incident radiation with the device.� 2008 Elsevier B.V. All rights reserved.

Keywords: High electron mobility transistor; Terahertz detector; Near field imaging

1. Introduction

A novel imaging technique which can probe the activeresponse of electronic devices has been demonstrated.

We have performed the measurement on a High Elec-tron Mobility Transistor (HEMT) which has also showna peculiar detection characteristic of the device in theTHz range. HEMT used as THz detectors have been inves-tigated recently where the detection mechanism is related toplasma wave excitations [1,2].

The reflective imaging system developed at the ENEAFEL Facility in Frascati has been used. The 150 GHzFEL radiation is coupled to a series of two WR6 direc-tional couplers terminated by microprobe open waveguideend, which directs the 130 GHz radiation to the surface ofthe sample under investigation [3]. Details on the imagingset-up are shown elsewhere [4]. The device is a double gatedepletion GaN HEMT which is a standard high frequencydevice with a unity gain frequency (Gmax) at 30 GHz [5].The dc output characteristics are shown in Fig. 1, in the

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doi:10.1016/j.infrared.2007.12.035

* Corresponding author. Tel.: +39 06 415221.E-mail address: andrea.coppa@ifn.cnr.it (A. Coppa).

same figure is also shown the load line of the measuringset up. Photograph of the device and the pcb layout isshown in Fig. 2. The measuring set-up is shown inFig. 3. The bias scheme is a standard common source con-figuration, with the detector output taken at the drain, fil-tered by a high pass filter. The images are takennormalizing the output voltage to the incident input signalthat comes out from the waveguide or, alternatively, to thereflected signal from the HEMT surface. The effect ofpolarization on the output signal has also been measuredby rotating the device by 90�. The guide has a rectangularsection of 1.3 � 0.6 mm2. The pulse has amplitude of 3microseconds and a repetition rate of 2 Hz. A schematicof the pulse is shown as an insert in the right-top side ofFig. 3. The electric field at the output of the guide is2 � 105 V/m. The output voltage is measured while thewaveguide scans over the pcb board on which the HEMTis bonded with Al wires to the copper lines. The first mea-surement is a map of reflectivity which is performed at afixed height, 0.7 mm from the pcb plane. This map has fea-tures, like the copper lines, which constitute referencepoints that are successively used to superimpose the opticalimage. By using this procedure we are confident to get apositioning error of ±50 lm.

Fig. 3. Schematic of the measuring set-up. The insert in the top-rightshows the shape of the input waveform radiation.

Fig. 4. Typical output voltage with Vg = �5 V.

Fig. 1. Output dc characteristics of the HEMT device. Also shown is theload line in red.(For interpretation of the references in colour in this figurelegend, the reader is referred to the web version of this article.)

Fig. 2. Left: photograph of the device. Right: the device glued on thePrinted Circuit Board.

A. Coppa et al. / Infrared Physics & Technology 51 (2008) 470–472 471

2. Experimental

A typical signal taken with the gate voltage equal to�5 V, i.e. close to the cut off region, is shown in Fig. 4.The red1 line corresponds to a signal voltage proportionalto the power of the incident radiation, which is measuredwith a commercial Schottky diode.

The green line corresponds to the output signal. It mustbe stressed that the measured output signal is a low fre-quency signal related to a rectifying action of the HEMTdevice. Typical output voltage range is also very high, ofthe order of various volts. Various measurements havebeen taken varying the gate voltage from the cut-off tothe linear region of the device characteristic.

The waveform output follows the input shape, which is apulse of a few microseconds, this should rule out any heat-ing effects at this level, which should obviously exhibit lar-ger time constants. The Fig. 4 shows an output signal outof phase with respect to the input signal. This polaritydepends on the voltage gate used. With values of gate biasclose to �1 V, corresponding to an operating point close tothe linear part of the curve, we obtain an output signal ofthe same shape of the input signal and in phase with it.

1 For interpretation of color in Fig. 4, the reader is referred to the webversion of this article.

The following Fig. 5 shows the image of the output sig-nal scanning the source over an area of 4 mm2. The devicepositioned approximately in the center of the scanned area.The image of the device has been superimposed on the out-put signal image with the procedure explained above. The

Fig. 5. Map of the output signal with Vg = �5 V, the polarization ofinput radiation is indicated by the arrow above the map.

Fig. 7. Output voltage with Vg = �4 V.

472 A. Coppa et al. / Infrared Physics & Technology 51 (2008) 470–472

polarization of the electric field is shown by the arrow. Inthis case the maximum of the signal is centered on thedevice, mainly over the drain region. This behavior is sim-ilar to all the maps performed with this kind ofpolarization.

There is in this case a direct interaction of the radiationwith the core of the device, particularly with the drainregion.

The situation changes significantly when the polariza-tion of the input signal is rotated by 90�. Although theoverall shape of the output voltage remains the same, i.e.like that in Fig. 4 if we used the same gate bias conditions.

The Fig. 6 shows the map of the signal output with arotated polarization.

The maxima of the signal are now in two lobes outside thedevice, in the region which correspond to the half distancebetween the device and the copper leads, along the linewhere the Al bonding wires are positioned. Clearly the Alwires act as a pick up antenna which leads the signal tothe HEMT. Also in this case there is again a rectifying effect.

Now we observe a positive signal with respect to the pre-vious negative one. This is just because the map has beentaken at a different bias gate voltage (Vg = 0 V) withrespect to Fig. 4 (Vg = �5 V). Setting the same bias gatevoltage leads to the same polarity of the output signals,independently from the polarization of the input radiation.Fig. 6 also shows that the signal on the drain region is moreintense than the one on the gate side. This effect has beenconsistently observed for all various gate bias voltages.

The reason of the observed rectifying behavior is understudy and it may depend on additional effects related to thedevice performance. However we have to stress that in thiscase, without any particular care paid in optimizing thedetector sensitivity, the device acts as a detector withresponsivity comparable to Schottky diodes in this fre-quency range.

Fig. 6. Map of the output signal with Vg = 0 V the device is now rotatedby 90� with respect to Fig. 5, consequently the polarization of inputradiation is also rotated by 90� with respect to the device.

When the bias gate voltage is adjusted to operate theHEMT in the active region with Vg ranging from �2 to�4 V the voltage output waveform changes significantly.Fig. 7 shows a case where Vg is equal to �4 V which cor-responds to a crossover between the negative and the posi-tive output response.

The output voltage swing extends with a long tail wellbeyond the region where the input radiation is on. Thereis an evident non ideal behavior which may depend onmany effects related to deep level traps in HEMT devicesthat have been consistently studied [6].

3. Conclusion

We have demonstrated a novel imaging technique whichcan probe the active response of electronic devices. Lateralresolution and precision can be easily optimized to getmore insight into the coupling mechanism between tera-hertz radiation and devices. The analysis of the GaNHEMT has shown a peculiar detection characteristic ofthe device which will be matter of further and deeperinvestigation.

References

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Valusis, A. Shchepetov, Y. Roelens, S. Bollaert, A. Cappy, S.Rumyantsev, Appl. Phys. Lett. 89 (2006) 131926.

[3] A. Doria, G.P. Gallerano, M. Germini, E. Giovenale, A. Lai, G.Messina, I. Spassovsky, F.Valente, L. d’Aquino, in: Proc. of the”Joint30th Int. Conference on Infrared and Millimeter Waves and 13thInternational Conference on Terahertz Electronics” IRMMW-THz2005, art.no. 1572505 (2005) 255-256.

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