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While these exotic devices offer great promise at higher frequencies, one of the most important devices for terahertz systems is still the common transistor, which is used throughout high-frequency systems. A number of researchers have focused on extending the frequency range of commonly used devices, such as InP HEMTs and HBTs.19 InP HEMTs with maximum frequencies between 0.5 and 0.65 THz and HBTs with frequencies from 0.3 to 0.4 THz have already been reported.20 Considering the results of these reports, and the research being conducted by DARPA, the upper frequency limits of these devices should only increase. Table 1 provides a summary of various research being performed on these two types of high-frequency transistors.21 Recently, a research team presented a new InP HEMT device with average frequency of 0.55 THz and maximum frequency of more than 1 THz.

To develop higher-frequency transistors for possible terahertz use, a number of studies have explored the use of 100-kV electron beam lithography (EBL) to fabricate devices with the fine dimensions needed for higher-frequency operation.21 In one report, researchers have fabricated an HBT device with emitter width of 0.15 μm. In addition, an advanced InP HEMT was fabricated with emitter area of 0.25 × 4 μm2 and breakdown voltage of 4 V with maximum current gain of 30. The measured S-parameters for this transistor indicate a maximum frequency of operation of more than 650 GHz.

Some InP HEMT and metamorphic HEMT (mHEMT) transistors with operating frequencies between 300 and 350 GHz have also been introduced.22 Based on these researches and technologies, an amplifier with gain of more than 11 dB and frequency of 480 GHz was proposed. Figure 6 shows the gain versus frequency for a 300-GHz amplifier. For a three-stage amplifier based on this amplifier, maximum gain of 15 dB at 15 THz was achieved. However, the three-stage amplifier suffers non ideal noise characteristics at its input amplifier stage.21

6. The gain of a 300-GHz amplifier is plotted across an approximate 50-GHz bandwidth.

Figure 7 summarizes recent activity on oscillator ICs operating at 100 to 600 GHz. All of these devices offer output power of more than 10 μW, considered a minimum power level for practical operation. The core research in this area focuses on transistors and MMICs, with HBT transistors having cutoff frequencies to 0.8 THz and HEMT transistors with frequencies to 1 THz already having been reported.23 The characteristics of InP-based devices indicates that they are more suitable for terahertz applications than silicon-based devices, although devices based on silicon-germanium (SiGe) substrates are showing great promise at higher frequencies.

7. These plots compare the output levels of IC oscillators based on different semiconductor technologies.

The challenges for producing terahertz transistors include decreasing device dimensions while also overcoming the effects of parasitic circuit/device elements at higher frequencies. For HBTs, small emitter dimensions are needed with low contact resistance between the base and emitter as well as reduced capacitance between the base and collector. Experimental HBTs have been developed to 500 GHz, compared to silicon circuits which reached a maximum of 96 GHz. In addition, an InP HEMT with breakdown voltage of 2.5 V and current of 0.25 mA/mm for an operating frequency of 1.2 THz has been proposed. A number of studies have focused on exploring the performance capabilities of InP HBT devices.24 One of the main challenges is balancing the current increase that occurs when the InP HBT device dimensions are reduced, and the need to dissipate the added heat generated by additional current flow. One possible solution involves changing the connection width and reversing the power, thus increasing the heat resistance capability. Practical terahertz bandwidths can only be achieved with these devices when the ohmic contact resistance is very low.

A proposed terahertz-frequency modulator is based on a HEMT device in which a two-dimensional electron gas (2DEG) is used as the interface layer for the GaAs/AlGaAs substrate material. The electron density of the 2DEG can be tuned and controlled by the external gate voltage. Experiments performed on this structure used an applied voltage between 0 and 10 V. The cutoff frequency based on the growth of received signal was calculated as 6 kHz. With an applied gate voltage of 10 V, a maximum modulated terahertz wave of 3% was obtained using terahertz time-domain spectroscopy (TDS). By creating forms of artificial metals known as metamaterials, it has been possible to create split ring resonators (SRRs) with extremely high frequencies.25 The metamaterials have been analyzed by means of Maxwell’s equations for different properties—such as electron permittivity and magnetic permeability—and these materials show great promise for use at terahertz frequencies.

8. These plots show the amplitude (top) and phase (middle) performance of a terahertz electronic modulator (bottom).

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