Fujitsu InP HEMT Helps To Close The Digital Divide
Japanese firm Fujitsu has developed an indium phosphide-based high electron mobility transistor (InP HEMT) by introducing a "space cavity" into the conventional structure. Operating at the millimeter-band frequency level of 94 GH, it reduces the noise level generated by the transistor by approximately 30% compared to previous technologies, to 0.7 decibels, resulting in the world's lowest noise levels.
The noise-lowering technology enables sensitivity enhancements in millimeter-band receivers, which in turn would enable the time required for image capture by image sensors - being deployed for anti-terrorist initiatives in major airports - to be reduced by nearly half.
In addition, as the new technology can enable range extension by 20% of high-capacity wireless transmission distance on par with optical transmissions, wireless networks can be used in place of trunk lines in locations where it is difficult to install fiber-optic cabling, thereby helping to close the digital divide.
Details of this technology were presented at the 22nd International Conference on Indium Phosphide and Related Materials (IPRM 2010), held May 31 - June 4 in the Japanese city of Takamatsu.
In contrast to infra-red or optical signals, radio waves in the millimeter-band frequency range - which extends from 30 GHz to 300 GHz - are permeable and able to travel through thin walls and fabric. As part of their anti-terrorism prevention efforts, major airports around the world are considering deployment of image sensors that take advantage of the permeability of millimeter-waves.
Additionally, millimeter-wave signals are ideal for transmitting very large volumes of data exceeding several gigabits per second and can be used in place of trunk lines in locations where it is difficult to install fiber-optic cabling. This also making such equipment useful in helping to close the digital divide. (Figure 1)
Image sensors must be able to detect very weak millimeter-waves emitted by people or objects, and high-capacity millimeter-band transceiver equipment needs to be able to detect signals that have weakened in the process of being transmitted and dispersed through the air. Thus, for both of these applications there is a need for high-sensitivity millimeter-band transceivers that can receive very weak millimeter-wave signals.
Figure 1: Features and applications of millimeter-waves
To make a millimeter-band transceiver highly sensitive (Figure 2), in the HEMT that is used in the transceiver's amplifier, it is necessary to control the noise that is generated along with high signal amplification rates. If the noise level within the HEMT is high, the noise interferes with reception, thus making detection of weak signals impossible.
Thus far, in order to achieve high signal amplification rates with low noise, the conventional method used was to miniaturize the HEMT (see Figure 3a). However, miniaturizing the HEMT requires specialized fabrication equipment, and progress was limited due to low yields and poor uniformity. Therefore, the challenge was to find an alternative method for achieving high signal amplification rates with low noise.
Figure 2: Structure of Fujitsu's millimeter-band receiver
In conventional InP HEMTs, if the coupling capacitance between the gate and the source or drain is large, the millimeter-band signal amplification rates fall. To resolve this problem, Fujitsu eliminated the inter-layer dielectric film around the gate electrode, forming instead a "space cavity" that reduces the coupling capacitance. This technology was developed under a research contract from Japan's Ministry of Internal Affairs and Communications.
Since the gate electrode on a HEMT is the source of noise, reducing gate resistance is an effective method for reducing noise. For that reason, HEMTs for lower frequencies that are used in transceivers for home satellite broadcast systems use a T-shaped gate, with a low-resistance head electrode placed on the upper part of a micro-electrode to reduce gate resistance and noise.
However, when using a low-resistance T-shaped gate in the 94 GHz band, because the distance between the T-shaped gate and the source or drain is narrowed, there is an increase in noise from coupling capacitance. With the application of the "space cavity" structure, Fujitsu has now confirmed that even in the 94 GHz band, coupling capacitance is reduced when using the T-shaped gate. Moreover, Fujitsu also verified that increasing the length of the T-shaped gate's head also serves to reduce noise.
Employing the newly-developed technology, Fujitsu created an InP HEMT with a gate length of 75 nanometers (see Figure 3b). This caused a reduction in noise within the transistor by 30% compared to previous technologies. The noise figure, which is a noise index for noise within the transistor, was reduced to 0.7 decibels, thus resulting in the world's lowest noise levels. (Figure 4)
Configuring an amplifier with the newly-developed HEMT greatly improves the sensitivity of millimeter-band receivers, holding the promise of reducing by half the time image sensors require for image capture and extending the transmission range of high-capacity millimeter-band transmission equipment by approximately 20%.
As a result, security checks at airports would be much quicker, and wireless systems with date transmission capacities equivalent to optical transmission systems could be created, thereby accelerating the widespread deployment of millimeter-band systems that deliver speed and convenience in our daily lives.
Figure 3: Cross-sectional views of (a) conventional InP HEMT and (b) InP HEMT with "space cavity"
Figure 4: Effect of Fujitsu's newly-developed technology for reducing transistor noise (94GHz)
Fujitsu now plans to equip high-performance image sensors and high-capacity transmission equipment with amplifiers that will employ the newly-developed low-noise InP HEMT, targeting practical-use commercialization for 2012. The firm will also strive to expand the range of applications into such areas as radio astronomy and the environmental monitoring of the earth.