IPRM showcases advanced InP devices and novel applications
Some of the key attractions included comprehensive sessions that covered HEMTs, HBTs, VCSELs and high-speed optoelectronics. Industry and academic leaders also took to the floor to present research on InP epitaxy, reliability and packaging, GaInAsN and related compounds, as well as more niche areas such as quantum-dot physics and devices, and novel materials.
Electronic developments
Satoshi Tsunashima and co-workers from NTT Photonics Laboratories revealed their latest digital InP/InGaAs digital-frequency divider, which they say has the highest reported toggle frequency to date (Tsunashima et al.). Digital-frequency dividers are a critical component of today s high-speed, wide-bandwidth optical fiber communication systems.
Fabricated with an InP HBT process, the 1/8 digital-frequency divider IC operated at 90 GHz and had an ft and fmax of 155 and 245 GHz, respectively. The researchers used undoped InP emitters to reduce emitter capacitance and designed the IC with a first toggle flip-flop stage that makes use of feed-forward techniques. Additional T-type and master-slave flip-flop stages contain a conventional series-gated emitter-coupled logic configuration.
The NTT device also has a clocked inverter design to enhance the dynamic circuit performance. By combining a feed-forward and clocked inverter the researchers increased the speed of the IC by 92%.
The chip was designed with a 0.5 VPP internal logic swing, compared with the usual 0.2-0.3 VPP logic swing found in SiGe HBT ICs. According to Tsunashima, this result illustrates that InP still has significant headroom for additional performance with respect to SiGe.
The IAF Fraunhofer Institute, Freiburg, Germany, has also been developing record-performance frequency dividers, but by using HEMTs rather than HBTs. Using 0.1 µm gate-enhancement mode metamorphic HEMTs on 4 inch GaAs substrates, A Leuther and colleagues have developed 4:1 dynamic-frequency dividers over the 58-82 GHz range (Leuther et al.).
The devices reached an ft of 200 GHz, which was achieved through careful attention to the gate recess process, and featured a 10 nm AlInAs barrier and a composite channel design with indium mole fractions of 0.53 and 0.65. The devices also contained NiCr resistors, MIM capacitors and a BCB dielectric to minimize the interconnect parasitic capacitance. The median lifetime of a divider, based on a 10% transconductance degradation, was 1.1 x 106 h at 125 °C with an activation energy of 1.5 eV.
Cooling devices
Ali Shakouri of the University of California, Santa Cruz, highlighted the use of thermoelectric coolers for applications such as spot cooling in electronic and optoelectronic devices. When a current passes through a metal-semiconductor junction, the junction is either warmed or cooled depending on the direction of flow. This phenomenon, known as the Peltier effect, can be used in thermoelectric (TE) conversion. In thermoelectric coolers, transferred heat is used to provide a temperature gradient that allows a refrigeration capability.
The usefulness of a material for TE applications is determined by its TE figure of merit Z, which is dependent on temperature and includes contributions from the Seebeck coefficient, and electrical conductivity, as well as λE and λL which are the electronic and lattice contributions to the material s thermal conductivity. Today s best TE materials are characterized by ZT ~ 1, but if this could be increased to around 3, TE coolers could become competitive with conventional refrigeration systems.
Shakouri explained that thermionic emission of energetic carriers over a barrier mimics the Peltier effect and acts in the same way as evaporative cooling. The researchers have developed experimental TE cooling devices that achieve up to ΔT = 4.5 °C near room temperature (figure 1). According to Shakouri, while these are still modest temperature drops they can be achieved with useful cooling capabilities of the order of 680 W/cm2 at an ambient temperature of 70 °C, which could be very advantageous in a number of applications. Shakouri added that continued technological improvements could lead to an order of magnitude improvement in the ZT product, with InGaAs/InGaAsP TE coolers providing cooling by almost 15 °C near room temperature.
