TECHNOLOGY MICROELECTRONICS
digital and optical functions on the same die for a variety of markets including, but not limited to, hand-held smartphones and tablets, PCs, servers, data centres, military and industrial applications.
With our technology, the incompatibility issue between transistors and the optical devices disappears, and it is possible to form high mobility channels for both the n-type and p-type transistors. But to do this, we have to challenge the assumption that these high-mobility materials have to be introduced on a silicon substrate.
In our case, we use substrates made of GaAs. These are currently available in diameters up to 200 mm, and there is no fundamental barrier to the production of 300 mm equivalents. Our preferred growth technique for depositing III-V layers on this foundation is MBE, and this can be applied to substrates of this size.
Tier 1 fabs already use this approach to deposit material on 300 mm wafers, so the only barrier to a switch of substrate is cost. Differences between the price of silicon and GaAs substrates will shrink as shipments of the latter rise, and costs could be further reduced through innovations in substrate release techniques. Note that the POET fabrication process employs the same set of foundry tools currently used for silicon CMOS, so only a reconfiguration is required.
The idea of using GaAs rather than silicon to make digital circuits is not new. During the nMOS era that spanned the 1970s and early 1980s, GaAs MESFET technology was a contender for silicon E/D logic applications. And later, during the development of CMOS, the GaAs HEMT was also considered for high-speed logic circuits.
Two fundamental flaws prevented these GaAs devices from making a significant impact. The first is that both the MESFET and the HEMT are inherently normally on (depletion) devices, and although the threshold can be shifted marginally to the positive, the swing is too small to be well controlled. The second is that it is not possible to make p-channel MESFETs and HEMTs, which prevents the construction of a complementary technology.
Our POET technology is not held back by these issues. In our case, the p-type form of the FET is a natural component in the POET device family. Although the pFET and the nFET are grown as enhancement devices, they can be transformed to depletion devices with a suitable implant. Consequently, the complementary inverter is just as compact as it is when built with silicon CMOS technology, but it can operate with a smaller supply voltage.
All of the devices in the POET family are formed with a single epitaxial growth step that creates multiple-quantum- well or multiple-quantum-dot layers. This is followed by a comprehensive fabrication sequence, which produces electronic and optical devices simultaneously.
Figure 3. POET technology can be used to create a complementary inverter, a key building block for an IC.
Electronic devices are realized as bipolars, FETs and thyristors. For the HFETs, gates are formed by refractory metals, with the multiple quantum wells becoming the channels of n-type and p-type devices, and self-aligned ion implants defining the source and drain regions. Like MOSFETs, the HFETs have a back gate contact, which is the substrate.
The gate metal of the HFET forms an ohmic contact. This eliminates static conduction because the gate input logic swing is always maintained below the knee voltage of the input diode. In contrast, both the pnp and npn bipolars are in the form of bipolar inversion channel transistors (BicFETs), with the refractory ohmic contact behaving as the emitter terminal operating in the conductive region.
Meanwhile, in the bipolar devices, the multi-quantum well channels function as control electrodes, while the collectors are quantum-well regions contacted by ion implantation. Finally the thyristor utilizes both HFETs and BicFETs. This four-terminal digital device has a high resistance off-state and a low resistance on-state, which makes it useful in synchronous circuits and memory applications.
The same sequence of steps creates optical devices. To form a quantum-well laser from the HFET structure, the refractory emitter is contacted and also the channel. Laser operation is possible with either p-type or n-type contacts. Note that it is also possible to operate the thyristor as a laser when it is switched to its conducting state.
Thanks to the flexibility of the POET platform, lasers can be produced as either in-plane emitters or VCSELs. To fabricate the latter class of device, the transistor structure has to be designed to fit within an integer number of half wavelengths. This requirement is easier to fulfil when the laser operates at longer
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