Technical Insight
IQE supplies InP and metaphoric wafers (Materials and equipment)
IQE supplies InP and metamorphic wafers IQE are to begin supplying production quantities of a range of InP HEMT epiwafers, as well as metamorphic HEMT and HBT epiwafers on GaAs substrates, designed for fiber optic and wireless communications applications. "It is well known that InP-based electronic structures provide significant performance advantages over today s GaAs-based structures, and that the high-volume commercialization of such devices is imminent," explains IQE s chief technology officer, Tom Hierl."The metamorphic approach represents an attractive alternative to InP-based electronics and is potentially the lowest cost solution to requirements for next generation applications." InP HBT wafers Hierl notes that the company s Be-doped InP HBTs have demonstrated ft and fmax values of over 210 and 300 GHz, respectively, in compositionally-graded structures doped at a level of 4 1019/cm3 in the base region. In addition, a high breakdown voltage of 6 V has been achieved at a current density Jc of 1.1 A/cm2. The company s commercial partners have obtained ft and fmax values of over 150 and 250 GHz on less complicated DHBT wafer designs with the same base doping. "Although IQE offers both MBE and MOCVD growth technologies, we have found that MBE offers a decided advantage over MOCVD for obtaining high base doping levels in InP HBT structures," says Hierl. "Our customers can choose between beryllium and carbon base doping. We have provided a number of structures which are currently undergoing testing; these feature base carbon doping levels up to 8 1019/cm3." Metamorphic HEMTs and HBTs Metamorphic HEMT epiwafers employ a buffer layer between the GaAs substrate and the active region to accommodate the difference in lattice constants. According to Hierl, GaAs substrates are less expensive, mechanically stronger, of higher quality and available in larger diameters than InP. GaAs substrates also benefit from more mature backside processing. "The metamorphic approach enables the production of a wider range of epitaxial structures due to the flexibility of designing the buffer layer for a range of active region lattice constants," says Hierl. "As a result, metamorphic HEMTs that operate at higher frequencies than a GaAs PHEMT and at higher power densities than InP HEMTs can be designed. The metamorphic approach also offers a GaAs HEMT solution to those customers using 6 inch diameter processing equipment." IQE is also developing metamorphic HBTs. The structure consists of a single heterojunction HBT with 1 1019/cm3 p-type doping in the InGaAs base (see figure). This device has achieved a current gain of 320, compared with 360 on structures grown lattice-matched to InP.
