We have grown C-doped GaAs and (In,Ga)As epitaxial layers of device quality in a standard solid source molecular beam epitaxy system using carbon tetrabromide (CBr4) as the carbon source. Dopant incorporation was relatively efficient for both GaAs and (In,Ga)As, requiring a CBr4 beam pressure of about 1×10−6 Torr to achieve a hole density of 1.5×1020/cm3. For doping in the 1019/cm3 range, hole mobilities were comparable to or slightly higher than those of Be-doped layers with the same carrier concentrations. Modulation-doped structures grown immediately after heavily C-doped GaAs layers exhibited reduced two-dimensional electron gas mobility, but the mobility recovered to previous values within 24 h. (Al,Ga)As/GaAs heterojunction bipolar transistors (emitter size=25 μm×50 μm) with C-doped bases (p=1.2×1019 cm−3) had common emitter small signal current gains averaging 86 at an emitter current density of 970 A/cm2. The relatively low gas load during growth, the lack of long-term memory effect, and the acceptable device performance indicate that CBr4 is an attractive alternative to Be for GaAs and (In,Ga)As devices grown by solid source molecular beam epitaxy.
Shifts of the band edge in GaAs layers as measured by photoreflectance (PR) spectroscopy have been accurately calibrated to the N-type doping level (Nd). Samples produced by controlled Si-doping experiments using ion implantation of GaAs substrates and GaAs doped with Si to known levels during growth by molecular-beam expitaxy have been investigated with this technique. A measurable change in the location of the band gap (E) determined from PR directly correlates with the maximum N-type doping level as determined via C-V for both types of samples with a change of band gap δE/δNd=5.8±0.5×10−20 eV cm3 for 1×1016 cm−3 ≤Nd≤8×1017 cm−3. Correlations were also made to sheet carrier concentration (Hall measurements). This method is shown to be fast, accurate, and easily applicable to uniformity studies and a viable alternative to either C-V or Hall measurements for nondestructive determination of Nd.
A low-cost 0.7 tum gate power pseudoinorplhic high-electron-mobility transistor (PHEMT) process wsas developed. PHEMT structure, etch profile and passivation conditions wxere optimized to yield a device with high breakdown combined with minimal gate lag. A 2 mm PHEMT exhibits dc Id. of 300 mA/mm, 1,,. of 500 mAmrnm, peak Gm of 360 mS/mm and 3-tenninal breakdown voltage of 13 V. At 0.85 GHz such a device exhibits a record output power density of 630 mW/mmn at V'4=5.8 V and 290 mW/mm at Vd=3.4 V with associated PAE of 60%.
INTRODUCTIONStringent specifications of modern wireless systems require development of new semniconductor technologies to address design challenges at the devicc level. Modem transistors for personal communication systems have to be lhiglhly efficient and deliver high power at low supply voltages. PHEMT devices were previously found to meet these criteria [ 1]-[4J. High-volumiie processing of suci devices at low-cost without sacrificing their performance is lhowever a challenge. This is partially attributed to the limited ability to control the surface states at AlGaAs and GaAs interfaces witl silicon nitride passivation. These surface states strongly effect PHEMT breakdown and transient
Compliance to the low electric field regime necessary for application of the third derivative functional form formalism is demonstrated for ion-implanted samples used to determine the doping level by photoreflectance spectroscopy. Due to field and doping inhomogeneity, the proper line shape is speculated to be described by a superposition of Franz–Keldysh oscillations.
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