Heterojunction bipolar transistors with low temperature Be-doped base grown by CBE

Abstract Growth of highly doped p-type InGaAs is required for low contact resistivity demanded by high performance microwave devices. Increasing the doping level in the base of HBTs is known to be reflected in better unity power gain cut-off frequency. Low temperature growth has been shown to significantly enhance the maximum doping levels obtainable in carbon-doped bases of Npn transistors [C.J. Palmstrom et al., Appl. Phys. Lett. 64 (1994) 3139]. However, low temperature growth of Be-doped InGaAs has been reported to significantly degrade surface morphology of CBE grown material [T.K. Uchida et al., J. Appl. Phys. 29 (1990) L2146]. In this study the opposite effect was found. As the growth temperature was lowered, the surface morphology improved. Co-optimized normal growth temperature of InP and InGaAs has been previously determined to be 525°C while the low temperature is approximately 460°C. Growth was performed using a Varian Gen II CBE reactor using triethylgallium, trimethylindium, 100% phosphine and 100% arsine as source materials. Elemental Si and Be were used as n- and p-type dopants. A factor of 2 improvement in the doping level was seen with a maximum level of 6 × 10 19 cm −3 measured by the Van der Pauw-Hall technique. A marked improvement in the surface morphology and X-ray spectra accompanies this reduction in temperature from a rough surface at normal temperature to specular at low temperature. SHBTs have been grown using both low temperature base structures and normal temperature base structures. The contact resistivity for the base improves by an order of magnitude (to 4 × 10 −6 Ω · cm 2 ) and the sheet resistance improves by a factor of 6 (to 324 Ω/□). DC current gains of 25, a common emitter breakdown of 6 V and a common base breakdown of 9 V are obtained. The unity current cut-off frequency for these heavily doped structures is above 90 GHz for a 4 × 4 μ m emitter geometry and the unity power gain cut-off frequency improves by more than a factor of 2 when compared to the lower-doped structure to above 100 GHz. Detailed X-ray investigation of the bulk low temperature InGaAs base and the InGaAs InP interface will be presented as will explicit HBT structures, s -parameter modeling of the HBT, and high frequency performance limitations.