HIGH EFFICIENCY AND HIGH LINEARITY MICROWAVE POWER AMPLIFIERS BASED ON ULTRA- HIGHfmm SELECTIVE BURIED SUB-COLLECTOR (SBSC) HBTS

This paper presents several microwave power ampl~ier designs that promise high power and broad bandwidth, while meeting the eficiency and linearity requirements of advanced radar and communication systems. Power amplifiers are designed based on an ultra-high f~u~ (>450 GHz) selective buried sub-collector (SBSC) HBT technology [1]. SBSC HBTs are fabricated on prepatterned sub-collector mesas. The active device region is care~lly designed so that the overlap between the extrinsic base and the sub-collector is totally eliminated for minimizing the Cbc, and maximizing the device f~a~ Simulations on SBSC HBT microwave amplifiers show that >1OW output power may be achievable with PAE>50% and 01P3=40 dBm at 7-11 GHz (X-band) and >4W output power with PAE=30Yo and 01P3=32 dBm at 28-32 GHz (Ku-band). These results show the great potential of using SBSC HBTs for broadband and high power microwave amplifiers with high efficiency and lineari~. SUMMARY Microwave power amplifiers have been implemented by using conventional GaAs HBTs. However, their performance in power, efficiency, linearity and bandwidth is usually limited by device parasitic. Among them, the base resistance (Z?~)and base-collector capacitance (C~C) are the most crucial, Reducing CbC is especially important because the output of HBTs with large C~c is difficult to be matched to the standard 50 Q impedance for broadband applications. The extrinsic basecollector capacitance (Cb..) under the base ohmic contact region usually contains the major part of the tOCd (k Reducing C~CXhas always received a great deal of attention in HBT device design for improved microwave performance. In the past, H+ or 0+ ions were implanted to the extrinsic collector region to reduce Cb. [2] (Fig. 1). However, the Cbm between the extrinsic base and sub-collector regions is still significant. Deep H+ implantation into the collector layer reduces Cbc, but may also increase the base resistance and degrade the device reliability. Transferredsubstrate HBTs with very high f~m are demonstrated recently [3], but their fabrication process is rather complicated for IC manufacturing and their thermal resistance is greatly increased due to the complete removal of the substrate. San Diego, La Jolla, CA* Figure 2 shows the cross section of a SBSC HBT. The sub-collector layer is grown first on the GaAs substrate. The subsequent HBT layers are then grown on the prepatterned selective buried sub-collector mesa. The active HBT region is carefully designed so that the overlap between the extrinsic base and sub-collector regions is negligible and the associated cba is substantially reduced. Experimental results showed that the Cb. was reduced by half comparing to that of conventional HBTs while the base resistance remained unchanged. As a result, a 4050% increase in f~~ was obtained [1]. The performance of SBSC HBTs can be further improved by scaling down device dimensions. For example, when reducing the emitter dimensions to 0.6x5 ~m2, the RF performance of both SBSC and conventional HBTs is calculated in Table 1 and 2 based on a common material structure as shown in Table 3. It is shown that although f,’s are similar for both types of HBTs, fu of the SBSC HBT is greatly increased due to the significant reduction of CbCX. In conventional HBTs (Fig. 1), the base contact width must be optimized to obtain a low RbCbC and high fm. But the value of Rb may not be the minimum for the lowest R~CbC. In SBSC HBTs, the base contact width can be designed large enough to achieve the lowest base resistance without increasing Cbti In other words, a minimum RbCbC can be obtained by selecting the lowest Rb and Cbc. Based on our simulation, SBSC HBTs with reduced device dimensions may obtain an ultra-high f(>450GHz) which is comparable to InP-based HBTs made with the transfer substrate technique [3].