The next generation of low V_CEsat bipolar transistors

Introduction The last few years have seen an astonishing renaissance in bipolar transistors. Higher-power switching applications were primarily the reserve of mosfets, but now bipolar transistors are appearing in a growing number of applications such as charging circuits and load switches for portable devices in consumer electronics and communications. A major reason for this is the greater success registered in reducing saturation resistance by increasing homogenisation of current distribution in the semiconductor chip, and thus in developing components that exhibit high and constant current gain. Current driving, an innate drawback of bipolar transistors, can at least be reduced sufficiently for bipolar advantages such as temperature stability, ESD strength, and reverse blocking to again come to the fore. NXP Semiconductors has assumed a leading role here by creating its BISS (breakthrough in small signal) transistor family. The new architecture of fourth-generation BISS transistors (BISS 4, Table 1) represents a milestone in the development of SMD-packaged medium-power bipolar transistors, helping to expand an attractive range of applications. Two product categories — portfolio architecture and product specification Producing the concept for a new medium-power bipolar family calls for close examination of the entire transistor architecture (choice of material, chip design, chip metallisation, chip/package connection, package architecture) because many constituents contribute substantially to the resistance of the product. Two separate categories of the portfolio were defined in creating the product family of the BISS 4 generation. In the first ultra-low V_CEsat category, the emphasis was on minimising saturation resistance R_CEsat. All concepts of the product architecture (chip design, semiconductor substrate resistance, chip metallisation, chip/package connection) were subjected to this requirement. The aim was to achieve saturation resistance of as low as 14 mO in an SMD package. In the second high-speed switching category, in addition to reducing saturation resistance R_CEsat, there was extra focus on faster switching and storage times t_s of the transistor (approx. 140ns) to serve applications at higher frequencies. In this context, it is essential to achieve the right balance and priorities between the specified resistance and switching time. In both categories, importance was attached to packaging the product architecture in standard SMDs (SOT23, SOT457/SC-74, SOT223; SOT89 and SOT96/SO-8). This concept produces a wide variety of standard applications and implementation for the customer, combined with the assurance of being able to supply the products in large quantities for volume applications. To start, NXP is rolling out the products in a 20 to 60V range aimed at applications in communications and automotive electronics, but the technology will also be produced for a larger range of 20 to 100V. Product design The architecture and design of the two product categories (AECQ-101 qualified for automotive applications) take different approaches to implementing the specification, and different priorities are set in the design of the devices. What is important here is to bear in mind which components of the product make which contribution to the resistance and thus to the saturation voltage, and which measures influence which proportions of the switching times. What particularly contributes to the saturation voltage is ohmic voltage drops plus recombination and injection components. The recombination and injection voltages are fairly insignificant in their sum, so the major focus is on the ohmic components. These result primarily from the semiconductor substrate resistance, the chip design, and resistances through the package and interconnect technology. Reduced saturation voltage: low-resistance semiconductor substrates, chip metallisation structures, and chip design The proportion of voltage drops across the semiconductor can be very much reduced by selecting low-resistance substrates such as phosphor- or arsenic-doped substrates, for example (Fig. 1). Also decisive is the current distribution, which should be as homogenous as possible, over the chip volume, and little spreading resistance in the metallisation on the front of the chip. In the case of BISS transistors, a homogeneous current distribution in the chip is achieved by what is called mesh design, which breaks the transistor down into corresponding cell structures. The patented dual-layer metallisation layout of BISS 4 transistors additionally maximises the possible metal thickness of the emitter tracks, thus reducing saturation resistance to a minimum (Fig. 2). Faster switching time: integrated clamping components to reduce diffusion capacitances To reduce switching time in addition to saturation voltage, it is essential to minimise the diffusion capacitances of the transistor in the switching operation. This is implemented by means of integrated parasitic clamping structures (Fig. 3), avoiding overdriving of the transistor in saturation, and in particular reducing the storage time t_s of the transistor. Typical applications Low V_CEsat transistors address general switching applications in which the advantages of bipolar transistors with low R_CEsat values are sought (comparable to the R_DSon of typical mosfets). These include load switches (Fig. 4) in battery-powered devices in which only a small driving voltage is available. The blocking of reverse currents and high energy efficiency are decisive for functionality, however, so bipolar transistors are often preferred to mosfets. The low V_CEsat transistors are used as a charge transistor in the power management unit (PMU) of mobile phones, for instance. The length of operation of notebooks can also be extended by using low V_CEsat transistors in that the losses over the load switch for the fan or the supply of the interfaces are reduced, for example. The product category optimised for switching time in addition to minimal V_CEsat is naturally used in switching applications at higher frequencies (50 to 200 kHz). Switches in PWM applications or switch-mode power supplies are typical of this. An example of the latter is the backlighting of displays in which cold-cathode fluorescent lamps (CCFLs) are used. Different power classes are necessary depending on the size of the supply unit, for which optimal solutions are possible through the large selection of different SMD packages from NXP. Prospects The development of this portfolio of low V_CEsat transistors opens up new areas of application for bipolar transistors, especially in portable devices, because high-efficiency solutions are offered for load and switching functionalities. To satisfy modern device requirements in this segment, there are plans to roll out the technology on new leadless packages of reduced height and demanding little board space. The further development of low V_CEsat bipolar transistors is likely to be well worth watching. Further information on the new portfolio is available here. Author profiles Soenke Habenicht - senior development engineer, NXP Detlef Oelgeschläger - senior development engineer, NXP Bjoern Scheffler - application support manager, NXP