![]() Also, don't forget that bipolar transistors block voltage in two directions, as specified by their BV EBO or BV ECO characteristic. A similarly rated MOSFET has such high specific on-resistance and poor current capabilities that it requires packages such as D-PAK to accommodate the necessary silicon area. For example, the 450V-rated FMMT459 NPN transistor has a current capability of 150mA and a typical R CE(sat) of just 1.4ïï that allows SOT-23 packaging. The difference in the exponent value in these formulae emphasizes the specific-area-resistance advantage that the bipolar transistors hold over MOSFETs as breakdown voltages increase. Breakdown voltage versus R CE(sat) for the Zetex third generation transistor series As is demonstrated in the straight-line plot in Figure 2, the relationship between breakdown voltage and collector-emitter saturation voltage for the Zetex third generation transistor series yields:įigure 2. ![]() MOSFETs don't possess any similar conductivity modulation mechanism, which accounts for one of the bipolar transistor's advantages. Furthermore, bipolar transistors benefit from conductivity modulation of the resistive collector region when operated as a saturated switch, significantly reducing the R CE(sat). By optimizing process technology and chip layout, voltage biasing and current flow is evenly distributed across the chip area to maximize silicon efficiency. Under the correct drive conditions, it's worth noting that bipolar transistors have matched or bettered MOSFETs in terms of die-area-specific on-state resistance (see Figure 1). Higher voltage MOSFETs also suffer from the high resistance of the lightly doped drain region so on-resistance typically increases proportionally with breakdown voltage: The results are spectacular – especially at low breakdown voltages – but current flow remains concentrated in narrow channel regions. Trench MOSFETs give lower on-resistance by allowing greater channel density. The first characteristic that designers most often consider is on-state resistance for a given breakdown voltage. It's therefore important to review the characteristics and benefits of each technology to extract the best system performance. Meanwhile, developments in bipolar transistors continue to position the technology to rival or exceed MOSFET performance in many applications. In recent years, MOSFETs have benefited from huge investments in technology and promotion alike, overshadowing bipolar devices to the degree that many designers view bipolars as old technology.
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