Epi and Schottky Diodes
Power can be efficiently converted from the mains to low voltage, high current, d.c. in the switched mode power supply. The output stages require low voltage, fast diodes to minimize the losses. Reverse recovery times of 30 ns, and reverse voltages of 200 V can be achieved by replacing the double-diffused structure with an epitaxial one. The well-controlled, narrow base width allows low stored charge to be achieved, together with low forward voltage drop. This technology is well established in the range 1 - 70 A. An extension of the technique to higher voltage is being studied, and it appears that it can be used with advantage at 400 V. Alternative lifetime killers to gold may give benefits, e.g. in the hot leakage current. A particularly difficult application is that of antiparallel diode to a power MOS switch, because of the very low forward voltage needed to avoid charge storage occurring in the switch.
For output voltages of < 12 V the forward losses of epi diodes are too great, and the Schottky diode is used. In these devices there is no charge storage, so switching times are only dependent on the capacitance of the device and the impedance in the circuit. The absence of conductivity modulation means that the epitaxial layer introduces a series resistance into the device. This limits the device to thin epi and, hence, low reverse voltage, < 60 V.
The barrier height can be varied by choosing the barrier metal, and values of 0 .5 0.9 eV have been used . It can be shown that values of 0.6 - 0.7 eV are optimum for typical applications. Molybdenum is thus a good choice for the barrier metal. The reverse voltage breakdown of the device can be optimized, with a simple structure, using 2-D computer calculations of the field distribution at the edge of the device. The future requirements of even lower output voltages ('" 2 V), to power advanced logic circuits, may need lower barrier height devices. In addition , higher frequencies may demand lower capacitance. Schottky devices are readily available for 1 to 70 A operation. Together with ep i diodes, there seems to be no basic reason why higher current operation cannot be achieved. However, the need for such devices is not apparent. This is probably because the development of the other components, e . g . power switches and transformers , is lagging behind . Also, the requirement for output currents of > 100 A at low d .c . voltages seems limited .
KeywordsBarrier Height Epitaxial Layer Breakdown Voltage Schottky Diode Base Width
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