Metal-semiconductor junctions, called Schottky diodes, can be designed for such a low-barrier potential. The traditional point-contact diode formed a metal-semiconductor or Schottky junction. Due to the way they are built, however, point-contact diodes are inherently fragile: mechanically due to whisker movement, and electrically due 10 the very small junction area resulting in high current densities and local hot spots. Besides the fragile nature of point contacts, origin resistance Ro and junction capacitance Co are quite variable from diode to diode. The junction capacitance and other parasitic reactances must be kept small in order to achieve frequency independent performance. 

Junction diodes of the low-barrier Schottky variety and with low enough junction capacitance for good performance to 18 GHz can be built with photometal-lurgical techniques (Figure 4-2). A thin, lightly-doped epitaxial layer is grown atop a heavily-doped, monocrystalline silicon substrate. The surface of the epitaxial layer is oxidized to form a protective, insulating layer called the passivation layer. A hole is then etched in the oxide, and a suitable metal is deposited to form a junction with the epitaxial layer. The result is a Schottky diode. The choice of metal controls the barrier height and therefore the high saturation current and low origin resistance so important for good performance. A low capacitance of about 0.1 pF is achieved by keeping the 

metal-semiconductor junction small and by etching the chip into a mesa-type shape. 
Such a low-barrier Schottky diode (LBSD) is included in the HP 8484A Power Sensor which covers the 10 MHz to 18 GHz frequency range and can measure power from -20 dBm (10 pW) to -70 dBm (100 pW). To achieve such frequency independent performance, other rf circuit elements must be included to compensate for the inevitable parasitic reactances associated with the diode, its connections, and the dc circuitry. 
The equivalent circuit for the rf portion of the HP 8484A Power Sensor is shown in Figure 4-3. The diode chip itself has three elements: Ro is the origin resistance of the junction; Co is the junction capacitance; and Rb is the bulk resistance of the semiconductor material. Cb is the bypass capacitor used also as a low-pass filter to integrate the current through the diode. Lw is the inductance associated with the bonding wires to the diode. Resistor R is about 30 ohms and intentionally added to increase the loss of the resonance of Lw with the capacitors. This loss makes the resonance of such low Q that it is not noticeable. Resistor RL is the terminating resistance for the transmission line. L is actually a short section of transmission line that compensates for other reactances to yield an overall flat performance with frequency. The rf circuitry, including the 50 ohm termination, is constructed on a sapphire substrate using thin-film techniques. 
Diode Mount for Sensing Power 
The advantage of the low-barrier Schottky diode over thermocouple power sensors, is that the detecting diode is some 3000 times more efficient in converting rf power to dc. In measuring power levels of 100 picowatts (-70 dBm) the diode detector output is about 50 nanovolts. This low signal level requires careful design to prevent leakage signals and thermocouple effects from dominating the desired signal (see Figure 4-4). The HP 8484A Power Sensor has gold leads from the diode to a chopper-amplifier. The chopper-amplifier, which presents a 5 K ohm load impedance to the diode, is somewhat different than for the thermocouple sensors, even though the power meter itself is the same for both types of sensors. 
The major thermal drift mechanism, the one that dominated the design of the diode mount, is the thermal emf generated by any temperature gradient that appears across the diode. The mechanical design of the 8484A (Figure 4-4) minimized this gradient by insulating the diode package and arranging the thermal paths so the primary heat source is at the rf input connector. Any