Slide 6 of 64
Here we see an OPTICAL micrograph of the Rockwell 50 GHz Baseline transistor and a typical full differential CML current steering circuit. These HBT devices are not submicron despite their high speed! Current enters the CML circuit from the power rail and is simply "steered" through one of the paths in the circuit. Voltage swings are only about 200 mV to turn on or off the differential switch. Differencing of logic signals provides additional noise immunity beyond the current steering strategy (against EMI or coupling for example). The top input level of this CML circuit can switch the output with a delay of only about 20 picoseconds for a FO=FI=1 ring oscillator. Low on resistance of the HBT and associated bias resistors makes wire loading effects less than in CMOS. DC power is lowered by shrinking the emitter and/or lowering Vbe (which is possible in InP/InGaAs graded base HBTs).