4.5GHz frequency dividers using GaAs/(Ga,AI) as heterojunction bipolar transistors

Abstract

THIS PAPER WILL DESCRIBE the design of frequency dividers implemented with GaAs/(GaAl)As heterojunction bipolar transistors (HBTs) in an emitter-coupled logic circuit configuration. Frequency division (+ 2 and + 4) was obtained with input frequencies as high as 4.5GHz, using devices with emitter dimensions of 1 . 6 ~ x 5 ~ . Interest in GaAs/(GaAl)As HBTs derives from their potential for higher cutoff frequency, lower base resistance and lower emitter-base capacitance than obtained with Si bipolar transistors'. At the same time, they are expected to provide higher current drive capability, higher transconductance, and lower sensitivity to process parameters than GaAs FETs'. I2L circuits employing HBTs have been described'. Recently, prototype ECL ringoscillators using HBTs have also been reported3. The ECL approach is oriented towards high frequency operation. At the same time, its differential structure reduces circuit sensitivity to supply voltage, temperature and device variations, and its constant supply current characteristic reduces noise generation due to parasitic inductances during high frequency operation. The divider circuits were based on master-slave flipflops constructed from D-latches of the type shown in Figure 1. Divide-by-2 operation was obtained by feeding the M/S flipflop output back to the D input. Divide-by-4 operation was obtained by combining two divide-by-2 sections on the same chip, with the output of the first providing the clock input of the second (after an emitter-follower stage for buffering and level-shifting). Both types of dividers were provided with output buffer amplifiers and emitter-follower output drivers. Divide-by-4 circuits utilized 32 transistors. A microphotograph of a completed chip is shown in Figure 2. Exclusive of pads, the chip occupies an area of 2 1 0 p x 450W. The structure of the transistors is shown schematically in Figure 3. MBE growth was utilized to deposit the device layers on a semi-insulating GaAs substrate. Base layers approximately 1000Athick doped to 3x1018,-3 with Be were utilized. Implanted Be was also used to make contact to the base. Device isolation was provided by boron bombardment. Resistors were made with evaporated Ni-Cr. Further details of the process have been reported3. Transistors (with the exception of output drivers) had emitter dimensions of 1 . 6 ~ x 5m and emitter-base contact separations of 1.6/un. The maximum current of the devices is 6-8mA. -