A GaAs/AlGaAs Heterojunction Bipolar Transistor (HBT) process was deve
loped to meet the speed, gain, and yield requirements for analog-to-di
gital converters (ADC's). The HBT has current gain of over 100 and f(T
) and f(MAX) of over 50 GHz, A 6-b, 4 GSa/s (4 giga-samples/s) ADC was
designed and fabricated in this process, The ADC uses an analog foldi
ng architecture, includes an on chip master-slave track-and-hold (T/H)
circuit, and provides Gray-encoded digital outputs, The ADC achieves
5.6 effective bits at 4 GSa/s, a faster clock rate than any noninterle
aved semiconductor ADC reported to date, It has a resolution bandwidth
(the frequency at which effective bits has dropped by 0.5 b) of 1.8 G
Hz at 4 GSa/s, higher than any published ADC, The chip operates at up
to 6.5 GSa/s, GaAs HBT IC's are especially prone to high operating tem
peratures, This led to reliability problems that were overcome by the
use of a fast dc thermal simulator written for this project. A SPICE m
odel for self-heating effects is also described.