A 6-B, 4 GSA/S GAAS HBT ADC

A GaAs/AlGaAs Heterojunction Bipolar Transistor (HBT) process was developed to meet the speed, gain, and yield requirements for analog-to-digital 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 folding 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 noninterleaved 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 GHz 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 temperatures, This led to reliability problems that were overcome by the use of a fast dc thermal simulator written for this project. A SPICE model for self-heating effects is also described.