Ken Poulton

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 (ADCs). The HBT has current gain of over 100 and f/sub T/ and f/sub 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 ICs 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. >

Thermal design and simulation of bipolar integrated circuits

Keeping device operating temperatures within reasonable limits is necessary for reliability of all ICs and important for achieving the expected performance for many ICs. GaAs heterojunction bipolar transistors (HBTs) offer high speed and good device matching characteristics that are attractive for many high-speed circuits, but they are more susceptible than other IC technologies to the unexpected generation of very high junction temperatures. The reasons for this tendency are discussed, and an HBT sample-and-hold (S/H) circuit that had device temperature rises of over 300 degrees C is described. To address this problem, a new thermal simulation tool called ThCalc was created. ThCalc calculates the temperature profile of an IC and runs fast enough to allow calculations on a whole chip. ThCalc was used to redesign the S/H IC to reduce the largest temperature rise by a factor of 2.7 with a minimal impact on circuit size. >

A 6-bit, 4 GSa/s ADC fabricated in a GaAs HBT process

A GaAs-AlGaAs Heterojunction Bipolar Transistor (HBT) process was developed to meet the speed, gain and yield requirements for Analog to Digital Converters (ADCs). A 6-bit, 4 GSa/s (4 giga-samples per second) ADC was designed and fabricated in this process. The standard HBT used has an emitter area of 1.4/spl times/3.0 /spl mu/m; it has current gain of over 70 at I/sub c/=1 mA and f/sub T/ and f/sub MAX/ of over 50 GHz at I/sub c/=4 mA. The process also includes Schottky diodes, thin-film NiCr resistors, MIM capacitors and three levels of metal interconnect. The ADC uses an analog folding architecture to reduce transistor count and power well below that of a straight 6-bit flash ADC. It includes an on-chip track-and-hold (T/H) circuit and Gray-encoded digital outputs for best immunity to dynamic errors. The ADCs measured differential nonlinearity is less than /spl plusmn/0.5 LSB and its integral nonlinearity is less than /spl plusmn/0.8 LSB. It has a resolution bandwidth (the frequency at which effective bits has dropped by 0.5 bits) of 2.4 GHz at 3 GSa/s and 1.8 GHz at 4 GSa/s, higher than any ADC published to date. The chip operates at up to 6.5 GSa/s, but linearity at that clock rate is much worse.

A 2Gs/s HBT sample and hold

The authors describe a Schottky-diode sample-and-hold (S/H) circuit fabricated in an AlGaAs-GaAs heterojunction bipolar transistor (HBT) process. The transistors exhibit an f/sub T/ of over 50 GHz. The S/H circuit operates at up to 2G samples/s, with distortion below-40 dBc up to and beyond the Nyquist input frequency of 1 GHz.<<ETX>>

Thermal simulation and design of a GaAs HBT sample and hold circuit

Methods which are used to predict and measure device temperatures within an IC are described, and their application to the design of an HBT (heterojunction bipolar transistor) sample and hold circuit (S/H) is discussed. A new thermal simulation tool called ThCalc is also described. ThCalc calculates the temperature profile of an IC and runs fast enough to allow calculations on a whole chip.<<ETX>>

Linearizing a 128 Msample/S ADC

We describe a 128 Msample/s 12-bit analog-to-digital converter IC that has special circuitry on-chip to reduce the nonlinearities that make most ADCs unsuitable for highdynamic-range spectrum analysis and digital receiver applications. Dither, dynamic element matching, and output data scrambling combine to achieve 0.05-LSB DNL.

AlGaAs/GaAs HBTs FOR ANALOG AND DIGITAL APPLICATIONS

AlGaAs/GaAs Heterojunction Bipolar Transistor (HBT) technology has emerged as an important IC technology for high performance electronic systems. Many outstanding circuits have been demonstrated as a result of the AlGaAs/GaAs HBTs high speed, high accuracy and its semi-insulating substrate. Several GaAs HBT manufacturing lines have been established; some of which are shipping products. In this paper, we describe AlGaAs/GaAs HBT technology, summarize some key and representative circuits in analog, A/D conversion and digital applications, and provide prospects of GaAs HBT research.