G.J. Sullivan

AlGaAs/GaAs heterojunction bipolar transistors fabricated using a self-aligned dual-lift-off process

This paper describes a self-aligned heterojunction-bipolar-transistor (HBT) process based on a simple dual-lift-off method. Transistors with emitter width down to 1.2 µm and base doping up to 1 × 1020/cm3have been fabricated. Extrapolated current gain cutoff frequency ftof 55 GHz and maximum frequency of oscillationf_{\max}of 105 GHz have been obtained. Current-mode-logic (CML) ring oscillators with propagation delays as low as 14.2 ps have been demonstrated. These are record performance results for bipolar transistors. The dual-lift-off process is promising for both millimeter-wave devices and large-scale integrated circuit fabrication.

Heterojunction Bipolar Transistors for Microwave and Millimeter-Wave Integrated Circuits

This paper reviews the present status of GaAIAs/ GaAs HBT technology and projects the impact of these devices on microwave and millimeter-wave integrated circuits. Devices with f/sub max/ above 100 GHz are described. Differential amplifiers are shown to have offset voltages with standard deviation below 2 mV and voltage gain of 200 per stage. Breakdown voltages (BV/sub CBO/) above 20 V are demonstrated. Frequency dividers operating above 20 GHz are described.

GaAs-based heterojunction bipolar transistors for very high performance electronic circuits

This paper reviews the principles and status of AlGaAs/GaAs heterojunction bipolar transistor technology. Comparisons of this technology with Si bipolar transistor and GaAs field-effect transistor technologies are made. Epitaxial materials, fabrication processes, transistor DC and RF characteristics, and modeling of AlGaAs/GaAs HBTs are described. Key areas of HBT application are also highlighted. >

A 20-GHz frequency divider implemented with heterojunction bipolar transistors

This paper reports a high-speed frequency divider implemented with AlGaAs/InGaAs/GaAs heterojunction bipolar transistors (HBTs). The divide-by-four static frequency divider was fabricated with a fully self-aligned dual-lift-off HBT process. A maximum operating frequency of 20.1 GHz was achieved. This is the highest frequency ever reported for static frequency dividers.

High power GaAlAs/GaAs HBTs for microwave applications

This paper reports the attainment of high microwave output, up to 0.4 W cw at 10 GHz, with GaAlAs/ GaAs heterojunction bipolar transistors. In addition to high power, the HBTs displayed excellent power-added efficiency (4896) and power gain (7 dB). A key factor in obtaining these high powers and efficiencies is the ability to support high collector-emitter voltages without breakdown. Breakdown voltage was up to 23 V (BVcbo) in the devices reported here. The experimental data are in good agreement with a theoretical model of I-V characteristics near breakdown. The cutoff frequency ftwas found to vary with Vceas expected for electron-drift at a saturation veloclty of 1 × 107cm/s across the base-collector depletion region.

A high-efficiency HBT MMIC power amplifier

An AlGaAs/GaAs heterojunction bipolar transistor monolithic microwave IC (HBT MMIC) power amplifier is developed that demonstrates very high power-added efficiency, high gain, and broad bandwidth. It uses a cascode structure with four 200- mu m common-emitter HBT cells driving four common-base cells of the same size. This amplifier achieves over 14-dB gain from 6 to 10 GHz, with a peak power-added efficiency (PAE) of 47% at 7.5 GHz at an output power level of 31 dBm. This corresponds to a power density of over 3 W per millimeter of emitter length. Input and output matching networks, as well as biasing networks, are all contained within the chip, which measures 80*80 mils (2*2 mm).<<ETX>>

Heating effects on the accuracy of HBT voltage comparators

Voltage comparators implemented with GaAs/(GaAl)As heterojunction bipolar transistors (HBTs) were examined for dynamic hysteresis effects. Heating effects were identified as the major source of hysteresis. An approximate model is proposed to explain the measured data. Suggestions for analog-to-digital converter design are discussed.

IVA-1 self-aligned AlGaAs/GaAs heterojunction bipolar transistors with improved high-speed performance

This paper presents the high-speed performance of HBT’s fabricated by a self-aligned dual-liftoff method. Transistors with emitter width down to 1.2 pm and base doping up to 1 X lOZ0/cm3 have been fabricated. Extrapolated current gain cutoff frequency ft of 67 GHz and maximum frequency of oscillationf,,, of 105 GHz have been obtained. Current-mode logic (CML) ring oscillators with propagation delays as low as 14.2 ps have been demonstrated. Frequency dividers (1/4) have operated up to 20.1 GHz. This is believed the highest operating frequency ever reported for semiconductor frequency dividers. MBE is used to form the HBT layer structure on a semi-insulating undoped LEC GaAs substrate. After device isolation, a single photoresist pattern is used to define an etch-down to the base layer of a HBT structure, thereby establishing the active emitter area. The same photoresist is used in masking the proton implantation for base-collector capacitance reduction and the base metal evaporation. Finally, the photoresist is used again to mask a low-temperature-deposited dielectric. The base metal and the dielectric are patterned by liftoff as the photoresist is dissolved. The resultant structure has base metal in base contact areas overlaid with a dielectric which protects both the metal and the sidewalls. This process is termed “dual liftoff” because unwanted base metal and dielectric are lifted off simultaneously. Following the liftoff, one can use noncritical alignment to define an emitter contact pattern because the emitter metal can overlap the covered extrinsic base area or the base contact area without concern for emitter-base shorting. Contact to the collector is made by etching a via hole through the uppermost layers down to the buried subcollector. Resistors are formed by depositing a nichrome film on the GaAs surface, and two levels of interconnect metal are provided. All lithographic work was carried out with an optical contact aligner. Dual-liftoff HBT’s have shown low device parasitics, high transconductance and high cutoff frequency. With a base layer doping of 1 X lOZ0/cm3, the base contact resistance is 7 X lo-’ D . cm’. The emitter contact resistance is on the order of 5 X Q 1 cmz. In the smallest transistor, with emitter dimensions of 1.2 pm X 3 pm, the measured peak transconductance is 35 ms. The current gain of these devices varies with device size. A current gain of 55 has been obtained for larger devices (emitter dimensions: 70 pm X 70 pm). For the smallest device, the current gain is 10-20. Microwave performance of HBT’s has been characterized by Sparameter measurements with Cascade Microtech probes. HBT’s with three emitter fingers of 1.2 pm X 9 pm dimensions were tested. By assuming a 6-dB/octave rolloff from the actually measured gain value at 26 GHz, f, was extrapolated to be 67 GHz, and fmaX was as high as 105 GHz. A 19-stage CML ring oscillator with emitter dimensions of 1.2 pm X 2.5 pm showed propagation delays down to 14.2 ps/gate. The l /4-frequency divider was configured from two cascaded 1 /2-stages based on master slave flipflops. The dividing operation was correct up to 20.1 GHz with wafer-probe testing. The total on-chip power consumption of the divider was 140 mW. To our knowledge, thef,, fmax, and the speed of IC’s for the CML ring oscillator and the frequency divider are the highest values ever reported for bipolar transistors.

A high-speed 1-kbit high electron mobility transistor static RAM

A 1-kbit static RAM with enhancement and depletion-mode devices was designed and fabricated using the high electron mobility transistor (HEMT) technology. The RAM circuit was optimized to achieve ultra-high-speed performance. A subnanosecond address access time of 0.6 ns was measured at room temperature for a total power dissipation of 450 mW. The minimum WRITE-ENABLE pulse width required to change the state of memory cell is less than 2 ns on probe testing. The best chip has 3 bits that failed to function, which corresponds to a bit yield of 99.7 percent. According to the simulation, variations of the threshold voltage inside the memory cell greatly reduce its stable functional range. High-speed operation requires more uniform threshold voltage control to achieve fully operational LSI memory circuits.

AlGaAs/GaAs pnp HBTs with high fmax and ft

AlGaAs/GaAs Pnp HBTs have the potential for high frequency performance approaching that of Npn HBTs. To achieve this performance, it is necessary to dope the base as heavily n-type as possible. This heavy base doping results in large degeneracy in the base, which reduces the heterobarrier to reverse injection of electrons from the base into the emitter. High A1 content in the emitter is desirable to maintain good injection efficiency. Incorporating a gradient in the base doping can introduce fields to sweep injected holes across the neutral base region, which reduces base transport time. DC and RF characteristics of Pnp HBTs with 40% and 75% Al in the emitter will be presented. ft of 17 GHz and fmax of 39 GHz has been achieved in 2 ?m x 11 ?m HBTs fabricated using a self-aligned ohmic contact process. Further improvement in performance should be possible.