IVA-2 high-speed performance of AlGaAs/GaAs heterojunction bipolar transistors with nonalloyed emitter contacts

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.