GaAs and InP-based Heterostructure Bipolar Transistors

The objectives under this program are the investigation of HBT transistor designs suitable for high-frequency operation. Both GaAs and InP-based HBTs are studied for this purpose. The theoretical understanding of the devices is supported by transient and steady-state Monte Carlo approaches, simulations of breakdown and distortion properties. Conventional and special collector HBTs are studied. A self-aligned technology using Reactive-Ion-Etching (RIE) has been developed for GaAs as well as InP-based HBTs. Speed-power characterizations and correlations to the theoretical expectations complete the device understanding. Accomplishments made in these areas are given below.

• Trench technology self-aligned HBTs

• InP HBT process schematic

• Experimental and theoretical demonstration of a collector transit-time reduction using AlGaAs/GaAs and InP/InGaAs HBTs with undoped-collector and inverted field designs. A Monte Carlo analysis was specially developed for this purpose.

• Development of a transient Monte-Carlo technique for studying the switching properties of HBTs Inverted field designs showed best performance in switching.

• Study of the effect of graded Emitter-Base design in InAlAs/InGaAs HBTs. Intermediate (x=0.5) gradings showed cut-off frequencies as high as 270 GHz and 2.2 psec switching times.

• Development of a harmonic-balance based HBT-model for analyzing the power saturation mechanisms. Nonlinearities in transconductance, input and output capacitance were identified. These depend on class of operation and device terminating conditions.

• Establishment of speed-power criteria for GaAs and InP-based HBTs demonstrating the potential limitations of special collector structures for power applications.

• First study and analysis of noise upconversion mechanisms in HBT oscillators. This provides the means of correlating the oscillator noise with 1/f and other types of noise existing in the discrete HBT devices, as well as with its nonlinearities.

• Evaluation of intermodulation characteristics of HBTs by Volterra-series analysis and demonstration of improved IMD3 performance through cancellation of various nonlinear current components at the base-emitter and base collector junctions.

• Demonstration of a hybrid optoelectronic technique for obtaining the small and large signal characteristics of HBTs.

• Development of a direct technique for extracting the equivalent circuit parameters of HBTs in view of providing more physicalinsight to the device characteristics.

• Development of a self-aligned technology for AlGaAs/GaAs and InGaAs/InP HBTs. Special RIE approaches were established for controlled emitter definition and base contacting without severe damage of the dry etched surfaces. Methane-based gases were used for the InP-based devices and submicron ``quantum-wire'' type tests were conducted for the damage studies.

• Demonstration of InP/InGaAs HBTs with a gain of 100 and realization of the first GaInP/GaAs HBTs with a gain of 440 and a collector current density of 150 A/cm^2.

• Identification of low frequency noise sources in AlGaAs/GaAs and InP/InGaAs HBTs. Surface recombination at the base periphery and diffusion contributed to the collector 1/f noise, while G-R noise showed enhanced impact at low base currents.

• Demonstration of a novel self-aligned InP/InGaAs HBT technology using integrated air-bridge technology for reduced emitter pad parasitics.
  fT values of ~35 GHz were obtained using this approach. Significant performance improvement is found by employing this approach in HBT layouts with reduced base-collector capacitance.

• Demonstration of speed-breakdown tradeoffs in InP-based HBTs and evaluation of such characteristics for devices with special collectors and double HBT designs.

• Verification of superior speed-breakdown tradeoff by p-n- collector for InP HBTs as compared with conventional designs.

• Prediction of impact ionization coefficients for a variety of InP-based materials using physical models in view of evaluating their values over a wide range of electric fields as required for electronic device simulations.

• Demonstration of the possibility of obtaining physically significant information on the HBT equivalent circuit characteristics.

• Application of the analytic HBT model extraction technique to newly developed expressions in order to evaluate which factors are limiting the device performance.

• Introduction of ion-implantation techniques for isolation of GaAs-Based HBTs and demonstration of 4.7 reduction of base collector capacitance down to 32 fF over trench isolation technology.

• Development of a cold measurement extraction technique to remove interconnect pad parasitics and allow accurate HBT characterization without the need of additional test structures.

• Development of analytic expressions for HBT gain and application to the explanation of non-idealities in gain roll-off characteristics.

• Demonstration of the impact that the first order zero fZERO in the |H21| characteristic has on current gain distortion and increase of the actual intercept frequency of HBTs. The results showed that GMAX extrapolation significantly underestimates fMAX.

• Demonstration of the better suitability of unilateral gain (U) extrapolation for reliable evaluation of fMAX in HBTs.

• Determination of rapid roll-off of unilateral HBT gain (U) at high frequencies and overestimation of fMAX from extrapolation caused by transitdelay effects. The presence of resonances and gain peaking in U is also shown to distort the characteristics.

• Development of a novel analytic approach to the modeling of velocity overshoot effects in HBT precollectors and their impact on signal delay times and switching speed. Incorporation of the new approach into an equivalent circuitrepresentation and demonstration of the possibility of extending peak velocities and mean free times in an experimental device.

• Evaluation of HBT low frequency noise by direct measurement of the noise spectra and by the indirect technique where the output noise is measured under a series of different base terminations. Demonstration of comparable results for the collector noise spectrum density using both techniques. Errors in the indirect technique appeared, however, to impact the accuracy of base noise spectrum density.

• Demonstration of the 1/f noise origin in self-aligned HBTs due to recombination based mechanisms. Verification of absence of diffusion 1/f noise in the tested devices and confirmation that the HBTs of the study were not at the fundamental noise limits.

• Development of a simplified intrinsic noise model for HBTs and demonstration that the terminal noise is due to at least two intrinsic noise sources: for low bias conditions the noise would only be approximated by one source in the B-E region dominating the base noise spectral density, and one source between C and E dominating the collector spectral density.

• Demonstration of CBE grown GaInP/GaAs HBTs using reduced toxicity, all metalorganic TBA and TBP precursors as As and P sources. DC characteristics (current gain, base, collector ideality factors) of the CBE grown HBT were found to be comparable to those grown by MOCVD with different group V sources. Good microwave characteristics were also demonstrated.

• Demonstration of tradeoff imposed by ledge length on 1/f noise and microwave power gain on AlGaAs/GaAs HBTs. Compared with conventional unpassivated devices, HBTs with 1.1 um ledge length improve the equivalent input base noise current spectral density at 100Hz by as much as 6.5dB and degrade the maximum available gain by 2.4dB at 18 GHz.

• Nonlinear analysis of HBTs by means of Volterra series and device analytical models. The results provided a physical insight to HBT nonlinearities and showed 3rd order nonlinear current cancellation of gbe at the base node.

• Development of a Gummel-Poon large-signal model incorporating self-heating and model implementation in LIBRA for harmonic balance AlGaAs/GaAs HBT analysis.

• Demonstration of a large-signal model for InP/InGaAs HBTs including breakdown effects in the Gummel-Poon formalism.

• Demonstration of large-signal modeling capability of InP-based transimpedance amplifiers and observation of gain and transimpendence degradation at relatively small input power levels.

Technical Developments

Review of Group Activities