Ali Khatibzadeh

Current gain collapse in microwave multifinger heterojunction bipolar transistors operated at very high power densities

The rapid development of heterojunction bipolar transistor (HBT) technologies has led to the demonstration of high power single-chip microwave amplifiers. Because HBTs are operated at high power densities, the ultimate limits on the performance of HBTs are imposed by thermal considerations. The authors address a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phenomenon, referred to as the collapse (of current gain), occurs when suddenly one finger of the HBT draws most of the collector current, leading to an abrupt decrease of current gain. A quantitative model and the condition separating the normal operation region and the collapse are presented. Critical difference of the collapse in the constant l/sub b/ and constant V/sub be/ modes of operation is discussed for the common-emitter l-V characteristics. The collapse in the common-base l-V characteristics and its relationship with avalanche breakdown are also described. A solution to eliminate the collapse is experimentally verified. >

The use of base ballasting to prevent the collapse of current gain in AlGaAs/GaAs heterojunction bipolar transistors

We propose the use of base-ballasting resistance to guarantee absolute thermal stability in AlGaAs/GaAs heterojunction bipolar transistors (HBTs). Base-ballasted HBTs are fabricated and the measured I-V, regression and S-factor characteristics are discussed. We present a numerical model which elucidates the reasons why the base-ballasting scheme is helpful to HBTs but is damaging to silicon bipolar transistors. We compare measured small-signal and large-signal performances of unballasted, emitter-ballasted, and base-ballasted HBTs.

The collapse of current gain in multi-finger heterojunction bipolar transistors: its substrate temperature dependence, instability criteria, and modeling

One undesirable phenomenon observed when AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are operated under high power density is the collapse (of current gain). The collapse manifests itself by a distinct abrupt decrease of collector current in the transistor common-emitter current-voltage (I-V) characteristics. In this investigation, we study the substrate temperature dependence of the collapse. A unified equation is introduced to relate the collapse instability criterion with other thermal instability criteria proposed for silicon bipolar transistors. The effects of the thermal instability on the collapse behavior of 2-finger and 1-finger HBTs are examined. We also present a numerical model to adequately describe the collapse in multi-finger HBTs having arbitrary geometry. The I-V characteristics and regression plots of both ballasted and unballasted HBTs are compared. >

Characterization of bias-stressed carbon-doped GaAs/AlGaAs power heterojunction bipolar transistors

We report on the performance of carbon-doped heterojunction bipolar transistors (HBTs) bias stressed at elevated temperatures. We have determined that in devices without a thin passivating layer of AlGaAs covering the extrinsic base, a tunneling-recombination current that increases in magnitude with the duration of the stress is generated. This current is seen in both the collector and the base at cryogenic temperatures. The variation of this current with temperature is primarily due to carrier freeze-out in the AlGaAs emitter. We hypothesize that this conduction mechanism is related to the generation of midgap traps in the base layer as a result of electron-hole recombination events.<<ETX>>

The role of baseband noise and its upconversion in HBT oscillator phase noise

The phase noise spectral density (#(f/sub m/)) of an 11.02-GHz heterojunction bipolar transistor (HBT) dielectric resonator oscillator (DRO) has been investigated in terms of the HBTs low frequency noise and the oscillators upconversion coefficient. Experimental studies have been used for this purpose and the measured #(f/sub m/) ranged from -89 dBc/Hz to -101 dBc/Hz at a 10-kHz offset frequency (best phase noise spectral density performance was -124 dBc/Hz at 100 kHz). It was shown that, in most test cases, #(f/sub m/) can be described by the upconversion of the HBTs baseband noise. As a result the frequency dependence, of #(f/sub m/), is dictated by the low frequency noise spectrum rather than the upconversion itself. Deviation from pure 1/f frequency dependence found for the HBTs baseband noise at frequencies above 100 Hz resulted in d#(f/sub m/)/d(f/sub m/) deviating from about -30-dB/decade rate. Reduced oscillator phase noise at high collector current is attributed to reduced upconversion in the oscillator. >

High-efficiency X-band HBT power amplifier

We report on the state-of-the-art performance of monolithic HBT amplifiers at X-band. Single-chip, two-stage amplifiers have been designed and fabricated using AlGaAs/GaAs HBT process. An output power level of 12.5 W, with 51% power-added efficiency and 13 dB associated gain have been achieved at 8.5 GHz. The amplifier delivers more than 10 W output power with minimum 41% PAE in the 8.3-9.5 GHz band, and 9 W power with minimum 38% PAE in the 8.3-10.0 GHz band. The amplifier measures 4.5/spl times/4.5 mm/sup 2/ in size and is thermally ballasted for reliable operation. To our knowledge, these results represent state-of-the-art performance in terms of the combination of power, bandwidth, and efficiency for any monolithic solid-state amplifier technology.<<ETX>>

Room Temperature Hot Electron Transistors with InAs-Notched Resonant-Tunneling-Diode Injector

Room temperature operation is achieved in In(GaAl)As/InGaAs resonant-tunneling hot electron transistors (RHET) grown by molecular beam epitaxy on InP substrates. RHETs with base widths of 10, 40 and 60 nm are fabricated and all exhibit room temperature dc current gain greater than 2, with gain as high as 12 observed at resonance in the 40 nm base device. To our knowledge these are the first In(GaAl)As hot electron transistors to exhibit 300 K gain of this magnitude. In addition, the transistors also exhibit strong negative transconductance and a unique negative peak-to-valley current ratio. S-parameter measurements of the 40(60) nm base RHET give values for fT and fMAX of 67(54) and 41(11) GHz respectively.

Low frequency noise characteristics of self-aligned AlGaAs/GaAs power heterojunction bipolar transistors

The low frequency noise characteristics of modern self-aligned AlGaAs/GaAs power HBTs have been studied as a function of bias, temperature, frequency, and circuit topology. The devices have a 1/f/sup /spl gamma// behavior between 10 Hz and 100 Hz with 0.78/spl les//spl gamma//spl les/1.65. Strong deviation from 1/f/sup /spl gamma// is measured at higher frequencies due to trapping. The bias dependence of the collector noise ranged from I/sub C//sup 1.5/-I/sub C//sup 2.6/, while that for the base noise ranges from I/sub B//sup 0.7/-I/sub B//sup 2.5/. In all cases the collector noise is greater than the base noise. The base noise is apparently dominated by surface recombination noise and generation-recombination (G-R) noise. The collector noise is due to recombination mechanisms and G-R noise. The activation energy (E/sub a/) of the most significant trap is approximately 0.58 eV. The noise of the devices tested was found to be dominated by material and fabrication related mechanisms and not by fundamental mechanisms. >

Tradeoff between 1/f noise and microwave performance in AlGaAs/GaAs heterojunction bipolar transistors

The 1/f noise characteristics and microwave gain of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) have been investigated as a function of surface passivation ledge length. These measurements clearly demonstrate that the ledge length imposes a tradeoff between the 1/f noise and microwave power gain performance. Compared to a conventional unpassivated self-aligned HBT, HBTs with 0.4 and 1.1 /spl mu/m ledge lengths improve the equivalent input base noise current spectral density at 100 Hz by as much as 2 dB and 6.5 dB, respectively; while degrading the maximum available gain at 18 GHz by 0.3 dB and 2.4 dB, respectively. >

High linearity power X-band GaInP/GaAs heterojunction bipolar transistor

We report for the first time two-tone test results measured on a GaInP/GaAs HBT. A 2/spl times/400 /spl mu/m/sup 2/ device delivered more than 1.3 W under one-tone testing at 7.5 GHz and output more than 1 W under two-tone. The corresponding intermodulation product is -14 dBc, and decreases to -21 and -36 dBc, respectively, at 3-dB and 10-dB back-off from the saturated two-tone output. These results demonstrate that GaInP/GaAs HBTs are suitable for microwave transmitter applications.<<ETX>>