Power HBT Research Articles

Characterization and modeling the effect of temperature on power HBTs InGaP/GaAs

The variation and stability of HBT’s parameters at different temperatures are important for utilizing these devices in high-power integrated circuits. The temperature dependence of the DC current gain of bipolar transistors, as a key device parameter, has been extensively investigated. A major issue of the power HBT’s is that the current gain is decreased with junction temperature due to self-heating effect. Hence, how to stabilize the DC current gain and RF performances is important issue to develop the power HBTs. This work describes the DC and high-frequency temperature dependence of InGaP/GaAs HBT’s. The substrate temperature (T) was varied from 25 to 150°C. The static and dynamic performances of the HBT are degraded at high temperature, due to the reduced of carrier mobility with increasing temperature. The current gain (β) decreases at high temperatures; from 140 to 127 at 25 to 150°C, while the decreases in the peak Ft and Fmax are observed from about 110 GHz to 68 GHz and from 165 GHz to 53 GHz respectively in the temperature range of 25 to 150°C.

Emitter ballasting resistor design of InGaP/GaAs power HBT for high linearity based on Volterra Series distortion analysis

A distortion analysis of InGaP/GaAs HBT is presented based on Volterra Series. The interdependence between emitter ballasting resistor and linearity has been quantitatively analyzed. An optimized emitter ballasted power amplifier (PA) is designed and fabricated. Third-order intermodulation (IM3) of the PA is suppressed by 3–8 dB without sacrificing power gain and PAE. The theoretical calculation and measurement results show the optimization of emitter ballasting resistor is an alternative technique to improve PAs' linearity.

Novel Emitter-Segmented Power HBT with 2-D Non-Uniform Segment Spacing for High Thermal Stability

With the aid of the thermal model, thermal resistance matrix of emitter-segmented power HBT is proposed to represent thermal effects. The effect of 2-dimensional (2-D) inter-segment spacing on thermal stability of device is studied. It is shown that the increase of inter-segment spacing could effectively decrease thermal coupling resistance, lower temperature of center segments, and hence improves the thermal stability. Furthermore, a novel emitter-segmented power HBT with 2-D non-uniform segment spacing is proposed, in which the non-uniformity of segment temperature is improved by 75.26% and the maximum power level difference of emitter segment is improved by 55.84%, when compared with uniform segment spacing HBT. Therefore the technique of 2-D non-uniform segment spacing is a better method for enhancing the thermal stability of power HBTs.

Structure optimization of multi-finger power SiGe HBTs for thermal stability improvement

The two-dimensional temperature profile of a power SiGe HBT with traditional uniform emitter finger spacing is calculated, which shows that there is a higher temperature in the central region of the device. With the aid of the theoretical analysis, an optimized structure of the HBT with non-uniform emitter finger spacing is presented. The peak temperature is lowered by 23.82 K, and the thermal resistance is also improved by 15.09% compared with that of the uniform one. The improvements above are ascribed to the increasing the spacing between fingers, and hence suppressing the heat flow from adjacent fingers to the center finger. Based on the analytical results, two types of HBTs with uniform emitter finger spacing and non-uniform emitter finger spacing are fabricated and their temperature profiles and thermal resistance are measured. The measured results agree well with the calculated results, verifying the accuracy of the calculations. For the HBT with non-uniform emitter finger spacing, the peak temperature and the thermal resistance are improved markedly over a wide biasing range compared with that of the uniform one. Therefore, both the calculated results and the experimental results verify that the optimized structure of power HBT with non-uniform emitter finger spacing is superior to the uniform emitter spacing structure for enhancing the thermal stability of power devices over a wide biasing range.

Large Signal Evaluation of Nonlinear HBT Model

The performance of recently developed Large Signal (LS) HBT model was evaluated with extensive LS measurements like Power spectrum, Load pull and Inter-modulation investigations. Proposed model has adopted temperature dependent leakage resistance and a simplified capacitance models. The model was implemented in ADS as SDD. Important feature of the model is that the main model parameters are taken directly from measurements in rather simple and understandable way. Results show good accuracy despite the simplicity of the model. To our knowledge the HBT model is one of a few HBT models which can handle high current & Power HBT devices, with significantly less model parameters with good accuracy.

Improved SiGe power HBT characteristics by emitter layout

A new emitter finger layout in SiGe HBT to reduce thermal-coupling is presented in this study. By redistributing the emitter fingers of various SiGe HBTs, the thermal resistance reduces significantly from 133 to 88K/W under room temperature. Thus, in dc performance, output conductance increases from −2.0 to −1.6mA/V. In ac performance, the fMAX shows an improvement over 10%. In power performance, the linear power gain, P1dB and PAEatP1dB increase by 0.5dB, 1.5dB and 6.2% at 1.8GHz, respectively.

Efficiency Improvement of HBT Class E Power Amplifier by Tuning-out Input Capacitance

This paper demonstrates an efficiency improvement of the class E power amplifier (PA) by tuning-out the input capacitance (CSUBIN/SUB) of the power HBT with a shunt inductance. In order to obtain high output power, the PA needs the large emitter size of a transistor. The larger the emitter size, the higher the parasitic capacitance. The parasitic CIN affects the distortion of the voltage signal at the base node and changes the duty cycle to decrease the PA's efficiency. Adopting the L-C resonance, we obtain a remarkable efficiency improvement of as much as 7%. This PA exhibits output power of 29 ㏈m and collector efficiency of 71 % at 1.9 ㎓.

High power density, high efficiency 1 W SiGe power HBT for 2.4 GHz power amplifier applications

In this paper, an improved layout and thermal management of eight unit-cells SiGe power HBT with emitter area of 8 × 0.6 × 10 μm 2 were designed for high power density and efficiency performance. The on-wafer power characteristics were measured using an ATN load-pull system under class-AB operation at 2.4 GHz. The power HBT achieved a 1-dB compression power ( P −1 dB ) of 27.3 dBm and a saturation output power ( P sat) of 30 dBm which was correspond to a power density of 2.6 mW/μm 2 for the emitter area. A high peak power added efficiency (PAE max) of up to 75% was obtained, with a power gain of 11.4 dB at a P 3-dB of 29.0 dBm. In addition, the real part of the source impedance ( R in) was measured to be as high as 28 Ω. The impedance transfer ratio, R in/ R System is only 0.56 which relaxes the need for a high quality passive component (inductor) for on-chip input matching. This advantage makes it easier for the HBT to be integrated with other silicon-based transceiver in an RF System-on-Chip (SoC) design.

Thermalelectro-feedback model for multi-emitter finger power heterojunction bipolar transistor Part II: current crush phenomenon

Current crush phenomenon, a heat-related mechanism that is commonly observed in experimental I–V characteristic of power HBT, is explained favourably by the thermoelectro-feedback model. A feasible method with proper device geometry or layout to suppress this undesirable phenomenon is provided and experimentally verified.

High power X-band (8.4 GHz) SiGe/Si heterojunction bipolar transistor

A high-performance double-mesa type Si/SiGe/Si power HBT at X-band (8.4 GHz) frequency is demonstrated. Under continuous wave operation, a single 20-finger common-base Si/Si0.75Ge0.25/Si (2 × 10 µm2 of each emitter finger) HBT has an output power of 27.4 dBm and an associated power gain of 7 dB at peak PAE of 32.3%. These parametric values represent the state-of-the-art power performance of SiGe-based HBTs.

Multiharmonic generators for relative phase calibration of nonlinear network analyzers

Multiharmonic generators are of interest for determining coupler phase dispersion of nonlinear network analyzers inherently dealing with nonsinusoidal waveforms for RF current-voltage (I-V) measurements. Supported by simple waveform analysis, two on-wafer devices are experimentally evaluated: a single diode and a nonlinear transmission line (NLTL). The investigation uses an active harmonic load-pull system with 2 GHz fundamental frequency. It turns out that the NLTL is well suited as a reference generator. We present measurement examples of phase dispersion for different directional couplers up to 18 GHz and the application to RF I-V waveforms of a 1-W power HBT with harmonic load tuning.

Simulation study of on-state losses as function of carrier life-time for a GaN/SiC high power HBT design

SiC has several properties that makes it more suitable than silicon for high power devices. One problem with SiC bipolar devices is the short carrier life times, and this problem becomes more severe when designing devices for high voltage applications since the dimensions are larger. This work investigates how the Shockley-Read-Hall lifetime influences the on-state characteristics of a HBT or BJT switch in 4H-SiC. The on-state characteristics were simulated with varying SRH lifetimes in the base and drift region. Comparisons were made at 100 A/cm2 collector current density, JC, and at the base current density, JB, where the total on-state power loss of the design is at minimum. The SRH lifetime in the drift region is the dominant parameter for on-state performance, whereas the SRH lifetime in the base is of much less importance. The simulations showed that to reach an acceptable JC/JB-ratio of 100 at power minimum a SRH lifetime of at least 100 µs in the drift region was needed for the HBT design. This lifetime is far from the experimental values reported for 4H-SiC. The advantages of the heterojunction in comparison to ordinary BJTs decreases with shorter SRH lifetimes, but an improvement could always be seen.

SiGe power HBT's for low-voltage, high-performance RF applications

Silicon-Germanium (SiGe) power heterojunction bipolar transistors (HBT's) are fabricated by using two or ten device unit cells with an emitter area of 5/spl times/0.5/spl times/16.5 /spl mu/m/sup 2/ each. The large power transistor features 1 W RF output power at 3-dB gain compression, 3.5 V bias, and 2.4 GHz with a maximum power-added-efficiency (PAE) of 48% for class A/B operation. At a supply voltage of 1.5 V, the transistor delivers a 3-dB RF output power of 150 mW with a PAE of 47%. It is shown that a high collector doping level is advantageous for low-voltage operation. Further, by using special bias sense ports, the interconnect losses are found to degrade the device performance to a considerable degree.

Transient analysis of collector current collapse in multifinger HBT's

The authors report for the first time a time-domain analysis of thermal instability in multifinger heterojunction bipolar transistors (HBT's). This is based on a transient quasi-three-dimensional (3-D) electrothermal model that selfconsistently solves the thermal and electrical equations. This model is designed to evaluate the thermal time constant of GaAs-based power HBT's employing emitter thermal shunt and emitter ballast resistance.

Self-aligned emitter power HBT and self-aligned gate power HFET for low/unity supply voltage operation in PHS handsets

A new power HBT and HFET were developed for low unity supply voltage operation in PHS handsets. The emitter region, the emitter electrode, the buried collector and the base electrodes in the power HBT are formed using the emitter electrode self-alignment process in order to reduce parasitic resistance and capacitance. The wirings on each electrode of the HBT are formed by Au plating technique for high current operation. The gate electrode in the power HFET is self-aligned to the drain/source electrodes by using the drain/source contact mesas as a mask, where the distance between the drain and the source is minimized and the parasitic resistances are reduced. In addition, an asymmetrical double-doped structure of AlGaAs/GaAs/InGaAs/AlGaAs is applied to the HFET in order to obtain a high current density. Both the power HBT and HFET exhibited the knee voltage less than 1 V with the maximum current more than 500 mA. The power HBT performed a power gain of 14.2 dB, an efficiency of 33.8% and the power HFET performed 12.5 dB and 34.5%, with a sufficient margin of distortion for PHS standard at an output power of 22 dBm, a supply voltage of 3.5 V and a frequency of 1.9 GHz under the unity operation.

HBT power devices and circuits

The status of the power HBT technology is reviewed. The advances made in the electrical and thermal design of unit-cells are examined to identify the present and near future applications in microwave power amplifications. The current status of the technology is such that AlGaAs/GaAs HBTs have become suitable for most microwave and especially for commercial L/S-band frequency applications. The performance is not yet adequate for mm-wave applications, which may require alternative material technologies.

Optimising a GaAs HBT amplifier for maximum gain efficiency

The authors report a GaAs HBT amplifier which achieves a gain efficiency of 2.14 dB/mW at 17 GHz which is believed to be among the highest reported for a self-biased MMIC in this frequency range. The amplifier's optimum gain bias was determined by finding the maxima of a simple bipolar gain-bias function which involves the graphical solution of a transcendental equation. This design calculation was experimentally verified for a four-stage Ku-band HBT amplifier which achieves 11.94 dB gain at 17 GHz while consuming only 5.64 mW of DC power through a 1.88 V supply. This method for optimising the gain efficiency can be applied to the design of ultra-low DC power HBT gain blocks for communication and radio-astronomy applications.

Degradation of an X-K/sub u/ band GaAs/AlGaAs power HBT MMIC under RF stress

This paper summarizes the observed degradation in the performance of a high-efficiency X-K/sub u/ band 1-W HBT power MMIC operating under /spl sim/2 dB compression for an extended period. The main finding of this study is that no new degradation mechanisms appeared, even under severe RF stress; the device degraded in the same manner as with DC stress only. The only significant change in device characteristics was increased base leakage current that resulted in a monotonic reduction in current gain after an initial period of stability, Output power of the MMIC remained essentially unchanged even after current gain had dropped to 60% of its initial value, RF properties of the device, both small and large signal, showed little change even after severe deterioration of DC characteristics. The results of this study suggest that HBT reliability can be effectively evaluated for most applications by applying only DC stress to the device.

A new nonlinear dc model for HBT and mesfet power devices

AbstractA new nonlinear empirical model that can be applied to the collector current source of power HBT devices and to the drain current source of power MESFETs is presented. The model includes the self‐heating effects present in both HBT and MESFET devices. It also accurately predicts the slope variation of the dc‐I‐V characteristics from positive to negative, which occurs in MESFET devices. The proposed model is validated with the measured dc and small‐signal characteristics of one HBT and one MESFET medium‐power device. Good agreement between measurement and simulation results was obtained for both devices. © 1995 John Wiley & Sons, Inc.

1 W Ku-band AlGaAs/GaAs power HBT's with 72% peak power-added efficiency

High power and high-efficiency multi-finger heterojunction bipolar transistors (HBT's) have been successfully realized at Ku-band by using an optimum emitter ballasting resistor and a plated heat sink (PHS) structure. Output power of 1 W with power-added efficiency (PAE) of 72% at 12 GHz has been achieved from a 10-finger HBT with the total emitter size of 300 /spl mu/m/sup 2/. 72% PAE with the output power density of 5.0 W/mm is the best performance ever reported for solid-state power devices with output powers more than 1 W at Ku-band.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>