Y.A. Tkachenko

Analysis and experimental waveform study on inverse class class-F mode of microwave power FETs

The new inverse class F operation mode for high-efficiency power amplifiers is analyzed. Unlike regular class F, it requires an open circuit termination at the second harmonic and a small impedance termination at the third harmonic. The inverse class F features higher PAE than class F but requires transistors with higher breakdown voltages. A study performed using the waveform measurement technique in conjunction with the active/passive load-pull system showed PAE=83% for the inverse class F compared to PAE=64% for the class F. The measured results are in good agreement with the analytical prediction.

Performance Comparison of HBTs and Quasi E-mode PHEMTs for Single Supply High Efficiency Power Amplifiers

Performance of an HBT and a Quasi Enhancement Mode PHEMT (QE-PHEMT) unit cell is compared for wireless telephone PA applications. While the HBT has advantages of smaller chip size and single supply with no drain switch operation, the QE-PHEMT has higher efficiency and better low voltage characteristics. PA power control and device design trade-offs for both technologies are also discussed.

High-performance Power PHEMT for Wireless Communications

A low-cost 0.7 ¿m gate power pseudoinorplhic high-electron-mobility transistor (PHEMT) process was developed. PHEMT structure, etch profile and passivation conditions were optimized to yield a device with high breakdown combined with minimal gate lag. A 2 mm PHEMT exhibits dc Idss. of 300 mA/mm, Imax of 500 mA/mm, peak Gm of 360 mS/mm and 3-terminal breakdown voltage of 13 V. At 0.85 GHz such a device exhibits a record output power density of 630 mW/mmn at Vds=5.8 V and 290 mW/mm at Vds=3.4 V with associated PAE of 60%.

Completely consistent 'no-charge' PHEMT model including DC/RF dispersion

A new large-signal PHEMT model based on independent fitting of DC currents, RF currents, and capacitances is developed. Unlike conventional models, it does not contain charge terms and therefore, avoids the problem of path-dependence in the charge-integration of capacitances. Similar consideration is given to the dispersion of conductances. Since each nonlinear element of the model corresponds to an element of the small-signal equivalent circuit, the model shows complete consistency over the bias range of the models extraction. The model features accuracy. and simplicity of extraction. It is especially useful for PHEMTs or MESFETs with dispersion.

Enhancement Mode PHEMT for Single Supply High Efficiency Power Amplifiers

An enhancement mode power PHEMT technology is demonstrated as a viable alternative to an HBT for single supply high efficiency power amplifiers. The newly developed E-PHEMT has Idss=0.5 ¿A/mm, Imax=190 mA/mm, Vp=+0.3 V, Gm=340 mS/mm and Vbdg=20 V. Various on-wafer loadpull results at 900 and 1800 MHz are presented. A 315 mW/mm output power with PAE=76.4% is demonstrated at 1800 MHz and 4V, while 37 mW/mm and PAE=71% is achieved at 900 MHz and 1.5V. These state of the art results for any single-supply technology were achieved by utilizing systematic harmonic loadpull measurements in conjunction with accurate large-signal modeling.

Waveform measurement technique and its applications to optimum loading studies on power FETs

Microwave waveform measurement techniques on power transistors and power amplifiers are reviewed. Techniques of both low-impedance technique in a load-pull system and high-impedance internal node probing, including calibration approaches are presented. Applications of the low-impedance waveform technique are demonstrated in finding the optimum harmonic loads of power PHEMTs to achieve best power added efficiency (PAE). Measured versus simulated results show very good agreement and therefore verify the measurement technique. It has been shown that as high as 83% power added efficiency can be achieved under the inverse-F harmonic loading condition. Also the good agreement of high-impedance probing on a GSM power amplifier with simulated results validates the internal-node probing technique.

Alpha owned PHEMT model and its verification by load-pull and waveform measurements

Alpha-owned PHEMT model (AOPM) is presented which takes into account high-order derivative fitting in I-V curves, charge-conservation, dispersion/self-heating, and gate current caused by different mechanisms. In order to assess the accuracy of the model, verification has been performed based on complex fundamental and harmonic load-pull behaviour and the device voltage and current waveform. The simulated results are in excellent agreement with the measured data. It is, therefore, important to satisfy all the discussed useful criteria in building an accurate device model to predict adequate power performance for optimum power amplifier design.

Scaleable small-signal MESFET/PHEMT models up to 30 mm periphery

This paper addresses the issue of scaleability in equivalent-circuit based small-signal models for MESFETs and PHEMTs with periphery up to 30 mm. It is shown that large-periphery devices have complex distribution effects and the conventional scaling rule can be approximately applied only to certain elements in the equivalent circuit, namely Cgs, Cgd, Gm, and Gds. Modified scaling rules are proposed for devices with lager peripheries. The simulated S-parameters based on the new scaling rules give excellent fitting to the measured data.

Large-signal model of triple-gate MESFET/PHEMT for switch applications

A large-signal model of triple-gate MESFETs/PHEMTs is developed for switch applications. The devices are represented as multiple transistors connected in series. Systematic extraction procedure, including extrinsic parameters, is described. The model has been verified by comparing simulated dc characteristics, S-parameters and power performance of switches with measured results. As an example of applications, a feed-through circuit is simulated and preliminary experimental data supports the validity of the model.

The effect of gate recess width on the linearity of GaAs MESFETs

The width of the gate recess was found to greatly effect the linearity of GaAs MESFETs under class AB operation at 850 MHz. The MESFETs with gate-recess spacing larger than 105 nm exhibit poor 3rd order intermodulation and significantly different 5th order intermodulation product dependence on the output power. Good correlation of the discrete device data with the digital amplifier circuit performance was obtained. This work provides insight into a critical parameter for optimum performance of high linearity amplifier applications.