[paper] Harmonic Distortion of GaN HEMT Varactors

Loukas Chevas 1,  Matthias Bucher 1,  Nikolaos Makris 1,2, Ioannis Spiridon Fosteris 1,  Nikolaos Fasarakis 1,  Antonios Stavrinidis 2,1, Maria Kayambaki 2,  Athanasios Kostopoulos 2 

and George Konstantinidis 2

Methodology for Harmonic Distortion Characterization and Modelling of GaN HEMT Varactors

Instruments 2026, 10(3), 37; 

DOI: 10.3390/instruments10030037

1 School of Electrical and Computer Engineering, Technical University of Crete, 73500 Chania, Greece
2 Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, 70013 Heraklion, Greece

Abstract: The bias-dependent capacitance of varactors can introduce harmonic distortion into the circuits where they are utilized. A gate capacitance model valid through inversion–depletion has been presented for GaN HEMT varactors in the drive for their utilization in monolithic GaN ASICs. This work focuses on the circuit and the methodology employed to accurately measure on wafer the harmonic distortion caused by one such device. The circuit is presented and its design considerations and operation trade-offs are discussed, followed by a presentation of the measurements resulting from its use. Second- and third-order harmonic distortion is recorded and presented, with Verilog-A model simulations used to fit the measured data. The model consists of a charge-based expression of the HEMT varactor capacitance, with a minimal number of parameters. The good fit of the model is demonstrated, proving both the suitability of the circuit used for the measurements and the validity of the capacitance model for real-world applications.  

Fig: Measurement setup schematic for the harmonic distortion characterization of HEMT varactors and V(f) characterization of the HEMT varactor. Inset is the parallel impedance combination measured by the LCR meter. (a) Cgc(Vg) for fixed frequencies. (b) Cgc(f) for fixed Vg levels. (c) Rp(Vg) for fixed frequencies. (d) Rp(f) for fixed Vg levels.

Aknowlegements: This research was partially funded by the European Union under the project AGAMI_EURIGAMI (ID:101102983).

Data Availability Statement: The measurement data generated and analyzed during the present study are available from the corresponding author upon reasonable request.

[paper] GCC Method for Determining MOSFET VTH

Matthias Bucher¹, Nikolaos Makris¹, Loukas Chevas¹

Generalized Constant Current Method for Determining MOSFET Threshold Voltage

arXiv:2008.00576v1 (2 Aug 202) 

has been submitted to the IEEE for possible publication

¹ School of Electrical and Computer Engineering, Technical University of Crete

Abstract: A novel method for extracting threshold voltage (VTH) and substrate effect parameters of MOSFETs with constant current bias at all levels of inversion is presented. This generalized constant-current (GCC) method exploits the charge-based model of MOSFETs to extract threshold voltage and other substrate-effect related parameters. The method is applicable over a wide range of current throughout weak and moderate inversion and to some extent in strong inversion. This method is particularly useful when applied for MOSFETs presenting edge conduction effect (subthreshold hump) in CMOS processes using Shallow Trench Isolation (STI).

Fig:  Application of the GCC method in presence of edge conduction phenomenon in STI MOSFETs. A constant current is applied to determine pinchoff voltage for the center transistor in moderate inversion at IC=2. To characterize the edge transistor, imposing a current criterion IC=1E−4 corresponds to ICe≈0.02. Pinchoff voltage (VP) and slope factor n characteristics illustrate the determination of parameters for center and edge transistors.

Acknowledgment: This work was partly supported under Project INNOVATION-EL-Crete

(MIS 5002772).