Sheikh Aamir Ahsan

Capacitance Modeling in Dual Field-Plate Power GaN HEMT for Accurate Switching Behavior

In this paper, a surface-potential-based compact model is proposed for the capacitance of an AlGaN/GaN high-electron mobility transistor (HEMT) dual field-plate (FP) structure, i.e., with gate and source FPs. FP incorporation in a HEMT gives an improvement in terms of enhanced breakdown voltage, reduced gate leakage, and so on, but it affects the capacitive nature of the device, particularly by bringing into existence in a subthreshold region of operation, a feedback miller capacitance between the gate and the drain, and also a capacitance between the drain and the source, therefore, affecting switching characteristics. Here, we model the bias dependence of the terminal capacitances, wherein the expressions developed for intrinsic charges required for capacitance derivation are analytical and physics-based in nature and valid for all regions of device operation. The proposed model, implemented in Verilog-A, is in excellent agreement with the measured data for different temperatures.

Modeling of source/drain access resistances and their temperature dependence in GaN HEMTs

In this paper, we present the modeling of source/drain access resistances in the surface potential based model named “Advanced SPICE Model for High Electron Mobility Transistor” (ASM-HEMT) for AlGaN/GaN HEMTs. From TCAD simulation, it is shown that nonlinear source and drain resistances increase with drain current which is due to the saturation of electron velocity in this region. Accurate modeling of this access resistance is of immense importance to correctly predict the drain current, transconductance (gm) and hence the transit frequency (fT) at higher current. The model shows excellent agreement with experimental data at room temperature. Variation of measured ON-resistance (Ron) with temperature is well predicted by model which justifies the accuracy of temperature dependence model of source/drain resistances.

Analysis and Modeling of Cross-Coupling and Substrate Capacitances in GaN HEMTs for Power-Electronic Applications

In this paper, we present a capacitance model for field-plate AlGaN/GaN High Electron Mobility Transistor (HEMTs) accounting for the contribution of substrate capacitances and cross-coupling between field plates. TCAD simulations are performed to analyze both these contributions and analytical expressions for charges corresponding to the cross-coupling and substrate capacitances are presented in terms of our existing surface-potential-based model. The modeled results are validated by comparing the time-domain waveforms of a test circuit using a mixed-mode simulation setup in which the impact of cross-coupling and substrate capacitances on accuracy of switching transients predicted by the model is discussed.

Effect of access region and field plate on capacitance behavior of GaN HEMT

Incorporation of Field Plate in High Electron Mobility Transistors (HEMTs) improves the device breakdown voltage but on the other hand, increases the device Capacitance. It has a direct impact on the device switching characteristics and hence the study of the capacitive behavior holds supreme importance for GaN HEMTs power switching application. Also, in GaN HEMTs, lower values of access region resistance improves the device output current but at the cost of increase in its capacitance, CGD. In this paper, using TCAD simulations on a field plated GaN HEMT, we present the physical explanation for the variation in C-V characteristics for different access region and field plate lengths.

Pole-Zero Approach to Analyze and Model the Kink in Gain-Frequency Plot of GaN HEMTs

In this letter, we present a novel approach toward understanding the Kink effect (KE) in the bode plot of short circuit current gain (

Physics-Based Multi-Bias RF Large-Signal GaN HEMT Modeling and Parameter Extraction Flow

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Modeling of kink-effect in RF behaviour of GaN HEMTs using ASM-HEMT model

) observed for microwave transistors, particularly gallium nitride (GaN) HEMTs. We ascribe the origin of the KE to the presence of a pair of complex conjugate poles at the frequency of interest, introduced due to the extrinsic parasitic inductances and their interaction with the device intrinsic elements, such as the capacitances and transconductance, and develop simplified mathematical expressions that govern the behavior of the kink. We also present a physics-based compact model that is capable of capturing the KE and extensively validate the model against measured data for a GaN device under multibias conditions, thereby advocating the strong physical background of the model. We conclude by demonstrating the impact of various elements of the small signal model on the kink based on the developed mathematical hypothesis for KE.

Modeling of trapping effects in GaN HEMTs

In this paper, a consistent DC to RF modeling solution for Al gallium nitride (GaN)/GaN high electron mobility transistors is demonstrated that is constructed around a surface-potential-based core. Expressions for drain current and intrinsic terminal charges in the form of surface-potential are used to simultaneously model the DC characteristics and the intrinsic capacitances of a commercial GaN device. Self-heating and trapping effects are incorporated to account for the non-linear nature of the device. We discuss the parameter extraction flow for some of the key model parameters that are instrumental in fitting the DC characteristics, which simultaneously determines the bias-dependent intrinsic capacitances and conductances that significantly eases the RF parameter extraction. Parasitic capacitances, gate finger resistance, and extrinsic bus-inductances are extracted, from a single set of measured non-cold-FET S-parameters, using the model process design kit. The extraction procedure is validated through overlays of broadband (0.5–50 GHz) S-parameters, load-pull and harmonic-balance (10 GHz) simulations against measured data, under multiple bias conditions to successfully demonstrate the model performance at large-signal RF excitations.

A New Small-Signal Parameter Extraction Technique for Large Gate-Periphery GaN HEMTs

In this paper, a physics-based compact model is reported that captures the kink-effect observed in S22 for AlGaN/GaN HEMTs. The presence of this kink in the Smith-plot of S22 severely affects the design of the output matching network for amplifiers based on these devices which calls for a precise consideration of this effect. The kink-effect originates due to the ambivalent nature of the output impedance of the intrinsic device, wherein it changes its nature from a low frequency series-RC network to a high frequency parallel-RC network, giving rise to a kink at the frequency where the contours corresponding to low and high frequency approximations intersect each other. The output impedance is a complicated function of the various intrinsic elements of the device small signal model, and all the device intrinsic characteristics in our model arise from a physics-based framework, therefore, making multi-bias validation of the kink-behaviour with measured data possible with sufficient ease.

Physics-based Compact Modeling of MSM-2DEG GaN-based Varactors for THz Applications

In this work, we study the trapping in GaN power HEMTs and discuss effect of traps on device characteristics. Simulation setups for analysis of switching collapse and current collapse observed in pulsed I-V are also presented. We propose an RC network based trap model to capture the effect of trapping in a surface potential based compact model for GaN HEMTs. The proposed model has been verified with the hardware data for various quiescent biases and frequencies, and the model results are in excellent agreement with the hardware data.