We have developed a new, zone-based compact physics-based AlGaN/GaN heterojunction field-effect transistor (HFET) model suitable for use in commercial harmonic-balance microwave circuit simulators. The new model is programmed in Verilog-A, an industry-standard compact modeling language. The new model permits the dc, small-signal, and large-signal RF performance for the transistor to be determined as a function of the device geometric structure and design features, material composition parameters, and dc and RF operating conditions. The new physics-based HFET model does not require extensive parameter extraction to determine model element values, as commonly employed for traditional equivalent-circuit-based transistor models. The new model has been calibrated and verified. We report very good agreement between simulated and measured dc and RF performance of an experimental C-band microwave power amplifier.

AlGaN/GaN heterojunction field-effect transistors (HFETs) are promising RF transistors for use in high-power and high-frequency circuit applications. These HFETs possess a combination of high current density capability and high breakdown voltage due to the desirable physical properties of the materials, such as high critical electric field for breakdown, high electron mobility and saturated carrier velocity, high carrier density in the channel, lower dielectric constant compared to the conventional materials, and high thermal conductivity. These parameters permit the HFET to operate at high RF voltage and current, which results in high power operation at high frequency [

The basic structure for an HFET is shown in Figure

Schematic cross-section of a basic AlGaN/GaN HFET structure, showing the physical parameters of the NCSU HFET model. Four describe layout:

Reported HFET dc models include empirical models [

In this work, we report for the first time the implementation of a physics-based compact HFET model in Verilog-A, and we used Microwave Office (MWO) to compare its predictions against the RF power performance of an experimental HFET. MWO is an EDA package that is available from Applied Wave Research (AWR) Corporation. Our first-generation Verilog-A HFET model is a single, self-contained Verilog-A module.

The new model is developed, based upon separating the conducting channel of the HFET into a series of zones, based upon operational physics [

As previously indicated, the model is formulated based upon separating the conducting channel of the HFET into a series of five zones, which are defined based upon the physics that dominate within each zone. The model operates in two modes, triode and saturation. The transition between the two modes is smooth and dependent upon device design and operation criteria. Figure

Cross-section of HFET model (a) triode operation with its four physical zones and the voltages at the boundaries between them. (b) Saturated operation with its 5 physical zones. Except for Z4, the dotted line indicates the electron path in the 2DEG just below the AlGaN/GaN interface. In Z4, the 2DEG is disrupted and the electrons disperse away from the interface to form a net space charge in the GaN, as discussed in the text.

Associated with each interval, we define its zone as the interval itself, initialization of the distance, potential, and electric field

When the HFET is in triode operation, it can be modeled with four zones. In saturated operation, the model requires five zones. For each operating mode, the terminal characteristics of the device must be consistent with a simultaneous solution of all the zones that exist in that mode.

Fortunately, it is possible to compute this simultaneous solution efficiently by setting up equations for the zones from left to right in sequence, as we will now show. Each zone is solved in three steps. First, the distance, potential, and electric field parameters

In either of the two operating modes, the terminal characteristics can be qualitatively related to the physics of each zone. At zero drain-to-source bias

For positive

For saturated operation,

With increasing

With increasing

The gradual channel approximation (GCA) is readily adapted to zones Z1, Z2, and Z5. In zone Z3, the GCA fails because the carrier trajectories are not one dimensional as discussed above. In zone Z4, the 2DEG has reestablished itself, but the sheet charge density of the 2DEG is insufficient to neutralize the fixed polarization sheet charge and electron velocity in this 2DEG is effectively saturated.

To the best of our knowledge, this charge deficit zone (or zone Z4) is unique to AlGaN/GaN HFETs, but it can dominate device operation over part of the RF cycle. In zone Z4, the 2DEG is stable but is incompletely filled. The length of zone Z4 is approximately proportional to the difference

Theoretical investigations of electron dynamics using Monte Carlo techniques have determined velocity-field characteristics associated with GaN materials. These theoretical simulations show that the electron drift velocity initially increases with the applied electric field but reaches a peak value, after which it gradually decreases to a saturated value at a high electric field [

The curvature of the transition from the low-field region to the high-field region is important in calculating the knee region of the dc current-voltage relationship for the device. We have found that the same

Various

The

Figure

We estimate the saturated velocity

The conducting channel in an AlGaN/GaN HFET is formed from the 2DEG just below the interface of the AlGaN barrier layer that is grown on a GaN layer. This 2DEG conducting channel is formed by spontaneous and piezoelectric polarization effects at the AlGaN/GaN interface [_{m}Ga_{1−m}N, but

In zone Z1 (and also in zone Z5), the current at location

The electric field

At the source-side gate edge, electrons leave zone Z1 and enter zone Z2. In zone Z1, the voltage on the upper AlGaN surface follows the voltage

In accordance with the GCA, we treat this gate region as an MIS capacitor and write the electron sheet density:

Electrons in zone Z2 drift toward the drain because

If

In saturated operation,

The SLZ zone only occurs when the device enters saturation. We approximate the one-dimensional Poisson equation:

In the triode mode, we can define

Zone Z4 occurs in both triode and saturated modes when

Within zone Z5, the nominal electron physics are identical to that in zone Z1 but the zone length varies dynamically and satisfies different boundary conditions. As in zone Z1, overall charge neutrality prevails in zone Z5 and the current is described by (

It may be possible to fabricate an HFET with sufficiently short drain access region and to bias it at high enough drain voltage to deplete the entire drain access region, but we have not observed this in any device we have considered. Currently, we treat this possibility as an error condition.

The physics-based compact HFET model described above has been written in the Verilog-A language and implemented in MWO. The flowchart in Figure

Input parameters for the AlGaN/GaN HFET model.

Parameter | Description | Value |
---|---|---|

Electron mobility | 1120 cm^{2}/V-s | |

_{
sat} | Saturation velocity | |

Curvature parameter in | 1.45 | |

GaN permittivity | 10.1 | |

Energy gap of GaN | 3.52 eV | |

Affinity of AlGaN | 3.8 eV | |

Affinity of metal | 4.3 eV | |

Al mole fraction in AlGaN | 0.3 | |

Gate length | 0.8 | |

Length of the source access | 1.2 | |

Length of the drain access | 2.0 | |

Gate width | 400 | |

Thickness of the AlGaN barrier layer | 30 nm | |

Thickness of the GaN buffer layer | 0.3 | |

Unintentional doping in AlGaN layer | 10^{16} cm^{−3} | |

Polarization sheet charge density | ^{−2} | |

Piezo | Piezo charge density | ^{−2} |

Channel breakdown voltage | 39 | |

Drain-to-source breakdown resistance | 13 | |

Bkdslp | Slope of | 0 |

Flowchart to calculate the current as a function of

Note that the model requires twenty (20) input parameters, as compared to the eighty (80) parameters required to fit conventional equivalent-circuit-based models. The input parameters consist of physical dimensions, doping and thickness levels, and so forth, rather than equivalent circuit element values determined from parameter extraction procedures. The new model is, therefore, easier to define than traditional equivalent circuit transistor models.

To calibrate the model, we have compared the simulation with the measurement data for AlGaN/GaN HFETs of 0.8 ^{−2}.

The model is implemented in the commercial circuit simulator Microwave Office developed by AWR Corporation. The results are compared with the experimental measurements of an

Current-voltage characteristics simulated by MWO for the HFET model (blue lines), along with experimental measurements (red lines) for an HFET with 0.8

The source and drain access regions, zones Z1 and Z5, introduce an extrinsic resistance to the intrinsic HFET and will affect the performance of the HFETs. It has been previously shown that under high-current operation and large-signal RF drive, these resistances become nonlinear as space-charge-limited (SCL) current transport conditions are approached [

An AlGaN/GaN HFET power amplifier is simulated using a single-tone power sweep. The amplifier was biased at a drain bias of

Large signal simulation results including output power, power gain and PAE obtained from the HFET model comparison to the measurement data, with and without (insertion graph) channel break down model. (“Red” is measurement data, “blue” is simulation results).

As is evident in Figure

Figure

Current-voltage characteristics for a 0.8

The effect of the breakdown voltage on large-signal PAE is shown in Figure

PAE with different breakdown voltage.

PAE with different breakdown resistance.

The harmonic impedances of the matching network affect the large signal outputs. The effect of the impedances at the first and second harmonics is shown in Figures

PAE with different 1st load harmonics (real part).

PAE with different second load harmonics (real part).

A new physics-based compact model for AlGaN/GaN HFETs has been developed. The new model is based upon separating the conducting channel into various zones determined by the physical operation principles that dominate in each zone. A simplified set of the semiconductor device equations are applied to each zone and the zones are then interfaced by forcing electric field and potential continuity at the zone interfaces. In this manner, a complete model for the device can be constructed. The compact model is suitable for integration into readily available harmonic-balance circuit simulators. The model has been formulated in Verilog-A and is integrated into the harmonic-balance simulator Microwave Office offered by AWR. However, the new compact model can be employed in any readily available circuit simulator.

The new model permits the dc and RF performance of the device to be determined as a function of device design parameters such as material parameters, the physical dimensions and doping concentrations of the various layers, and the charge transport characteristics. About twenty parameters are required to define the model, but most of these consist of handbook data. The new model does not require the parameter extraction procedures commonly employed to determine equivalent-circuit-based models. The new model, therefore, requires significantly less effort to define.

The new model has been calibrated and verified by comparison with measured data for experimental AlGaN/GaN HFET amplifiers operating at S-band. Excellent agreement between simulated and measured dc and large-signal RF data is obtained.

This work was supported by ARO grant DAA019-03-1-0148 and by ONR grant N0014-05-0419.