This article describes origins of the use of high electron mobility field-effect transistors (HEMT) based on AlGaN/GaN technology as control components for high power microwave and RF control applications.
Currently, the two main technologies for microwave and RF semiconductor control are silicon (Si) PIN diodes and gallium arsenide (GaAs) MESFETs. PIN diode control devices provide a good trade-off between high power handling capability, high isolation, low insertion loss, good switching speed performance and good bandwidth, but do so at the expense of DC power budgets and external (to the device) biasing networks. GaAs FET control devices, on the other hand, provide low insertion loss, high isolation, high switching speed performance and broad bandwidth with minimal DC bias power needed for the switching and control actions. One of the factors limiting the power handling capability of the GaAs MESFET is its relatively low breakdown voltage. This breakdown voltage limit manifests itself during operation when the transistor is in its high impedance state: any microwave or RF voltage dropped across the drain-source terminals higher than breakdown will cause unwanted conduction. This unwanted conduction can decrease the circuit isolation (in a series configuration), increase the insertion loss (in a shunt configuration), or possibly result in device damage. This limitation leads to safe power levels for individual transistors in the range of a few watts.
Wide bandgap (WBG) semiconductors, such as those based on the III-V nitrides, are promising candidates for use in microwave and RF semiconductor control because of their higher breakdown voltages (2 * 106 V/cm breakdown electric field). Technological progress in recent years has seen GaN-based HEMTs (primarily AlGaN/GaN HEMTs) successfully implemented for use in high power microwave amplifiers. WBG semiconductors differ from bulk-type devices such as GaAs MESFETs in that conduction is governed by a two-dimensional electron gas (2DEG) that exists at the interface of the AlGaN/GaN layer rather than bulk majority carrier flow. The density of this 2DEG influences the on-state impedance of these devices. In addition, the electrons travel more freely in the 2DEG rather than in bulk material, leading to an enhancement in mobility in the interface region. Large electron saturation velocities (1.5 * 107 cm/s) further allow high speed and high frequency operation. Several types of AlGaN/GaN devices utilizing doped and undoped structures are being studied,13 each with their own specific current-voltage-power and frequency characteristics. While much work has been done on AlGaN/GaN microwave devices for high power generation and amplification, there have been few studies to date on the use of the technology for high power microwave and RF control applications. This article provides an overview of modeling efforts underway to study the use of AlGaN/GaN devices in the microwave and RF control environment.
AlGaN/GaN HEMT MODEL FOR CONTROL PURPOSES
For any semiconductor component used in microwave control devices applications such as switches or phase-shifters, behavior in both the high impedance (off) and low impedance (on) states are important for the microwave and RF circuit designer. In addition, the influence of the incident microwave power level on the on- and off-state behavior is informative, and any model of semiconductor component should take these aspects into account.
The typical AlGaN/GaN device consists of a high thermal conductivity substrate (silicon carbide or sapphire) with a GaN epitaxial layer. Another layer of AlGaN is grown on the GaN layer, with a 2DEG being induced at the interface between the AlGaN and GaN layers. Gate, source and drain contacts are added as shown in Figure 1. It is this 2DEG located at the AlGaN/GaN interface that plays the major role in the device behavior. In microwave and RF control applications, the AlGaN/GaN device is used in a similar fashion as the MESFET: a three-terminal device with the source and drain in the RF path and the third terminal (gate) connected to the DC control voltage. The structure of the AlGaN/GaN high electron mobility field-effect transistor (HEMT) with the main network of resistors and capacitors forming the HEMT equivalent circuit representation is also shown. For illustrative purposes, red elements of the network appear in the off-state, the green elements only in the on-state, and the blue elements are present in both transistor states.
AlGaN/GaN HEMT RESISTANCE MODEL
From the AlGaN/GaN HEMT cross section, the overall source-drain resistance RSD is governed by the conducting properties of the 2DEG and can be described using a combination of several resistance components as
RSD = RD + RS + Rch (1)
where
|
Rch |
= |
interface resistance |
|
RS |
= |
parasitic source resistance |
|
RD |
= |
drain contact resistance |
The three contributors to the channel resistance are the resistance Rch in the 2DEG directly under the gate, and two equal components (in the case of a symmetrical transistor) Rsg and Rdg located in the interface between the source and gate or drain and gate, respectively. There is also a resistance Rmax governed by the bulk GaN resistivity; this parameter is introduced to characterize the maximal channel resistance beyond the gate threshold voltage when the 2DEG is suppressed.
This three-section resistance model of the interface has two sections (Rsg and Rdg ) that are not influenced by the applied DC gate bias voltage, but are influenced by the applied microwave signal. The third section models the region directly under the gate (Rg ) and displays behavior that is dependent on both the gate bias and applied microwave signal amplitude.
These three resistances are functions of the 2DEG density, interface mobility and device geometry, and make up the total HEMT resistance such that
Rch = Rsg + Rg // Rmax + Rdg (2)
where the channel resistance under gate is given by
and the source-gate and drain-gate interface resistances are given by
where
|
Lg |
= |
gate length |
|
W |
= |
gate width |
|
Lsd |
= |
source-drain distance |
|
ns |
= |
2DEG sheet density in the appropriate region |
|
µ |
= |
carrier mobility |
|
q |
= |
electron charge |
In Equation 3, ns is the 2DEG density which is controlled by the DC voltage applied directly to the gate as well as any microwave voltage coupled to the gate by the device capacitance; in Equation 4, ns is assumed to be controlled only by the applied microwave signal. Frequently, the 2DEG carrier density is not referred to directly; rather, the sheet resistance is used as a metric to describe the conducting properties of the 2DEG. When the sheet resistance is known, the region or layer resistance can be found simply by multiplying the sheet resistance value by the region aspect ratio (length-to-width ratio). Sheet resistances used in this study ranged from 400 to 2000 Ω/sq, values indicative of the current technology. Appendix A presents additional information relative to the HEMT resistance model.
HEMT CAPACITANCE MODEL
The capacitance model includes both intrinsic and extrinsic capacitances. The extrinsic parasitic capacitance of the source and drain metal coupling to the gate metal in air (see the HEMT structure) Cext sg(dg) exists in both transistor states, and a parasitic capacitance Cext sd couples the source and drain above the semiconductor in off-state. The capacitance Cint sd represents the parasitic capacitance coupling the drain-source terminals through the substrate, and also exists in both switch states. These capacitances are estimated using standard geometrical-based relationships used in modeling MESFETs4 and are strongly dependent on the layout of the device. The capacitance Cig is included for the off-state only and is estimated from MOSFET expressions.5 The voltage-dependent gate capacitance Cg between the highly conductive 2DEG and the gate plays a major role in this capacitance network.6 This capacitance is estimated using the expression
where
Ci = above-threshold gate-channel capacitance, estimated as
|
di |
= |
AlGaN layer thickness |
|
Δd |
= |
2DEG effective thickness |
The model also accounts for additional capacitance due to donor neutralization described in an earlier publication.7 Finally, the capacitance C SD represents the total extrinsic and intrinsic capacitances between the drain and the source.
The resulting AlGaN/GaN HEMT equivalent circuit is shown in Figure 2. The additional impedance Zgg represents a model of the gate bias circuitry and consists of a gate resistsance Rgg and parasitic gate capacitance Cgg to ground. The resistance Rgg has several origins and may include the inherent resistance of the gate material, or it may include an intentionally added resistance to aid in keeping the gate floating at high frequencies. The gate bias circuit has been shown in MESFET and pHEMT-based control circuits to influence the nonlinear behavior in these devices and is therefore included in the AlGaN/GaN HEMT model.4,8 For the microwave and RF design engineer, the two design parameters of interest for microwave signal control are based on a simple RC-model for the switching element: the on-state resistance RON and off-state capacitance COFF . The results shown in the remainder of this article are in terms of these two equivalent circuit parameters.
SIMULATION RESULTS
The AlGaN/GaN HEMT control circuit model previously described was used to compute the impedance properties for a typical AlGaN/GaN HEMT. The model AlGaN/GaN device used for computer simulations had a gate length of 0.3 µm, a source-drain spacing of 2.0 µm and sheet resistance values in the 400 to 2000 Ω/sq range. The results presented are in terms normalized to the gate width W (resistance in Ω-mm, capacitance in pF/mm). The model presented here has been previously verified with experimental data on equivalent resistance RSD and capacitance CSD as functions of frequency, gate bias voltage and incident power level.9,10
Small-signal Impedance Properties
Figures 3 and 4 show typical computed DC gate bias voltage characteristics of normalized source-drain resistance and capacitance at a frequency of 2.5 GHz. The simulated results show that, in the off-state, RSD is typically on the order of kiloohms, while in the on-state this value is in the single ohm range, depending on 2DEG sheet resistance and gate width. The simulation results show the channel resistance is relatively constant on either side of the threshold voltage. The resistance shows an abrupt transition in the vicinity of the threshold voltage of approximately 1.00 V for the modeled device. The varying resistance around zero DC gate bias voltage indicates that the control device may also be used as a voltage variable resistance, a key factor for the development of variable microwave and RF attenuators. The CSD vs. DC gate bias voltage plot shows that the source-drain capacitance CSD does not exceed approximately 0.3 pF/mm of gate width in both control states. The slight increase of the capacitance CSD with gate voltage increase in the on-state is caused by ionized donors in the AlGaN layer.7
Figures 5 and 6 present theoretical resistance and capacitance versus frequency at two gate biases 2.0 V and +2.0 V for off- and on-state operation, respectively. Simulated high frequency properties of the on-state and off-state HEMT resistances for sheet resistances of 400 and 1850 Ω/sq are displayed. For the on-state device (Vg = +2.0 volts), the simulation results show a nearly constant resistance up to X-band for the 400 Ω/sq device. The higher sheet resistance device shows a slight decrease in resistance beginning at approximately 7 GHz. The AlGaN/GaN HEMT device capacitance variation with frequency for both switch states shows that the off-state capacitance CSD is constant up through X-band. The on-state capacitance shows greater variation; however, it should be noted that the on-state resistance will dominate the overall device impedance.
The HEMT on-state resistance RON and an off-state capacitance value, COFF , are used to define the broadband switch cutoff frequency4 such that
This cutoff frequency can be used as a control circuit figure of merit in comparing different microwave and RF control technologies. For the HEMT sample with the displayed RSD and CSD characteristics the cutoff frequency, estimated for the DC gate bias voltage of 2 V (COFF ) and +1.5 V (RON at 400 Ω-mm sheet resistance), is approximately 300 GHz. Higher cutoff frequency values can be achieved with smaller sheet resistance rather than large gate widths since COFF increases with these parameters.
Large-signal Impedance Properties
Large-signal properties of AlGaN/GaN HEMT are important for the microwave designer to know because the insertion loss or isolation of circuits using these devices may be a function of the applied power if this power is above a particular threshold. Several physical mechanisms are involved in this behavior in AlGaN/GaN devices. The two most prominent physical mechanisms involved in this behavior are the 2DEG sheet resistance and the carrier mobility. There is a slight increase in the 2DEG sheet resistance with increasing input microwave power level caused by the enhancement of charges in the channel due to the increased gate voltage. On the other hand, the electric field increase due to high power causes a reduction in the channel mobility, which further study shows is larger than the increase in 2DEG sheet resistance. These factors combine to yield an overall on-state resistance increase with increasing microwave power. This phenomenon is shown in Figure 7 where resistance is plotted at 3 GHz versus power level and applied gate voltage. The resistance increase with increasing incident power level can be up to 30 percent at the 1 W (+30 dBm) level. The degree of circuit performance degradation depends on the application. For example, a control element that exhibits a 5 Ω on-state resistance has a series insertion loss of 0.42 dB (for a 50 Ω system). A 30 percent increase in resistance will correspond to a modest 0.1 dB increase in insertion loss. However, in attenuator applications, the degradation is more noticeable. A 6 dB series attenuator will have its attenuation increase more than 1 dB with a 30 percent increase in resistance caused by increasing power levels.
Figure 8 shows the results of simulations using the model, and the degree of insertion loss and isolation variation with power level in the range from 20 dBm to +20 dBm is shown in Figure 9. Plotted are the shunt-connected HEMT isolation and series-connected transistor insertion loss versus applied microwave power at 1 GHz for the sheet resistance of 500 Ω/sq. For both switch types, there is a significant degradation in control device performance based on this phenomenon above power levels in the range of +10 dBm. For series-connected devices, the degradation is strongly dependent on the initial insertion loss, with less degradation observed at low insertion loss levels. The shunt-connected device's isolation follows similar behavior.
TECHNOLOGY COMPARISON
One of the factors governing power handling in the non-conducting state of the FET-based switch is the device breakdown voltage. As mentioned at the outset of this article, AlGaN/GaN-based devices exhibit higher breakdown voltages than conventional FETs; Table 1 lists a comparison between typical ranges of the two main FET-based control elements' breakdown voltage and broadband switch cutoff frequency, and the AlGaN/GaN control element.
It should be noted that the breakdown voltage in AlGaN/GaN HEMTs is dependent on the gate-drain and drain-source spacing, a separation that also influences the on- and off-state resistance and capacitance, respectively, and hence the broadband switch cutoff frequency. The table shows the usual engineering tradeoffs between high power applications used at high frequencies. While the AlGaN/GaN HEMTs have lower broadband switch cutoff frequencies than their GaAs counterparts at the current time, the much higher breakdown voltage allows for a dramatic increase in a switch's power handling capability based on this technology. As noted earlier, the broadband cutoff frequency is a function of the 2DEG sheet resistance, so as the AlGaN/GaN technology improves, the broadband cutoff frequency should approach, or even exceed, that of GaAs-based technology. This indicates that the future is bright for AlGaN/GaN technology for use in high power microwave control and switching applications.
CONCLUSION
This article explored the use of AlGaN/GaN technology for use in high power microwave and RF control circuits. The results discussed show that sheet resistances in the 400 Ω/sq range provide good cutoff frequency behavior, with values of cutoff frequency in the range of 300 GHz. This cutoff frequency will increase with each new step as technology improves. The results also show the feasibility of using AlGaN/GaN HEMTs for both high power microwave amplification and control.
ACKNOWLEDGMENT
This work was partially supported by grants from the Office of Naval Research and the National Science Foundation. The material for this article was first presented at the RAWCON2000 IEEE Radio and Wireless Conference held in Denver, CO in September 2000. *
References
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Robert Caverly received his PhD in electrical engineering in 1983 from The Johns Hopkins University in Baltimore, MD. He currently holds an appointment on the faculty at Villanova University in Villanova, PA. Prior to 1997 he was on the faculty at the University of Massachusetts at Dartmouth, starting his academic career in 1983. He is the author of more than 50 technical articles on a variety of topics ranging from microwave control devices (primarily PIN diodes and MESFETs) to microwave and microelectronics education. He is a senior member of the IEEE and on the editorial board of MTT.
Nikolai Drozdovski received his diploma in electrical engineering (with honors) and his Candidate of Technical Science degree in microelectronics from the Moscow Power Engineering Institute (Technical University), Russia, in 1983 and 1993, respectively. He is now with the ECE Department of the Villanova University, PA. He has authored or co-authored some 20 journal papers, 33 conference presentations and 10 patents. His main research interests are in RF and microwave control devices and semiconductor components such as PIN diodes, MESFETs and HFETs.
Michael Quinn is enrolled at Villanova University working toward a Master's degree in electrical engineering. He is currently working on a project involving the modeling of properties of microwave control devices such as PIN diodes and GaN HEMTs. He holds a Bachelor's degree in electrical engineering from Villanova and has been a student member of IEEE since 1998.
