Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Gallium-nitride (GaN) semiconductor devices offer great promise for designers of high-frequency active circuits, especially when high output-power levels are needed at higher frequencies. GaN devices offer the mobility and transconductance of gallium arsenide (GaAs) active devices, with the added capability of operating at high voltage levels to achieve high output-power levels. As with silicon laterally diffused metal-oxide-semiconductor (LDMOS) devices, GaN active devices are capable of output-power levels of tens to hundreds of watts, but with a fraction of the input and output capacitances of those Si LDMOS devices, for high output power over broad bandwidths and with high efficiency.

With +48-VDC GaN devices now available, design engineers have further options using for GaN devices in commercial and military applications. Understanding the differences between these emerging +48-VDC GaN devices compared to existing +28-VDC GaN devices can help in matching these higher-voltage devices to suitable applications.

These higher +48-VDC drain supply voltages offer both obvious and subtle benefits. Because device channel size is largely set by peak current requirements, a higher supply voltage reduces the transistor size required for a given RF power rating. Table 1 shows how power density, as measured by gate periphery for typical GaN-on-Si devices, approximately scales by the voltage ratio.

High-Voltage GaN-on-Si Devices Deliver High Power, Table 1

Migrating existing +28-VDC processes to allow for +48-VDC operation requires redesigning the device structure to improve reliability due to the increased electric field. In the X-Y direction, the underlying substrate, epitaxial structure, and channel characteristics remain largely the same for higher-voltage operation, with increased gate-to-drain spacing needed to raise the breakdown voltage. In the Z-direction (vertically), the gate oxide is thickened and changes to the gate metal allow for both higher reliability and increased standoff voltages.

Of greater importance to RF/microwave amplifier designers, Table 1 shows that device output capacitance (COUT) scales by device size. Because of the increased power density, the resulting COUT) for a given power level is reduced. This reduction in output capacitance is the largest differentiator for higher-voltage operation since the resulting change in load impedance offers a major advantage in device operation.

Perhaps the biggest challenge that designers face at higher power levels and frequencies is matching the low output impedances of larger devices. A parallel resistor-capacitor (R-C) circuit is a good model for the transistor output, corresponding to the equivalent R with COUT as specified in Table 1. Given the supply voltage (V) and desired power (P) of an ideal amplifier, there is a simple equation that estimates the output resistance (ROUT):

P ≈ V2/2ROUT   (1)                                                                                                                        

Figure 1 plots ROUT for both +28- and +48-VDC device operation across a typical output-power range. It shows how an amplifier designed for +28-VDC operation is well optimized for output-power levels of 8 to 10 W, requiring minimal impedance transformation for 50-Ω systems. It becomes obvious from Fig. 1 how +48-VDC operation is capable of nearly 25 W output power with minimal impedance matching needed for the same 50-Ω system compared to +28-VDC operation.

High-Voltage GaN-on-Si Devices Deliver High Power, Fig. 1

In addition to increased ROUT, +48-VDC operation also provides further benefits that can ease matching complexity. All things being equal, the usable bandwidth for an amplifier is fundamentally limited by the metrics of impedance transformation ratio and device quality factor (Q):

Impedance transformation ratio = Re(ZDEVICE)/50 Ω Device Q, where Q ≈ Im(ZDEVICE)/Re(ZDEVICE) and ZDEVICE ≈ ROUT + 1/jωCOUT   (2)

The impact of these limitations varies with frequency, but there are general trends. With +28-VDC devices, the transformation ratio becomes the limiting factor—i.e., low ROUT tends to limit the available bandwidth. With +48-VDC devices, smaller COUT and larger ROUT mitigate both factors, and device Q tends to set the limit. Parameter COUT limits the usable maximum frequency in all cases.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.