much debate exists about whether galliumnitride (GaN) devices can replace or even outperform devices based on other technologies, such as laterally diffused metal oxide semiconductor (LDMOS) devices. In an application note titled, "GaN For LDMOS Users," Nitronex Corp. compares LDMOS against GaN for RF power-amplifier (PA) stages. The 15-page document evaluates equivalent outputpower devices to illustrate both the differences and similarities between LDMOS and GaN.
The input and output impedances of GaN devices can be estimated and modeled the same way as LDMOS and gallium-arsenide (GaAs) ferroelectric transistors (FETs). For power devices, however, small-signal stability can be a difficult problem. Vertical metal-oxide semiconductor (VMOS), LDMOS, and GaAs FETs as well as GaN high-electron mobility transistors (HEMTs) have large amounts of gain at low frequencies and similar challenges from a stabilization standpoint. When designing amplifiers in both small- and large-signal scenarios, careful attention must therefore be paid to stability.
In terms of capacitance-voltage characteristics, GaN HEMTs have some of the same capacitance behaviors as other FET devices. Compared to the other technologies, however, GaN's higher power density results in smaller devices and thus smaller absolute capacitance values. Output capacitance plays a significant role in determining the bandwidth limitations. To compare various devices, it is best to compare capacitance per watt of output power.
From a biasing perspective, GaN has two special requirements: The gate and drain must be sequenced during startup and shutdown and the designer must accommodate current flow both into and out of the gate. Otherwise, bias network topology and general design practices are the same for LDMOS and GaN devices. Compared to silicon LDMOS devices, the paper asserts that GaN HEMT linearity is less sensitive to changes in the bias point.
The note advises GaN users to possibly operate their devices above P1dB for continuouswave (CW) applications. For the firm's devices to be used past the 1-dB compression point, three criteria need to be considered: junction temperature, gate current, and device variation over process and temperature. The paper ends with a discussion of insertion-phase variation, which impacts how multiple devices are combined. Although GaN HEMT devices are very different from LDMOS devices, this application note asserts that they can be designed and treated just like LDMOS FETs.
Nitronex Corp., 2305 Presidential Dr., Durham, NC 27703; (919) 807-9100, Fax: (919) 807-9200, Internet: www.nitronex.com.
