Tantalum Polymer use in GaN based applications

Introduction

We are living in a world where increased electronic content controls, monitors and connects items commonly found in our daily lives. Examples range from our transportation systems to appliances, personal electronics to industrial process control systems. Regardless of the end item – electronics is everywhere.

Electronic proliferation into end devices is responsible for the high levels of performance that we all are very accustomed to and expect.

Additionally, careful implementation of electronics into end systems also provides efficiency gains and conservation of limited resources.

It is important to note that the control and communications circuits themselves are getting more efficient due to the low loss features of advanced semiconductor technologies. This makes it easier to utilize electronics in end devices since the electronics is smaller and requires less power to operate. In turn, electronic content in end systems accelerates further.

One of the most important recent semiconductor advancements to note is that of Gallium Nitride devices – GaN.

GaN devices show significant advantages over similar silicon devices in terms of efficiency, speed, operating frequency, power levels and temperature.

Equivalent sized GaN to Silicon die comparisons show massive current, voltage, power, and switching speed advantages, consistently favoring GaN. Alternatively, a GaN based semiconductor could be dramatically smaller than its Si counterpart if equivalent performance is the goal.

Regardless of the route taken – GaN has the potential to change both RF and power electronics design tremendously because of significantly reduced conduction losses (lower RDS-ON) and faster switching capacity due to reduced material capacitance and enhanced electron mobility. [1]  

In short, use of GaN semiconductors may enable new applications to become possible – even practical or in less stringent applications make the circuit smaller, lighter & less power hungry.

For the purpose of this paper, we will concentrate on GaN semiconductors in RF power amplifier (PA) applications.

Three main areas considered where GaN technology improves the efficiency of the RF systems are:

• Reducing power supply distribution loss through higher voltage bias voltages thereby reducing current levels of similar power operation circuitry
• Reduced heat generation creates an opportunity for smaller thermal management solutions. That minimized need then results in smaller, lighter and potentially more reliable systems.
• Faster conduction /switching times

The GaN PA semiconductors in this paper are depletion mode devices that require a negative gate bias whenever there is a drain bias applied to prevent device damage.

Negative gate-source voltage must be present anytime the drain bias voltage is applied. [2] There are certain power up & power down sequences that must be followed in order to not damage GaN transistors. End users must insure those sequences always occur and many times utilize added hardware, software or a combination of both to achieve the correct conditions.

From a hardware point of view – banks of capacitors are used to filter noise in the bias lines and to provide charge during high di/dt demands of the amplifier during various operation conditions. The capacitor banks are chosen for stability over applied voltage & temperature as well as time/age of the device.

The frequency response of the capacitor bank is determined by both large value bulk capacitors as well as high frequency MLCCs (sometimes de-Q’d with series resistors). An example of capacitor banks used in GaN bias is shown in figure 1 below. [2]

The capacitors used in filtering these two bias lines play a critical role in PA operation are the area of concentration of this paper.

Filter Bank Capacitor options

Present day bulk capacitor options for the voltage bias banks range from high CV MLCC to Tantalum, Tantalum Polymer and Aluminium and Aluminium Polymer electrolytic capacitors.

High CV MLCC capacitors can achieve the capacitance ranges used in many bias networks but do not provide the stability required for optimum GaN PA efficiency across operating conditions.

For example, capacitance stability of a 100uf X5R MLCC can vary from 100uf at 25°C to a capacitance of ~ 55uf at negative 55°C and 80uf at 125°C. In comparison, Tantalum, Tantalum Polymer, Aluminium and Aluminium Polymer electrolytic capacitors have adequate capacitance stability. (Near ideal stability in the case of Tantalum & Tantalum Polymer capacitors and acceptable stability in the case of Aluminium based devices). [3] Further, high CV MLCCs suffer from DC bias effects that can greatly reduce the capacitance value present in the actual circuit. This paper will not investigate high CV MLCCs any further for use as the bulk capacitor in bias networks due to the unstable capacitance associated with real world use cases.

The capacitance stability across common operating temperature range for all capacitor families discussed above are compared in figure 2.

Relatively low parasitic parameters (ESR and ESL) of bulk capacitors are advantageous to the response of the capacitor bank. One-step to reducing ESR of the bulk capacitor in the filter bank is to consider conductive polymer technology. However, the low ESL target eliminates wound aluminum electrolytic capacitors from further investigation in this paper. Though tantalum capacitors are used extensively in these bias applications – our attention will turn to polymer based capacitors where further reductions to already acceptable tantalum capacitor technology is possible from a ESR and ESL point of view.