Also indicated in Figure 4 is the emitter ballast resistor. As the power increases in the transistor, the junction temperature also increases. If the current is not perfectly distributed over the active transistor area, hot spots will appear, which will quickly lead to thermal runaway and catastrophic failure. To ensure even current distribution, resistors are added in series with the emitter fingers. One resistor can be connected to an individual emitter finger or a small group of fingers. Any undesired increase in the current through a particular emitter or group of emitters will be limited by the resistor (effectively, V _BE will be decreased) [5]. This is called emitter ballasting and can be implemented as NiCr resistors, diffused resistors or polysilicon resistors.

Ericsson s transistors use diffused resistors, which are easy to fabricate and control. The resistor value can be optimized for different products by changing one ion implantation dose during the fabrication. Drawbacks are larger CE capacitance contribution compared to polysilicon resistors, and the risk of lower BV _CES values if the resistor doping profile is too shallow or too deep.

The most common cause of device failures is high current standing wave ratios (VSWR) which occur during load mismatch. The emitter ballasting must therefore be chosen not only to ensure thermal balance but also to prevent damage during load mismatch. The resistance of the epitaxial silicon also limits high currents, effectively a collector ballasting. It increases the saturated V _CE and...