# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 1 Introduction  
In RF technology, a power amplifier (PA) is a system that is fed with a low-power RF signal. This circuit increases the signal power to the necessary level without causing distortions. Sometimes an additional filter is introduced between the PA input and the load to minimize unwanted signals due to design imperfections. In the RF signal path, the PA is located between the transmitter signal source circuits and the antenna [1, 2]. Companies designing RF electronic modules for telecommunications applications need the appropriate power devices that are able to power supply TRX (Transmitter-Receiver) circuits in order to transmit signals with the required power level in the appropriate frequency range. The efficiency of the PA is a function of the level and shape of the signal [1, 2]. For high power levels (in the range 500-1000 W) a single PA may not be able to ensure the required power level. In such cases, multiple PA modules may be switched on in parallel. Additionally, in PA circuits, energy efficiency influences the lifetime of batteries used in devices such as smartphones, in which the PA circuit is the major consumer of energy. The PA also determines the value of heat which must be dissipated by the telecommunications system [1, 2, 3, 4, 5]. Although the use of several PA modules connected in parallel can solve the power supply problem, there is a problem of power balance, current sharing, thermal matching, and preventing a failure or overheating of the components. All PAs exhibit gain compression at higher power levels, which means that the gain is not constant but it decreases when the input power increases. The above-mentioned limitations mean that we are looking for discrete power semiconductor devices that will meet a wide range of requirements, taking into account the optimization of four parameters: output power, gain, operating frequency,and energy efficiency. Power transistors included in PAs that meet these requirements belong to the group of LDMOS (Laterally Diffused Metal Oxide Semiconductor), HVLDMOS (high-voltage LDMOS) [6], or GaN transistors [7,8]. The technology used allows achieving a high gain, high efficiency, and linearity for the operation at the frequency up to 12 GHz [9, 10, 11, 12]. One of the applications of power transistors in RF circuits is the Doherty amplifier. The Doherty amplifier topology is used to maximize the effective use of DC power, when the transmitted signal has a high peak-to-average ratio [4]. Typically, using the Doherty amplifier, PA circuits operate at the frequency of 2.14 GHz using input signals of the W-CDMA (Wideband Code Division Multiple Access) type [3, 4, 5]. An example of such an amplifier can be a single-band amplifier containing two PAs. The first one is efficient at the lower power (and disables the other one in this range), and the other one--with smooth transition to the one that is less efficient with the low power and better adapted to work at the higher power [2]. The principle of operation of the Doherty amplifier is presented in detail in [2, 3, 4, 5]. All semiconductor devices lose power during their operation. The heat not dissipated outside the semiconductor device leads to an increase in its junction temperature, i.e., overheating of semiconductor devices is one of the most common causes of damage to electronic devices, because the intensity of damage to electronic components increases with an increase in the junction temperature [13,14]. Not only the reliability and the lifetime of semiconductor devices, but also material properties, mechanical and electrical strength (viscosity, loss, dielectric permittivity, etc.) depend on temperature [15]. Moreover, the permissible power load of practically all devices is severely limited by an increase in their junction temperature. For practical calculations of the cooling system, it can be assumed that in consumer electronics devices all the electricity supplied to the device, and in transmitting devices (about 75% of the supplied energy) is converted into heat [15].