Gain Compression of n-MESFET for High Power Applications in MIMO

In this paper, a new n-type recessed Metal semiconductor eld effect transistor (MESFET) with GaAs/ SiC materials is designed for high power applications in Multi Input Multi Output (MIMO) systems. Based on electrical characteristics of MESFET, a SPICE model of the proposed device is developed. For power switches, the power MESFETs are used. The feasibility of the technology is validated by the electrical measurements of the device. The operational technology has been shown by the characterizations done on the proposed device. To optimize the electrical performance, the contact resistance technique has to be enhanced. In this work, the output power and Gain compression of proposed n-channel MESFET at 100 MHz and 1 GHz for high input power is obtained. The output power at fundamental frequency of operation for high input power is also obtained.


I. Introduction
In present days, the compound MESFET structures are in high demand because of its use in microwave power applications like power oscillators and power ampli ers. For high power applications, the nonlinear behavior of device becomes dominating [1]. Thus a lot of previous research is taken on large signal behavior of MESFET i.e nonlinear behavior. To facilitate the operation of MESFET at high voltage, large energy bandgap is needed [2]. Also high electron velocity of transistor increases the frequency of switching action. To facilitate the device to operate in harsh environments and high temperatures, high melting point and high thermal conductivity are required [3]. Due to variation in channel conditions, for high switching of operations MESFETs are prone to self heating effects. This self heating is due to variation of conditions in channel. The temperature in the channel increases as the current increases inside the channel [4]. This decreases the mobility of carriers inside the channel. When the device is used in places of higher surrounding temperature, the degradation of mobility is still increased [5]. But when the MESFET has either GaAs or SiC material in the channel, these effects are comparatively less when it is compared with commercially available materials like Si or Ge [6]. Hence the MESFETs with GaAs/ SiC materials operate at wide range of temperatures.
Basic logic gates like NAND, NOR, combinational circuits and phase locked loops (PLLS) based on Bipolar junction transistors (BJT), MOSFET are studied by several researchers. The studies are done till high temperatures [7]. However, these advancements mostly concentrated on design of logical blocks and not on integration of power circuits. Primitively dual gate MESFET is designed for power IC's [8]. This is found to be outbreaking technology in harsh environments for power applications. The advantage of MESFET over MOSFET is mainly due to poor interface capacity. Also the MESFET technology exhibits good response at high temperature (500° C). The integration of all the devices becomes asset. There is a reduction in number of components and also the economic cost of the system. Also, an improvement of reliability of device is observed. The enhancement effect of charges in MESFETs has found to be same for GaAs MESFET and SiC MESFETs. For instance, when an electron hits the interface of gate-drain then a large signal is observed [9]. The effect of channel modulation occurs at the pinch-off con guration. The effect of charge enhancement depends more on amount of injected charge by striking of electrons.
The best semiconductor material is diamond. This diamond has large band gap (5.5 eV), high breakdown eld (> 10 MV/cm), higher thermal conductivity (> 3000 W m -1 k -1 ), greater electron mobility (2000 cm 2 v -1 s -1 ). The wider band gap has many advantages but also results in critical issues. These lead to limited inonized dopant concentrations at lower temperature. In this paper, a new n-type recessed MESFET is proposed. The gain compression of wide bandgap MESFETs is studied using GaAs/SiC materials [10].
The output power using GaAs/SiC materials is analyzed for high input power. This paper gives an insight into integrated circuit technology based on planar con guration of MESFET, where gate width can be scaled.
The organization of the paper is as follows: Section II shows the proposed n-MESFET using two different materials. The gain compression and output power of n-MESFET for two different materials is discussed in Section III. The conclusion of the paper is presented in Section IV.
Ii. Proposed Structure This type of simulation is usually not required when the circuit simulation is having either the DC or small signal behavior. The MODELS n-DEPLETION tells the Mercury simulator that device is n-type depletion. To optimize the simulation, Mercury uses this information [11]. To characterize the channel, the POISSON command is used by Mercury as a function of surface conditions. To calculate the transport mechanisms, Mercury uses 1D simulation [12]. Small number of mesh points are used by Mercury module. Thus, to simulate large signal behavior of FET, Mercury uses harmonic balance method. When compared with the matrices of DC simulation of FET, the matrices are 25-100 times bigger for harmonic balance method. The harmonic balance simulation at 100 MHz and 1 GHz of n-type MESFET is characterized in this paper.

Iii. Results And Discussion
To have the suitability of MESFET for high power applications, proper gate length and width are selected. Table I presents the parameters used in the proposed device. The proposed n-MESFET is formed on 100 nm buried oxide (BOX) layer. This BOX layer provides the support for the MESFET and also used for current transport [13]. The study of Gain compression is studied by using two different materials i.e GaAs and SiC for the proposed n-MESFET.  compression ratio of gain is higher for SiC than GaAs. Also the rolloff factor is also higher for SiC.
The output power (in dBm) at 1 GHz plotted against input power (in dBm) is shown in Fig. 4. The output power increases linearly for GaAs MESFET than SiC MESFET. This is mainly due to lower compression ration of GaAS MESFET. The output power saturates after -15 dBm for both GaAs MESFET and SiC MESFET. Similarly the output power (in dBm) at 100 MHz versus input power (in dBm) is shown in Fig. 5.
The performance of GaAs MESFET and SiC MESFET is unaltered at 100 MHz when compared to their performance at 1 GHz.

Iv. Conclusion
The MESFET devices has shown dominating performance for high power applications especially in MIMO structures. To suit MESFET for high power applications, large bandgap materials are needed. In this paper, two different bandgap materials are taken and the study of MESFET with GaAs and SiC is analyzed for high input power. The output power estimation (in dBm) for two different materials at 1 GHz and 100 MHz is studied. The gain compression ration is high for SiC material than GaAs material. Thus the SiC MESFET is highly suitable for applications of higher power rating and multi input multi output systems. Declarations