Carrier transport and noise in devices

To understand microwave devices or to design a circuit incorporating microwave devices we must possess some knowledge about them. A coherent body of knowledge is called a model. A device model tells us approximately how a device reacts to an electromagnetic field in which it is placed. For semiconductor devices, which in general are small compared with a wavelength, this simplifies to the response of the terminal currents to the terminal voltages. Depending on what the purpose is we distinguish two kinds of models: physical models and equivalent circuits.