PN Junction Theory In View Of Excess Majority Carriers

The traditional PN junction theory poses big stress on minority carriers,which makes misunderstandings that minority carriers are responsible for the current transport in PN junction and majority carriers do not contribute to the current transport.In addition,the total current given by the well-known diode equation is not derived directly by the original definition of the current.Furthermore,based on the framework of traditional theory,some basic physical problems in PN junctions,such as electric neutrality and current continuity,can not be elucidated properly,which results in many contradictions or even wrong understandings.In this paper,the key points of traditional PN junction theory are reviewed and its vague contradictions and shortcomings are presented.In the new theory,the concept of “excess majority carriers” in PN junction is proposed to solve these contradictions and shortcomings.The existence of excess majority carriers in PN junction is proved theoretically and it is in excellent agreement with simulation results.It is found that excess majority carriers and excess minority carriers have the same distribution(concentration gradient)in P-and N-quasi-neutral region,which indicates that excess majority carriers take part in the diffusion motion.For the first time,the“accompanying electric field” associated with excess carriers is proposed and the expressions of the applied electric field and the accompanying electric field are obtained,which indicates that excess majority carriers also take part in the drift motion.By taking excess majority carriers and electric fields in quasi-neutral region into account,the contradictions and misunderstandings of the traditional theory about the electric neutrality and the current continuity are eliminated fundamentally.On this basis,the analytical expressions of four current components in PN junction are presented,thus the total current through PN junctions is obtained directly in the light of the four current components(sum of them).It is also found that the total current of traditional PN junction theory in AC condition is still the sum of the minority diffusion currents at boundaries of space charge region,and the effect of the displacement current is missed,thus the capacitance characteristics of PN junction can not be described accurately.In our theory,the total current of PN junction is derived with consideration of both conduction current and displacement current and expressions of them are presented,thusthe total capacitance of PN junction is obtained.It is shown that not only the parallel relationship between junction capacitance and charge storage capacitance is indicated by the expression of total current given by new theory,but also it can be applied to positive or reverse bias condition.Further analysis and discussions on the physical effects of several junction devices are demonstrated.In conclusion,based on the proof of the existence of the excess majority carriers,a new systematic theory of PN junction is proposed,which eliminates the contradictions and shortcomings of traditional theory,moreover,the current transport in DC and AC conditions and the capacitance-voltage characteristic of PN junctions are clarified.Therefore,a new and thorough understanding of PN junction operation is presented.