In this chapter, you will learn about: Thermal generation/excitation, optical generation/excitation, particle bombardment and other external sources. | Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Carrier Transport in Semiconductors Lecture No: 4 Drift and Mobility Conductivity and Resistance Continuity Equations Einstein Relation Nasim Zafar 2 In the first few lectures we discussed and calculated the equilibrium distribution of charges in a semiconductor. = ni2, n ~ ND for n-type last lecture showed how the system tries to restore itself back to equilibrium when perturbed, through R- G processes. R = (n p - ni2)/[tp(n+n1) + tn(p+p1)] In this lecture we will explore the processes that drive the system away from equilibrium. Introduction: 3 Nasim Zafar 3 The carrier transport or the mechanisms which cause charges to move in semiconductors can be classified into two categories. Both these mechanisms will be discuss in this lecture. The two mechanisms are: Drift: Drift-Motion under an applied electric field. diffusion: Diffusion-Motion due to the concentration gradient of the charges. An applied electric field will cause drift, while thermal motion and the carrier concentration gradient will cause diffusion. Introduction: 4 Nasim Zafar The Drift Motion 5 Nasim Zafar 5 An Applied Electric Field Across: n-type Si + - V n – type Si e- Electric field Electron movement Current flow Current carriers are mostly electrons. 6 Nasim Zafar + - V p– type Si hole Electric field Hole movement Current flow Current carriers are mostly holes. An Applied Electric Field Across: P-type Si 7 Nasim Zafar The Thermal Velocity: For free charge carriers the thermal energy and the thermal velocity is given by: From classical thermal physics, or 107 cm/s in Si where vth is the thermal velocity, which is the average velocity of carriers due to thermal excitation. The Concept of Drift-under an applied Electric Field: Random scattering events (R-G centers) The electric field gives a net drift, superposed on top 9 Nasim Zafar 9 The | Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Carrier Transport in Semiconductors Lecture No: 4 Drift and Mobility Conductivity and Resistance Continuity Equations Einstein Relation Nasim Zafar 2 In the first few lectures we discussed and calculated the equilibrium distribution of charges in a semiconductor. = ni2, n ~ ND for n-type last lecture showed how the system tries to restore itself back to equilibrium when perturbed, through R- G processes. R = (n p - ni2)/[tp(n+n1) + tn(p+p1)] In this lecture we will explore the processes that drive the system away from equilibrium. Introduction: 3 Nasim Zafar 3 The carrier transport or the mechanisms which cause charges to move in semiconductors can be classified into two categories. Both these mechanisms will be discuss in this lecture. The two mechanisms are: Drift: Drift-Motion under an applied electric field. diffusion: .
