Chapter 5: Bipolar Junction Transistor's Goals is Explore the physical structure of bipolar transistor, Study terminal characteristics of BJT, Explore differences between npn and pnp transistors, Develop the Transport Model for bipolar devices. | Chapter 5 Bipolar Junction Transistors Chapter Goals Explore the physical structure of bipolar transistor Study terminal characteristics of BJT. Explore differences between npn and pnp transistors. Develop the Transport Model for bipolar devices. Define four operation regions of the BJT. Explore model simplifications for the forward active region. Understand the origin and modeling of the Early effect. Present a PSPICE model for the bipolar transistor. Discuss bipolar current sources and the current mirror. Physical Structure The BJT consists of 3 alternating layers of n- and p-type semiconductor called emitter (E), base (B) and collector (C). The majority of current enters collector, crosses the base region and exits through the emitter. A small current also enters the base terminal, crosses the base-emitter junction and exits through the emitter. Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates the i-v characteristics of the BJT. | Chapter 5 Bipolar Junction Transistors Chapter Goals Explore the physical structure of bipolar transistor Study terminal characteristics of BJT. Explore differences between npn and pnp transistors. Develop the Transport Model for bipolar devices. Define four operation regions of the BJT. Explore model simplifications for the forward active region. Understand the origin and modeling of the Early effect. Present a PSPICE model for the bipolar transistor. Discuss bipolar current sources and the current mirror. Physical Structure The BJT consists of 3 alternating layers of n- and p-type semiconductor called emitter (E), base (B) and collector (C). The majority of current enters collector, crosses the base region and exits through the emitter. A small current also enters the base terminal, crosses the base-emitter junction and exits through the emitter. Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates the i-v characteristics of the BJT. Transport Model for the npn Transistor The narrow width of the base region causes a coupling between the two back to back pn junctions. The emitter injects electrons into base region; almost all of them travel across narrow base and are removed by collector. Base-emitter voltage vBE and base-collector voltage vBC determine the currents in the transistor and are said to be positive when they forward-bias their respective pn junctions. The terminal currents are the collector current(iC ), the base current (iB) and the emitter current (iE). The primary difference between the BJT and the FET is that iB is significant, while iG = 0. npn Transistor: Forward Characteristics Forward transport current is IS is saturation current VT = kT/q = V at room temperature Base current is given by Emitter current is given by In this forward active operation region, npn Transistor: Reverse Characteristics Reverse transport current is Emitter current is given by is reverse current gain Base current is
