Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp Introduction to Electronic Devices (Course Number 300331) Fall 2006 Bipolar Transistors Information: http://www.faculty.iu- Dr. Dietmar Knipp bremen.de/dknipp/ Assistant Professor of Electrical Engineering Source: Apple Ref.: Apple Ref.: IBM Critical 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 101 dimension (m) Ref.: Palo Alto Research Center Bipolar Transistor 1 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp Introduction to Electronic Devices 5 Bipolar Transistors 5.1 Introduction 5.2 Basic transistor operation 5.3 Transistor under zero bias 5.4 Transistor under bias conditions 5.4.1 Shockley Assumptions 5.4.2 The ideal transistor equation 5.4.3 The transistor equation in its general form 5.4.4 Modes of Operation 5.8.1 The active mode 5.8.2 The Saturation mode 5.8.3 The cutoff mode 5.8.4 The inversion mode 5.5 Transport and gain factors 5.5.1 The Emitter efficiency 5.5.2 The Transport factor 5.5.3 The Common-base current gain 5.5.5 The summary of the transport and gain factors Bipolar Transistor 2 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.6 Transistor Design 5.6.1 The transport factor 5.6.2 The Emitter Efficiency 5.7 Bipolar Transistors as Amplifiers 5.7.1 Common base circuit 5.7.2 Common emitter circuit 5.7.3 The Early Effect 5.8 Transfer characteristic and gain 5.9 Device parameters 5.10 Equivalent circuit of a bipolar junction transistor References Bipolar Transistor 3 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.1 Introduction The transistor (Germanium point contact transistor) was invited by Barttain, Bardeen and Shockley in 1947. As the name already implies it is a bipolar device (like a diode), which means that both electrons and holes (minority and majority carriers) contribute to the overall current flow. The bipolar junction transistor (BJT) is one of the most important semiconductor devices. The transistor is used for high speed circuits, analog circuits and power applications. The underlying electronic transport mechanisms of bipolar junction transistors (BJT) and diodes are similar. Both devices are diffusion controlled devices. Therefore, the influence of the drift current on the total current is negligible. The operating principle of bipolar devices is different from the behavior of unipolar device like a Field Effect Transistors (FETs), where the current is either controlled by electrons or holes. Furthermore, a field effect transistor is a drift controlled electronic device. Bipolar Transistor 4 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.1 Introduction A bipolar transistor (like all other transistors) is a three (four) terminal device. The device consists of an input and an output loop. The device is designed in such a ways that small input changes of a current or/and a voltage result in large changes of the output current or/and voltage. Schematic cross section of a pnp The BJT device structure consists of two transistor. The transistor is pn junctions. The device can be implemented in a p-type implemented as an pnp or a npn substrate. The n-type and the p+- structure. Each of the doped regions is type regions are formed by connected with one of the terminals: diffusion of dopants in the p-type Base, Emitter or Collector. The three substrate. The electrodes are regions of a BJT are formed by the formed by metal contacts. diffusion of a doping profiles in a substrate. Ref.: M.S. Sze, Semiconductor Devices Bipolar Transistor 5 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.1 Introduction The operation principle of a bipolar transistor relies on the fact that the base region of the transistor is a very thin region, so that the two diodes affect each other. The thickness of the base in controlled by the manufacturing (diffusion) process. In terms of applications pure analog integrated circuits are getting less important. Nowadays the technology shifts towards BiCMOS technology. BiCMOS is a combination of bipolar technology and metal oxide semiconductor technology (technology required to manufacture field effect transistors). It allows the realization of analog and digital circuits on a single chip. 5.2 Basic transistor operation The two possible device structures of a bipolar transistor or Pnp- and a npn- structures. Bipolar transistors can operate in four modes of operation, depending on the voltage applied to the base-emitter and the base-collector junction. The four modes of operation are: Active mode, inversion mode, cutoff mode, Saturation mode. Bipolar Transistor 6 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.2 Basic transistor operation In order to realize an amplifier the BJT will be operated in the active mode. In the following the basic operating principle of a bipolar transistor in the active mode will be presented. The + and – signs indicate the polarities pnp-transistor of the voltages applied to the terminals under normal operating conditions (active mode). In the active mode the emitter base diode is forward biased (VEB>0) and the base collector diode is reverse biased (VCB<0). npn-transistor Ref.: M.S. Sze, Semiconductor Devices Bipolar Transistor 7 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.2 Basic transistor operation On this slide the + and – signs indicate the direction of the current flow. IB = IE − IC The npn-structure is complementary pnp-transistor to the pnp structure. As a consequence the current flow and the voltage polarities are reversed. In the following we will discuss the electronic transport of a pnp transistor. npn-transistor Ref.: M.S. Sze, Semiconductor Devices Bipolar Transistor 8 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.3 Bipolar Junction Transistors under zero bais At first the BJT will be studied in thermal equilibrium. All three terminals of the device are grounded. Under thermal equilibrium the Fermi level is constant throughout the entire device structure. As a consequence the derivative of the Fermi level is zero, so that the overall current flowing through the device is zero. The following device structure is used for the discussion: The emitter is (much) heavier doped than the base and the base is again heavier doped than the collector. The base region is much shorter than the emitter and the collector region. The base width of the base region is much shorter than the diffusion length of the minority carriers. Therefore, the two pn-junctions affect each other. If the base-region would be much longer than the diffusion length of the minority carriers in the base region of the two pn-junctions would behave like two separate diodes. The operation of the diodes would be independent of each other. The electric field distribution in the BJT can be calculated by solving the Poisson equation. As a consequence of the doping profile in the individual regions of the device the maximum electric field and the built-in voltage for the base/emitter junctions is higher than the maximum electric field and the built-in voltage for the base/collector junction (under thermal equilibrium). Bipolar Transistor 9 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.3 Bipolar Junction Transistor under zero bias Throughout the following discussion it is assumed that two abrupt junctions are formed. As a consequence the electric field distribution outside of the depletion regions is zero. The formation of the space charge region for a BJT is comparable with the formation of the space charge region of a diode. pnp-transistor (a) pnp-transistor under thermal equilibrium (all terminals grounded). (b) Doping profile of an abrupt pnp structure, (c) Electric field profile, (d) Energy band diagram Ref.: M.S. Sze, Semiconductor Devices Bipolar Transistor 10 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.3 Bipolar Junction Transistor under zero bias There are technological and a physical reasons for the different doping profiles in the individual regions of the device: Technological: The fabrication of a BJT requires two diffusion steps to form three regions with different doping concentrations. In the first diffusion step the base has to be formed and the already existing dopants in the material (in the substrate) have to be compensated or over compensated. In the second diffusion step the doping concentration has to be increased again to compensate the incorporated dopants of the first diffusion step. Physical: The goal of the transistor design is the realization of transistors with high current and/or voltage gain. High current and voltages gains can be achieved if the doping concentration in the base is lower than the doping concentration in the emitter. The underlying physical reasons for this particular behavior will be discussed in the chapter on transport and gain factors. Bipolar Transistor 11 Introduction to Electronic Devices, Fall 2006, Dr. D. Knipp 5.4 Transistor under bias conditions Before deriving the ideal bipolar junction transistor equations the basic operating principle of BJT under biasing conditions will be described. Here we will concentrate on a common base bipolar transistor in active mode as this is the most important mode of operation. In this case the base/emitter junction is in forward bias and the base/collector junction operates under reverse bias conditions. The input and the output loop share the base terminal. Therefore, the circuit is called a common base circuit. As a consequence of the applied bias voltages the width of the depletion regions is changed. Since the base emitter junctions is under forward bias holes are injected via the emitter and electrons are injected via the base.