Solid State Devices13February 2004 Solid State Devices Objectives • Understand the basic structure of semiconductors and how they conduct current • Describe the characteristics and biasing of a pn junction diode • Describe the basic diode characteristics • Analyze the operation of a half-wave rectifier and a full-wave rectifier • Describe the basic structure and operation of bipolar junction transistors • Describe the basic structure and operation of JFETs and MOSFETs • Discuss the basic op-amp • Explain the basic operation of a differential amplifier 13 February 2004 Solid State Devices 2 Professor Andrew H. Andersen 1 Solid State Devices 13February 2004 Diodes Introduction to Semiconductors • Two types of semiconductor materials are silicon and germainium – both have four valance electrons • When silicon and germanium atoms combine into molecules to form a solid material, they arrange themselves in a fixed pattern called a crystal – atoms within the crystal structure are held together by covalent bonds (atoms share valence electrons) • An intrinsic crystal is one that has no impurities • In an intrinsic semiconductor, there are relatively few free electrons – pure semiconductor materials are neither good conductors nor good insulators 13 February 2004 Solid State Devices 4 Professor Andrew H. Andersen 2 Solid State Devices 13February 2004 Introduction to Semiconductors • Intrinsic semiconductor materials must be modified by increasing the free electrons and holes to increase its conductivity and make it useful for electronic devices – by adding impurities, n-type and p-type extrinsic semiconductor material can be produced • Doping is the process of adding impurities to intrinsic semiconductor materials to increase and control conductivity within the material – n-type material is formed by adding pentavalent (5 valence electrons) impurity atoms • electrons are called majority carriers in n-type material • holes are called minority carriers in n-type material – p-type material is formed by adding trivalent (3 valence electrons) impurity atoms • holes are called majority carriers in p-type material • electrons are called minority carriers in p-type material 13 February 2004 Solid State Devices 5 The PN Junction Diode • A diode consists of an n region and a p region separated by a pn junction – the n region has many conduction electrons – the p region has many holes As a result of recombination, a large number of positive (in the n region) and negative (in the p region) ions builds up near the pn junction, essentially depleting the region of any conduction electrons or holes - termed the depletion region • 13 February 2004 Solid State Devices 6 Professor Andrew H. Andersen 3 Solid State Devices 13February 2004 The PN Junction Diode • The barrier potential, VB, is the amount of voltage required to move electrons through the electric field – At 25°C, it is approximately 0.7 V for silicon and 0.3 V for germanium – As the junction temperature increases, the barrier potential decreases, and vice versa 13 February 2004 Solid State Devices 7 The PN Junction Diode • Forward bias is the condition that permits current through a diode – the negative terminal of the VBIAS source is connected to the n region, and the positive terminal is connected to the p region 13 February 2004 Solid State Devices 8 Professor Andrew H. Andersen 4 Solid State Devices 13February 2004 The PN Junction Diode • The negative terminal of the bias-voltage source pushes the conduction-band electrons in the n region toward the pn junction, while the positive terminal pushes the holes in the p region toward the pn junction • When it overcomes the barrier potential (VB), the external voltage source provides the n region electrons with enough energy to penetrate the depletion region and move through the junction 13 February 2004 Solid State Devices 9 The PN Junction Diode • Reverse bias is the condition that prevent current through the diode – the negative terminal of the source voltage is connected to the p region, and the positive terminal is connected to the n region • If the external reverse-bias voltage (VZ) is increased to a large enough value, reverse breakdown occurs – minority conduction-band electrons acquire enough energy from the external source to accelerate toward the positive end of the diode, colliding with atoms and knocking valence electrons into the conduction band 13 February 2004 Solid State Devices 10 Professor Andrew H. Andersen 5 Solid State Devices 13February 2004 Diode Packages 13 February 2004 Solid State Devices 11 PN Junction Diode Transfer Characteristics 13 February 2004 Solid State Devices 12 Professor Andrew H. Andersen 6 Solid State Devices 13February 2004 Diode Characteristics • The “arrowhead” in the diode symbol points in the direction opposite the electron flow – The anode (A) is the p region – The cathode (K) is the n region 13 February 2004 Solid State Devices 13 Diode Characteristics • The simplest way to visualize diode operation is to think of it as a switch – When forward-biased, the diode ideally acts as a closed (on) switch – When reverse-biased, it acts as an open (off) switch 13 February 2004 Solid State Devices 14 Professor Andrew H. Andersen 7 Solid State Devices 13February 2004 Half Wave Diode Rectifiers • A diode is connected to an ac source that provides the input voltage, Vin, and to a load resistor, RL, forming a half-wave rectifier • on the positive half-cycle – the diode is forward biased and current flows – VD = .7V – V R = ID R – VR = VS - VD • on the negative half-cycle – the diode is revers biased and no current flows – VR = 0V – VD = VS 13 February 2004 Solid State Devices 15 BJT Professor Andrew H. Andersen 8 Solid State Devices 13February 2004 DC Operation of Bipolar Junction Transistor • The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctions • Regions are called emitter, base and collector 13 February 2004 Solid State Devices 17 DC Operation of Bipolar Junction Transistors • • • • There are two types of BJTs, the NPN and PNP The two junctions are termed the base-emitter junction and the base-collector junction The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structure In order for the transistor to operate properly, the two junctions must have the correct dc bias voltages – the base-emitter (BE) junction is forward biased – the base-collector (BC) junction is reverse biased 13 February 2004 Solid State Devices 18 Professor Andrew H. Andersen 9 Solid State Devices 13February 2004 DC Operation of Bipolar Junction Transistors • Transistor Currents: IE = IC + IB • beta (βDC) IC = βDCIB – βDC typically has a value between 20 and 300 13 February 2004 Solid State Devices 19 Operation of Bipolar Junction Transistors • A common-emitter (CE) amplifier – capacitors are used for coupling ac without disturbing dc levels 13 February 2004 Solid State Devices 20 Professor Andrew H. Andersen 10 Solid State Devices 13February 2004 FET’s Operation of Field-Effect Transistors • The junction field-effect transistor (JFET) is operated with a reverse biased junction to control current in a channel – the device is identified by the material in the channel, either n-channel or p-channel • When shown in a drawing, the drain is at the upper end and the source is at the lower end • The channel is formed between the gate regions – controlling the reverse biasing voltage on the gate-to-source junction controls the channel size and the drain current, ID 13 February 2004 Solid State Devices 22 Professor Andrew H. Andersen 11 Solid State Devices 13February 2004 Operation of Field-Effect Transistors • The metal-oxide semiconductor field-effect transistor (MOSFET) differs from the JFET in that it has no pn junction; instead, the gate is insulated from the channel by a silicon dioxide (SiO2) layer • MOSFETs may be depletion type (D-MOSFET) or enhancement type (EMOSFET) – D-MOSFETs have a physical channel between Drain and Source, with no voltage applied to the Gate – E-MOSFETS have no physical Drain-Source channel 13 February 2004 Solid State Devices 23 JFET 13 February 2004 Solid State Devices 24 Professor Andrew H. Andersen 12 Solid State Devices 13February 2004 JFET Schematic Diagram 13 February 2004 Solid State Devices 25 13 February 2004 Solid State Devices 26 Professor Andrew H. Andersen 13 Solid State Devices 13February 2004 JFET Fixed Bias RG is used to limit current in case VGG connected with wrong polarity 13 February 2004 Solid State Devices 27 DMOSFET 13 February 2004 Solid State Devices 28 Professor Andrew H. Andersen 14 Solid State Devices 13February 2004 DMOSFET Schematic Symbols 13 February 2004 Solid State Devices 29 EMOSFET 13 February 2004 Solid State Devices 30 Professor Andrew H. Andersen 15 Solid State Devices 13February 2004 EMOSFET Schematic Symbol 13 February 2004 Solid State Devices 31 Operational Amplifiers Professor Andrew H. Andersen 16 Solid State Devices 13February 2004 Introduction to Operational Amplifiers • The standard Operational amplifier has two input terminals, inverting (-) and noninverting (+) 13 February 2004 Solid State Devices 33 Introduction to Operational Amplifiers • The ideal op-amp has: – infinite voltage gain – an infinite input impedance (open) • does not load the driving source – zero output impedance • The practical op-amp has: – high voltage gain – high input impedance – low output impedance 13 February 2004 Solid State Devices 34 Professor Andrew H. Andersen 17