PHYS 3050 Electronics I Chapter 4. Semiconductor Diodes and Transistors Earth, Moon, Mars, and Beyond Dr. Jinjun Shan, Associate Professor of Space Engineering Department of Earth and Space Science and Engineering Room 255, Petrie Science and Engineering Building Tel: 416-736 2100 ext. 33854 Email: jjshan@yorku.ca Homepage: http://www.yorku.ca/jjshan Introduction In order to understand the extremely fast diode switching speeds or the offset voltage, we have to understand and view the diode as a pn junction. This in turn requires an elementary understanding of electron and hole motion in semiconducting materials. Semiconductor Diodes and Transistors 2
Intrinsic Semiconductors Semiconductor Diodes and Transistors 3 Intrinsic Semiconductors It is interesting to observe that conduction is by electrons and by positively charged carries, called holes, which are created when a bond is broken and an electron is freed. This is commonly referred to as production of a hole-electron pair. Semiconductor Diodes and Transistors 4
Intrinsic Semiconductors This leaves behind a vacancy or a hole. Another electron from some adjacent broken bond can jump into the hole and fill the vacancy, leaving a hole somewhere else. This is commonly referred to as elimination of a hole-electron pair by recombination. Semiconductor Diodes and Transistors 5 Intrinsic Semiconductors Silicon conduction? Semiconductor Diodes and Transistors 6
Intrinsic Semiconductors Example: Determine the conductivity for intrinsic silicon (Si) at room temperature (300 K). Semiconductor Diodes and Transistors 7 Intrinsic Semiconductors Conduction by holes and electrons in silicon Semiconductor Diodes and Transistors 8
Extrinsic Semiconductors The ability to vary the conductivity of semiconducting material over a large range leads directly to many useful devices, including the diode and transistor. Semiconductor Diodes and Transistors 9 Extrinsic Semiconductors There is a better way. The conductivity of a semiconductor can be substantially increased by adding some (typically 1 in 10 million) impurity atoms (called dopants) to the pure crystal structure. Semiconductor Diodes and Transistors 10
Semiconductor Semiconductors materials such as silicon (Si), germanium (Ge), have electrical properties between those of a conductor and an insulator. They are not good conductors nor good insulators (hence their name semi - conductors). They have very few fee electrons because their atoms are closely grouped together in a crystalline pattern called a crystal lattice. Semiconductor Diodes and Transistors 11 Semiconductor However, their ability to conduct electricity can be greatly improved by adding certain impurities to this crystalline structure thereby, producing more free electrons than holes or vice versa. By controlling the amount of impurities added to the semiconductor material it is possible to control its conductivity. Semiconductor Diodes and Transistors 12
Semiconductor These impurities are called donors or acceptors depending on whether they produce electrons or holes respectively. This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping. Semiconductor Diodes and Transistors 13 Semiconductor Silicon Semiconductor Diodes and Transistors 14
Semiconductor Germanium Semiconductor Diodes and Transistors 15 Semiconductor Pure Silicon Semiconductor Diodes and Transistors 16
Semiconductor As there are very few free electrons available to move around the silicon crystal, crystals of pure silicon are therefore good insulators, or very high value resistors. A crystal of pure silica is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons. Semiconductor Diodes and Transistors 17 N-type Semiconductor In order for silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or Phosphorus into the crystalline structure making it extrinsic. These atoms have five outer electrons in their outermost orbital to share with neighbouring atoms and are commonly called Pentavalent impurities. Semiconductor Diodes and Transistors 18
N-type Semiconductor This allows four out of the five orbital electrons to bond with its neighbouring silicon atoms leaving one free electron to become mobile when an electrical voltage is applied (electron flow). As each impurity atom donates one electron, pentavalent atoms are generally known as donors. Semiconductor Diodes and Transistors 19 N-type Semiconductor Antimony Atom and Doping Semiconductor Diodes and Transistors 20
N-type Semiconductor A semiconductor material is classed as N- type when its donor density is greater than its acceptor density, in other words, it has more electrons than holes thereby creating a negative pole. Semiconductor Diodes and Transistors 21 P-type Semiconductor If we go the other way, and introduce a Trivalent (3-electron) impurity into the crystalline structure, such as Aluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Semiconductor Diodes and Transistors 22
P-type Semiconductor Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as holes in the structure of the crystal where electrons are effectively missing. Semiconductor Diodes and Transistors 23 P-type Semiconductor As there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move into the hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turn attracts another electron which in turn creates another hole behind it, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure. Semiconductor Diodes and Transistors 24
P-type Semiconductor This movement of holes results in a shortage of electrons in the silicon turning the entire doped crystal into a positive pole. As each impurity atom generates a hole, trivalent impurities are generally known as Acceptors as they are continually accepting extra or free electrons. Semiconductor Diodes and Transistors 25 P-type Semiconductor Boron (symbol B) is commonly used as a trivalent additive as it has only five electrons arranged in three shells with the outermost orbital having only three electrons. The doping of Boron atoms causes conduction to consist mainly of positive charge carriers resulting in a P-type material with the positive holes being called Majority Carriers while the free electrons are called Minority Carriers. Semiconductor Diodes and Transistors 26
P-type Semiconductor Boron Atom and Doping Semiconductor Diodes and Transistors 27 P-type Semiconductor Then a semiconductor basics material is classed as P-type when its acceptor density is greater than its donor density. Therefore, a P-type semiconductor has more holes than electrons. Semiconductor Diodes and Transistors 28
Doping Antimony (Sb) and Boron (B) are two of the most commonly used doping agents as they are more feely available compared to other types of materials. However, the periodic table groups together a number of other chemical elements all with either three, or five electrons in their outermost orbital shell making them suitable as a doping material. Semiconductor Diodes and Transistors 29 Doping Semiconductor Diodes and Transistors 30
Conduction in Doped Semiconductors The product of electrons and holes in doped silicon is equal to electron squared (or holes squared) in pure silicon. Increasing the majority carriers by increasing the doping level will decrease the minority carriers proportionally. We conclude that in doped silicon, conduction is primarily by the impurity carriers. Semiconductor Diodes and Transistors 31 Conduction in Doped Semiconductors Conductivity for n-type semiconductors Conductivity for p-type semiconductors Semiconductor Diodes and Transistors 32
Conduction in Doped Semiconductors Example: (a) Find the conductance of arsenic- and indium-doped silicon if the doping level is 10 22 atoms/m 3. (b) Find the resistance of a cube of the above material if the cube measures 1 mm on a side. Semiconductor Diodes and Transistors 33 pn-junction How about if we join (or fuse) N-type and P-type semiconductor materials together? Semiconductor Diodes and Transistors 34
pn-junction, diode-junction, the diode The uncircled quantities (holes and electrons) are the free-charge carriers which, when in motion, constitute an electric current. Semiconductor Diodes and Transistors 35 pn-junction, diode-junction, the diode Near the junction, the charge distribution is unstable and can exist only for a very brief time during manufacture of the junction. The free charges on opposite sides of the junction will immediately combine. Semiconductor Diodes and Transistors 36
pn-junction, diode-junction, the diode As a result, the charge density of the P- type along the junction is filled with negatively charged acceptor ions ( N A ), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion. Semiconductor Diodes and Transistors 37 pn-junction, diode-junction, the diode Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers. This area around the pn-junction is now called the Depletion Layer. Semiconductor Diodes and Transistors 38
pn-junction, diode-junction, the diode Semiconductor Diodes and Transistors 39 pn-junction, diode-junction, the diode Semiconductor Diodes and Transistors 40
pn-junction, diode-junction, the diode We now have four currents in the junction. Majority current by holes and electrons Minority current by holes and electrons Fortunately, in most practical situations we can ignore drift current as being negligible. Semiconductor Diodes and Transistors 41 pn-junction, diode-junction, the diode Semiconductor Diodes and Transistors 42
pn-junction, diode-junction, the diode We observe that the V-field increases when moving from the p-region to the n- region. We have now obtained the potential jump V 0 across the junction Semiconductor Diodes and Transistors 43 pn-junction, diode-junction, the diode It should now be clear that this voltage is due to the internal electric field in the depletion zone. The region near the junction is called a depletion region or a depletion zone since it is depleted of all free carriers. In that sense, it is a nonconducting region a thin insulating sheet between p and n halves. Semiconductor Diodes and Transistors 44
Forward Bias Semiconductor Diodes and Transistors 45 Forward Bias Battery of voltage V across the pn-junction with the positive of battery on the p-side and the negative to the n-side. The battery will inject holes into the p- region and electrons into the n-region. This is referred to as forward-biasing a pn-junction. Semiconductor Diodes and Transistors 46
Reverse Bias Semiconductor Diodes and Transistors 47 Reverse Bias Battery connection: plus goes to n-side and minus goes to p-side. Electrons and holes to be repelled further from the junction, greatly increasing the depletion zone. A reverse bias increases the contact potential to V 0 +V at the junction, thus increasing the barrier height for majority carriers. Semiconductor Diodes and Transistors 48
Reverse Bias Under reverse bias, the diode is not an open circuit, but the small drift current, usually referred as reverse saturation current I 0, is the only current present under reverse bias, gives the diode a finite but large resistance. Semiconductor Diodes and Transistors 49 Rectifier Equation To derive a quantitative relationship for current in a pn-junction. The equation is better known as the diode equation. For reverse bias, a very small drift current, the reverse saturation current I 0, flows across the junction, as the majority diffusion current is blocked by the reverse bias. Semiconductor Diodes and Transistors 50
Rectifier Equation Semiconductor Diodes and Transistors 51 Rectifier Equation Example: If the reverse saturation current I 0 for a silicon diode at room temperature is 10-12 A. (a) Find the current at biasing voltages of V = -0.1, 0.1, and 0.5 V. (b) Should the temperature of the diode rise by 30 o C, find the new currents for the same biasing voltages. Semiconductor Diodes and Transistors 52
pn-junction and The Transistor Bipolar Junction Transistor (BJT) Semiconductor Diodes and Transistors 53 pn-junction and The Transistor Bipolar Junction Transistor (BJT) Bipolar because holes and electrons are involved in its operation. For most part, we ignore the contribution of the small minority current. The input region is referred to as the emitter. The center region is referred to as the base. The output region is the collector. Semiconductor Diodes and Transistors 54
pn-junction and The Transistor Bipolar Junction Transistor (BJT) With this type of transistor, we have two junctions with the input junction always forward-biased and the output junction always reverse-biased. Semiconductor Diodes and Transistors 55 pn-junction and The Transistor Differences in Diodes and Transistors Diode Transistor Semiconductor Diodes and Transistors 56
BJT Base, Emitter and Collector Labeling Semiconductor Diodes and Transistors 57 BJT Common base, Common collector or Common emitter (Emitter follower)? Common base? Common emitter? Common collector? Semiconductor Diodes and Transistors 58
BJT Example: Common base, collector or emitter (emitter follower)? Semiconductor Diodes and Transistors 59 BJT Example: Common base, collector or emitter (emitter follower)? Semiconductor Diodes and Transistors 60
pn-junction and The Transistor Grounded-Base Transistors (Common base) Semiconductor Diodes and Transistors 61 pn-junction and The Transistor Grounded-Base Transistors (Common base) Semiconductor Diodes and Transistors 62
pn-junction and The Transistor Grounded-Base Transistors (Common base) Semiconductor Diodes and Transistors 63 pn-junction and The Transistor Grounded-Emitter Transistors (Common emitter) Semiconductor Diodes and Transistors 64
pn-junction and The Transistor Grounded-Emitter Transistors (Common emitter) Semiconductor Diodes and Transistors 65 pn-junction and The Transistor Semiconductor Diodes and Transistors 66
pn-junction and The Transistor Example: Using the collector characteristics of the grounded-emitter transistor show in Fig. 4.7b in textbook, determine the current gain β for this transistor. Semiconductor Diodes and Transistors 67 pn-junction and The Transistor Example: If β for a BJT is given as 150, find the emitter current I e if the collector current I c is given as 4 ma. Semiconductor Diodes and Transistors 68
pn-junction and The Transistor Field Effect Transistor (FET) They are simpler in concept but were invented after the bipolar transistors. Unlike BJT, which is a current amplifier. The FET is basically a voltage amplifier, Semiconductor Diodes and Transistors 69 pn-junction and The Transistor Field Effect Transistor (FET) The ends of the N-type rod are referred to as drain and source, and the ring as a gate. Semiconductor Diodes and Transistors 70
pn-junction and The Transistor Field Effect Transistor (FET) Source of electrons Drain of electrons Semiconductor Diodes and Transistors 71 pn-junction and The Transistor Field Effect Transistor (FET) The direction of the arrow is the direction of current flow of a forward-biased junction. Semiconductor Diodes and Transistors 72
pn-junction and The Transistor Field Effect Transistor (FET) How much reverse voltage we should apply before the gate and source cut the current in the channel? Semiconductor Diodes and Transistors 73 pn-junction and The Transistor Field Effect Transistor (FET) Semiconductor Diodes and Transistors 74
pn-junction and The Transistor Field Effect Transistor (FET) Ohmic Region (V ds < V ov ) This is the region in which the FET acts as a variable resistor and obeys ohm s law. Semiconductor Diodes and Transistors 75 pn-junction and The Transistor Field Effect Transistor (FET) Saturation Region or Constant Current Region (V ov < V ds ) In this region, the curves become flat and horizontal. The transistor acts as a voltage-controlled current source. Compared to BJT, which is a currentcontrolled current source. Semiconductor Diodes and Transistors 76
pn-junction and The Transistor Field Effect Transistor (FET) BJT: current gain. FET: transconductance g m. g m = I d / V gs The effectiveness of current control by the gate voltage is given by g m : Semiconductor Diodes and Transistors 77 pn-junction and The Transistor FET - Transfer Characteristics Alternative to drain characteristics for describing the electrical properties of a FET. drain current vs. gate voltage Semiconductor Diodes and Transistors 78
pn-junction and The Transistor FET - Transfer Characteristics Semiconductor Diodes and Transistors 79 pn-junction and The Transistor Other Types of FETS Metal-oxide-semiconductor FET (MOSFET) depletion-mode (DEMOSFET) enhancement-mode Semiconductor Diodes and Transistors 80
The Transistor As Amplifier An amplifier is a device which consists of interconnected transistors, resistors, inductors and capacitors. Now we are ready to integrate the active and passive elements into an amplifying device. Semiconductor Diodes and Transistors 81 The Transistor As Amplifier Elements of an Amplifier Active element Resistor DC power supply Semiconductor Diodes and Transistors 82
The Transistor As Amplifier Basic Design Considerations How about if we replace the active element by a npn transistor? Semiconductor Diodes and Transistors 83 The Transistor As Amplifier Basic Design Considerations What is the correct voltage of the biasing battery? A good design criterion is to choose a voltage for V EE that sets the output voltage at one-half of the battery voltage V B when V s = 0. Semiconductor Diodes and Transistors 84
The Transistor As Amplifier Basic Design Considerations Common-base transistors, which have a low input impedance and good voltage gain but no current gain, are used as special-purpose amplifiers. They are applicable when the driving source has an inherently low impedance and maximum power transfer is desired. Semiconductor Diodes and Transistors 85 The Transistor As Amplifier The BJT as Amplifier The common-emitter configuration is the most widely used for amplifiers as it combine high-gain with a moderately high input impedance. Semiconductor Diodes and Transistors 86
The Transistor As Amplifier The BJT as Amplifier To choose a good transistor operation, we need to choose battery voltage, load resistor, load line and Q-point. Good transistor operation can be defined as optimal use of the operating region. How to use this area optimally?? Semiconductor Diodes and Transistors 87 The Transistor As Amplifier DC Self-Bias Design and Thermal Runaway Protection Semiconductor Diodes and Transistors 88
The Transistor As Amplifier Fixed-Current Bias If stabilization and drift of the Q-point are not of primary importance, a much simpler biasing circuit that injects the correct amount of base current into the transistor for a desired Q-point may suffice. Semiconductor Diodes and Transistors 89 The Transistor As Amplifier The FET as Amplifier The design of a FET amplifier is similar to that for the BJT. After picking a transistor with the desired characteristics, the DC design is carried out next. Semiconductor Diodes and Transistors 90
The Transistor As Amplifier The FET as Amplifier This involves choosing a suitable battery or DC power supply voltage, choosing a suitable load resistance which will determine the load line, and finally designing a biasing circuit to give a suitable Q-point. Semiconductor Diodes and Transistors 91 The Transistor As Amplifier The FET as Amplifier To avoid complicated algebra due to nonlinearity, we can use transfer characteristics in addition to the output characteristics to set the Q-point graphically. A more practical technique is simply an approximate cut-and-try, which makes the design of the operating point for FET not complicated. Semiconductor Diodes and Transistors 92
The Transistor As Amplifier Graphical Method Semiconductor Diodes and Transistors 93 The Transistor As Amplifier Approximate Method What is a good design? Semiconductor Diodes and Transistors 94
The Transistor As Amplifier Example: Design the DC bias circuit that would fix the Q-point at V gs = -0.6 V, when V DD = 10 V and R L +R s = 2.5 KΩ. Semiconductor Diodes and Transistors 95