Bipolar Junction Transistors ( BJT ). Transistors. Two main categories of transistors: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs).
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Transistors • Two main categories of transistors: • Bipolar Junction Transistors (BJTs) and • Field Effect Transistors (FETs). • Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region. • The physics of "transistor action" is quite different for the BJT and FET.
In analog circuits, transistors are used in amplifiers and linear regulated power supplies. • In digital circuits they function as electrical switches, including logic gates, random access memory (RAM), and microprocessors.
The First Transistor: Point-contact transistor A point-contact transistor was the first type of solid state electronic transistor ever constructed. It was made by researchers John Bardeen & Walter Houser Brattain at Bell Laboratories in December 1947. The point-contact transistor was commercialized and sold by Western Electric and others but was rather quickly superseded by the junction transistor.
The Junction Transistor • First BJT was invented early in 1948, only weeks after the point contact transistor. • Initially known simply as the junction transistor. • The term “bipolar” was tagged onto the name to distinguish the fact that both carrier types play important roles in the operation.
Bipolar Junction Transistors (BJT) • A bipolar transistor essentially consists of a pair of PN Junction diodes that are joined back-to-back. • There are therefore two kinds of BJT, the NPN and PNP varieties. • The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.
The First BJT Transistor Size (3/8”L X 5/32”W X 7/32”H) No Date Codes. No Packaging.
BJT Fabrication • BJT can be made either as discrete devices or in planar integrated form. • In discrete, the substrate can be used for one connection, typically the collector. • In integrated version, all 3 contacts appear on the top surface. • The E-B diode is closer to the surface than the B-C junction because it is easier make the havier doping at the top.
BJT Structure - Discrete • Early BJTs were fabricated using alloying - an complicated and unreliable process. • The structure contains two p-n diodes, one between the base and the emitter, and one between the base and the collector.
BJTs are usually constructed vertically • Controlling depth of the emitter’s n doping sets the base width
How the BJT works • Figure shows the energy levels in an NPN transistor under no externally applying voltages. • In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. • In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. • However, in the absence of any externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another. NPN Bipolar Transistor
How the BJT works • What happens when we apply a moderate voltage between the collector and base parts. • The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart. • This widens the depletion zone between the collector and base and so no current will flow. • In effect we have reverse-biassed the Base-Collector diode junction. Apply a Collector-Base voltage
Charge Flow • What happens when we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction. • This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. • Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region. • As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region. • Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied. • Hence an external current flowing in the circuit. Apply an Emitter-Base voltage
Charge Flow • Some of free electrons crossing the Base encounter a hole and 'drop into it'. • As a result, the Base region loses one of its positive charges (holes). • The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction. • The current flow would then stop. Some electron fall into a hole
Charge Flow • To prevent this happening we use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes. • The effect, some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector. • For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way. • Hence a Base current, IB, which is typically around one hundred times smaller than the Emitter current, IE. Some electron fall into a hole
Terminals & Operations • Three terminals: • Base (B): very thin and lightly doped central region (little recombination). • Emitter (E) and collector (C) are two outer regions sandwiching B. • Normal operation (linear or active region): • B-E junction forward biased; B-C junction reverse biased. • The emitter emits (injects) majority charge into base region and because the base very thin, most will ultimately reach the collector. • The emitter is highly doped while the collector is lightly doped. • The collector is usually at higher voltage than the emitter.
Operation Mode • Active: • Most importance mode, e.g. for amplifier operation. • The region where current curves are practically flat. • Saturation: • Barrier potential of the junctions cancel each other out causing a virtual short. • Ideal transistor behaves like a closed switch. • Cutoff: • Current reduced to zero • Ideal transistor behaves like an open switch.
BJT in Active Mode • Operation • Forward bias of EBJ injects electrons from emitter into base (small number of holes injected from base into emitter) • Most electrons shoot through the base into the collector across the reverse bias junction (think about band diagram) • Some electrons recombine with majority carrier in (P-type) base region
Band Diagrams (Active Mode) • EBJ forward biased • Barrier reduced and so electrons diffuse into the base • Electrons get swept across the base into the collector • CBJ reverse biased • Electrons roll down the hill (high E-field)
I-V Characteristics • Collector current vs. vCB shows the BJT looks like a current source (ideally) • Plot only shows values where BCJ is reverse biased and so BJT in active region • However, real BJTs have non-ideal effects
I-V Characteristics Collector-emitter is a family of curves which are a function of base current. Base-emitter junction looks like a forward biased diode
Common-emitter It is called the common-emitter configuration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point.
Common-collector It is called the common-collector configuration because both the signal source and the load share the collector lead as a common connection point. Also called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance.
Common-base This configuration is more complex than the other two, and is less common due to its strange operating characteristics. Used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.
Base Resistance, rB • Mainly effects small-signal and transient responses. • Difficult to measure since it depends on bias condition and is influenced by rE. • In the Ebers-Moll model (SPICE’s default model for BJTs), rBis assumed to be constant.
BJT Analysis • Here is a common emitter BJT amplifier: • What are the steps?
Input & Output • We would want to know the collector current (iC), collector-emitter voltage (VCE), and the voltage across RC. • To get this we need to fine the base current (iB) and the base-emitter voltage (VBE).
Input Equation • To start, let’s write Kirchoff’s voltage law (KVL) around the base circuit.
Output Equation Likewise, we can write KVL around the collector circuit.
Base-Emitter Circuit Q point The Load Line intersects the Base-emitter characteristics at VBEQ = 0.6 V and IBQ = 20 µA
Collector-Emitter Circuit Q point Now that we have the Q-point for the base circuit, let’s proceed to the collector circuit. The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at VCEQ = 5.9 V and ICQ = 2.5mA, then β = 2.5m/20 µ = 125
BJT DC Analysis - Summary • Calculating the Q-point for BJT is the first step in analyzing the circuit • To summarize: • We ignored the AC (variable) source • Short circuit the voltage sources • Open Circuit the current sources • We applied KVL to the base-emitter circuit and using load line analysis on the base-emitter characteristics, we obtained the base current Q-point • We then applied KVL to the collector-emitter circuit and using load line analysis on the collector-emitter characteristics, we obtained the collector current and voltage Q-point • This process is also called DC Analysis • We now proceed to perform AC Analysis
BJT - AC Analysis • How do we handle the variable source Vin(t) ? • When the variations of Vin(t) are large we will use the base-emitter and collector-emitter characteristics using a similar graphical technique as we did for obtaining the Q-point. • When the variations of Vin(t) are small we will shortly use a linear approach using the BJT small signal equivalent circuit.
BJT - AC Analysis • Let’s assume that Vin(t) = 0.2 sin(ωt). • Then the voltage sources at the base vary from a maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -0.2 = 1.4 V • We can then draw two “load lines” corresponding the maximum and minimum values of the input sources • The current intercepts then become for the: • Maximum value: 1.8 / 50k = 36 µA • Minimum value: 1.4 / 50k = 28 µA
AC Analysis Base-Emitter Circuit Note the asymmetry around the Q-point of the Max and Min Values for the base current and voltage which is due to the non-linearity of the base-emitter characteristics From this graph, we find: At Maximum Input Voltage: VBE= 0.63 V, iB= 24 µA At Minimum Input Voltage: VBE= 0.59 V, iB= 15 µA Recall: At Q-point: VBE= 0.6 V, iB= 20 µA ∆iΒmax = 24-20 = 4 µA; ∆iBmin = 20-15 = 5 µA
AC Characteristics-Collector Circuit Using these max and min values for the base current on the collect circuit load line, we find: At Max Input Voltage: VCE= 5 V, iC= 2.7mA At Min Input Voltage: VCE= 7 V, iC= 1.9mA Recall: At Q-point: VCE= 5.9 V, iB= 2.5ma
BJT AC Analysis - Amplifier Gains From the values calculated from the base and collector circuits we can calculate the amplifier gains: