1. BJT Bipolar Junction Transistors (BJT) EEE 3607 Dr. Mohammad Aminul Islam Department of Electrical and Electronic Engineering, IIUC, Bangladesh

2. Introduction 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. M A Islam, EEE, IIUC

3. Introduction First BJT was invented early in 1948, only weeks after the point contact transistor. It did not become practical until the early 1950s. The term “bipolar” was tagged onto the name to distinguish the fact that both carrier types play important roles in the operation. Field Effect Transistors (FETs) are “unipolar” transistors since their operation depends primarily on a single carrier type. M A Islam, EEE, IIUC

4. Introduction 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. M A Islam, EEE, IIUC

5. 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. M A Islam, EEE, IIUC

6. BJT Structure - Planar In the planar process, all steps are performed from the surface of the wafer M A Islam, EEE, IIUC

7. 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 M A Islam, EEE, IIUC

8. 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 M A Islam, EEE, IIUC

9. 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 M A Islam, EEE, IIUC

10. 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 M A Islam, EEE, IIUC

11. 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, I B, which is typically around one hundred times smaller than the Emitter current, I E. Some electron fall into a hole M A Islam, EEE, IIUC

12. 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. M A Islam, EEE, IIUC

13. 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. M A Islam, EEE, IIUC

14. Operation Mode M A Islam, EEE, IIUC

15. 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 M A Islam, EEE, IIUC

16. Band Diagrams ( In equilibrium) No current flow Back-to-back PN diodes M A Islam, EEE, IIUC

17. 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) M A Islam, EEE, IIUC

18. Minority Carrier Concentration Profiles Current dominated by electrons from emitter to base (by design) b/c of the forward bias and minority carrier concentration gradient (diffusion) through the base  some recombination causes bowing of electron concentration (in the base)  base is designed to be fairly short (minimize recombination)  emitter is heavily (sometimes degenerately) doped and base is lightly doped Drift currents are usually small and neglected M A Islam, EEE, IIUC

19. Diffusion Current Through the Base Diffusion of electrons through the base is set by concentration profile at the EBJ Diffusion current of electrons through the base is (assuming an ideal straight line case): Due to recombination in the base, the current at the EBJ and current at the CBJ are not equal and differ by a base current M A Islam, EEE, IIUC

20. Collector Current Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector b/c of the higher potential applied to the collector. Note that i C is independent of v CB (potential bias across CBJ) ideally Saturation current is  inversely proportional to W and directly proportional to A E Want short base and large emitter area for high currents  dependent on temperature due to n i 2 term M A Islam, EEE, IIUC

21. Collector Current Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector b/c of the higher potential applied to the collector. Note that i C is independent of v CB (potential bias across CBJ) ideally Saturation current is  inversely proportional to W and directly proportional to A E Want short base and large emitter area for high currents  dependent on temperature due to n i 2 term M A Islam, EEE, IIUC

22. Collector Current Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector b/c of the higher potential applied to the collector. Note that i C is independent of v CB (potential bias across CBJ) ideally Saturation current is  inversely proportional to W and directly proportional to A E Want short base and large emitter area for high currents  dependent on temperature due to n i 2 term M A Islam, EEE, IIUC

23. Breakdown Voltages M A Islam, EEE, IIUC

24. The Common Emitter Amplifier Circuit In the Bipolar Transistor the most common circuit configuration for an NPN transistor is that of the Common Emitter Amplifier circuit and that a family of curves known commonly as the Output Characteristic Curves, relate the transistors Collector current (I c ), to the Collector voltage (V ce ) for different values of Base current ( I b ).Bipolar Transistor All types of Transistor Amplifiers operate using AC signal inputs which alternate between a positive value and a negative value so some way of “presetting” the amplifier circuit to operate between these two maximum or peak values is required. This is achieved using a process known as Biasing. Biasing is very important in amplifier design as it establishes the correct operating point of the transistor amplifier ready to receive signals, thereby reducing any distortion to the output signal.Transistor Amplifiers Transistor Amplifiers

25. M A Islam, EEE, IIUC The Common Emitter Amplifier Circuit The single stage common emitter amplifier circuit shown above uses what is commonly called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as a potential divider network across the supply with their center point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits.

26. M A Islam, EEE, IIUC This method of biasing the transistor greatly reduces the effects of varying Beta, ( β ) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, R1, R2 and the power supply voltage Vcc as shown with the current flowing through both resistors. Then the potential divider network used in the common emitter amplifier circuit divides the input signal in proportion to the resistance. This bias reference voltage can be easily calculated using the simple voltage divider formula below: Then the total resistance R T will be equal to R1 + R2 giving the current as I T = Vcc/R T. The voltage level generated at the junction of resistors R1 and R2 holds the Base voltage (Vb) constant at a value below the supply voltage.

27. M A Islam, EEE, IIUC Bias Voltage The same supply voltage, (Vcc) also determines the maximum Collector current, I c when the transistor is switched fully “ON” (saturation), Vce = 0. The Base current I b for the transistor is found from the Collector current, I c and the DC current gain Beta, β of the transistor. Beta Value Beta is sometimes referred to as h FE which is the transistors forward current gain in the common emitter configuration. Beta has no units as it is a fixed ratio of the two currents, Ic and Ib so a small change in the Base current will cause a large change in the Collector current. One final point about Beta. Transistors of the same type and part number will have large variations in their Beta value for example, the BC107 NPN Bipolar transistor has a DC current gain Beta value of between 110 and 450 (data sheet value) this is because Beta is a characteristic of their construction and not their operation.

28. M A Islam, EEE, IIUC Common Emitter Amplifier Example 1 A common emitter amplifier circuit has a load resistance, R L of 1.2kΩs and a supply voltage of 12v. Calculate the maximum Collector current (Ic) flowing through the load resistor when the transistor is switched fully “ON” (saturation), assume Vce = 0. Also find the value of the Emitter resistor, R E with a voltage drop of 1v across it. Calculate the values of all the other circuit resistors assuming an NPN silicon transistor. We have,. This then establishes point “A” on the Collector current vertical axis of the characteristics curves and occurs when Vce = 0. When the transistor is switched fully “OFF”, their is no voltage drop across either resistor R E or R L as no current is flowing through them. Then the voltage drop across the transistor, Vce is equal to the supply voltage, Vcc. This establishes point “B” on the horizontal axis of the characteristics curves.

29. M A Islam, EEE, IIUC Generally, the quiescent Q-point of the amplifier is with zero input signal applied to the Base, so the Collector sits about half-way along the load line between zero volts and the supply voltage, (Vcc/2). Therefore, the Collector current at the Q- point of the amplifier will be given as: This static DC load line produces a straight line equation whose slope is given as: -1/(R L + R E ) and that it crosses the vertical Ic axis at a point equal to Vcc/(R L + R E ). The actual position of the Q-point on the DC load line is determined by the mean value of I b.

30. M A Islam, EEE, IIUC As the Collector current, Ic of the transistor is also equal to the DC gain of the transistor (Beta), times the Base current (β x Ib), if we assume a Beta (β) value for the transistor of say 100, (one hundred is a reasonable average value for low power signal transistors) the Base current Ib flowing into the transistor will be given as: Instead of using a separate Base bias supply, it is usual to provide the Base Bias Voltage from the main supply rail (Vcc) through a dropping resistor, R1. Resistors, R1 and R2 can now be chosen to give a suitable quiescent Base current of 45.8μA or 46μA rounded off. The current flowing through the potential divider circuit has to be large compared to the actual Base current, Ib, so that the voltage divider network is not loaded by the Base current flow.

31. M A Islam, EEE, IIUC A general rule of thumb is a value of at least 10 times Ib flowing through the resistor R2. Transistor Base/Emitter voltage, Vbe is fixed at 0.7V (silicon transistor) then this gives the value of R2 as: If the current flowing through resistor R2 is 10 times the value of the Base current, then the current flowing through resistor R1 in the divider network must be 11 times the value of the Base current. The voltage across resistor R1 is equal to Vcc – 1.7v (V RE + 0.7 for silicon transistor) which is equal to 10.3V, therefore R1 can be calculated as:

32. M A Islam, EEE, IIUC The value of the Emitter resistor, R E can be easily calculated using Ohm’s Law. The current flowing through R E is a combination of the Base current, I b and the Collector current Ic and is given as:Ohm’s Law Resistor, R E is connected between the Emitter and ground and we said previously that it has a voltage of 1 volt across it. Then the value of R E is given as: So, for our example above, the preferred values of the resistors chosen to give a tolerance of 5% (E24) are: Then, our original Common Emitter Amplifier circuit above can be rewritten to include the values of the components that we have just calculated above as drawn in the next slide.

33. M A Islam, EEE, IIUC Completed Common Emitter Circuit

34. Base Current Base current i B composed of two components:  holes injected from the base region into the emitter region  holes supplied due to recombination in the base with diffusing electrons and depends on minority carrier lifetime  b in the base And the Q in the base is So, current is Total base current is M A Islam, EEE, IIUC

35. Beta Can relate i B and i C by the following equation and  is  Beta is constant for a particular transistor  On the order of 100-200 in modern devices (but can be higher)  Called the common-emitter current gain For high current gain, want small W, low N A, high N D M A Islam, EEE, IIUC

36. Base current I b controlled physically by three dominant mechanism

37. Emitter Current Emitter current is the sum of i C and i B  is called the common-base current gain M A Islam, EEE, IIUC

38. Current flow in a pnp transistor biased to operate in the active mode. The PNP Transistor M A Islam, EEE, IIUC

39. Ebers-Moll Model

40. Introduction The bipolar junction transistor can be considered essentially as two pn junctions placed back-to-back, with the base p-type region being common to both diodes. This can be viewed as two diodes having a common third terminal as shown in Fig.1. Fig. 2.1 Bipolar Transistor Shown as Two Back-to-Back p-n Junctions

42. Fig. 2 The n-p-n Transistor Considered as Combined p-n Junctions

43. The Ebers-Moll transistor model is an attempt to create an electrical model of the device as two diodes whose currents are determined by the normal diode law but with additional transfer ratios to quantify the interdependency of the junctions as shown in Fig. 3. Two dependent current sources are used to indicate the interaction of the junctions. The interdependency is quantified by the forward and reverse transfer ratios, α F and α R. The diode currents are given as: Ebers-Moll Equations

46. The Ebers-Moll BJT Model is a good large-signal, steady-state model of the transistor and allows the state of conduction of the device to be easily determined for different modes of operation of the device. The different modes of operation are determined by the manner in which the junctions are biased. The charge profiles for each mode are shown in Fig. 4. Modes of Operation Fig. 4 Charge Profiles for Modes of Operation of n-p-n BJT

47. Fig. 5 Charge Profiles for Modes of Operation of n-p-n BJT