LECTURE 7 BIPOLAR JUNCTION TRANSISTOR BJT PREVIEW The

LECTURE 7 BIPOLAR JUNCTION TRANSISTOR (BJT)

PREVIEW Ø The transistor is a multifunction semiconductor device Ø The transistor is integrated with other curicuit element for voltage gain, current gain or signal power gain. Ø It’s known as an active device (apply bias) Ø Application : high speed circuit, analog circuit and power application. Ø Three type of transistor : a) bipolar transistor (BJT) b) metal-oxide-semiconductor field-effect transistor (MOSFET) c) junction field-effect transistor (JFET).

INTRODUCTION Ø Bipolar junction transistor (BJT) –used extensively in high-speed circuits, analog circuits and power applications Ø Both electrons & holes participate in the conduction process Ø Was invented by a research team at Bell Lab in 1947 Ø Modern bipolar transistors –replaced the germanium with Si& replaced the point contacts with two closely coupled p-njunctions in the form of p-n-p & n-p-n structures Ø Sections to be discussed in this chapter: • The current gain & modes of operation of bipolar transistors • The cutoff frequency and switching time of a bipolar transistor • The advantages of heterojunction bipolar transistor • The power handling capability of thyristor and related bipolar devices

DMT 234 Semiconductor Physic & Device The term Bipolar is because two type of charges (electrons and holes) are involved in the flow of electricity The term Junction is because there are two p-n junctions There are two configurations for this device

DMT 234 Semiconductor Physic & Device NPN AND PNP TRANSISTORS NPN is more widely used Majority carriers are electrons so it operates more quickly PNP is used for special applications The terminals of the transistor are labeled (Base, Emitter, and Collector) The emitter is always drawn with the arrow.

DMT 234 Semiconductor Physic & Device Figure 7 -1. Perspective view of a silicon p-n-p bipolar transistor.

DMT 234 Semiconductor Physic & Device DIFFERENCES BETWEEN NPN & PNP Type of BJT PNP-Type NPN-Type 1 If the base is at a lower voltage than the emitter, current flows from emitter to collector If the base is at a higher voltage than the emitter, current flows from collector to emitter. 2 Small amount of current also flows from emitter to base. Small amount of current also flows from base to emitter. 3 Emitter is heavily p-doped compared to collector. So, emitter and collector are not interchangeable. Emitter is heavily N-doped compared to collector. So, emitter and collector are not interchangeable. 4 The base width is small compared to the minority carrier diffusion length. If the base is much larger, then this will behave like back-to-back diodes. 5 Voltage at base controls amount of current flow through transistor (emitter to collector). Voltage at base controls amount of current flow through transistor (collector to emitter). Follow the arrow to see the direction of current flow 6 7

DMT 234 Semiconductor Physic & Device Active mode: • E-B junction is forward biased (VEB>0) • B-C junction is reverse biased (VCB>0) Figure (a) Idealized one-dimensional schematic of a p-n-pbipolar transistor and (b) its circuit symbol. (c) Idealized one-dimensional schematic of an n-p-nbipolar transistor and (d) its circuit symbol.

DMT 234 Semiconductor Physic & Device

DMT 234 Semiconductor Physic & Device 7. 2 Transistor action The following transistor theory is developed by considering the npn transistor [the same basic principles also apply to pnp device, we need only to reverse the polarities and conduction type] Forward active mode: • The B-E pn junction is forward , the B-C pn junction is reverse-biased Fig: Biasing of an npn bipolar transistor in the forward-active mode

DMT 234 Semiconductor Physic & Device Fig: Minority carrier distribution in an npn bipolar transistor in the forward-active mode • The B-E pn junction is forward biased (so electrons from the emitter are injected across the B-E junction into the base) while the B-C pn junction is reverse-biased (so the minority carrier electron concentration at the edge of B-C junction is ideally zero) • The width of the base needs to be small compared with minority carrier diffusion length (so that as many electrons as possible to reach the collector w/o recombining with any majority carrier holes in the base)

DMT 234 Semiconductor Physic & Device 7. 3 Modes operation of Bipolar Transistor. Active mode: • E-B junction is forward biased, B-C junction is reverse -biased Saturation mode: • both junctions are forward biased • corresponds to small biasing V and large output I – transistor is in a conducting state & acts as a closed (or on) switch Cutoff mode: • both junctions are reverse-biased • corresponds to the open (or off) switch Inverted mode: • inverted active mode • E-B junction is reverse-biased, C-B junction is forward biased

DMT 234 Semiconductor Physic & Device 7. 4 Bias Mode BIASING MODE BIASING POLARITY E-B JUNCTION BIASING POLARITY C-B JUNCTION Saturation Forward Active Forward Reverse Inverted Reverse Forward Cutoff Reverse

DMT 234 Semiconductor Physic & Device • A bipolar transistor has four modes of operation, depending on the voltage polarities on the emitter-base junction and the collector-base junction. VCB Cutoff Forward active VBE Inverse active Saturation Figure: Junction voltage conditions for the four operating modes of a npn bipolar transistor

DMT 234 Semiconductor Physic & Device 7. 5 Notation use for bilopar analysis Mnorhafiz 2011

DMT 234 Semiconductor Physic & Device 7. 6 minority carrier distribution § In calculating currents in the bipolar transistor, we need to determine the minority carrier diffusion. §Since diffusion currents are produced by minority carrier gradients, we need to determine the steady-state minority carrier distribution in each of the three transistor regions. 1. Forward-active mode Figure: Minority carrier distribution in an npn bipolar transistor operating in the forward-active mode

DMT 234 Semiconductor Physic & Device Notation Definition p. E 0, n. B 0, p. C 0 Thermal equilibrium minority carrier conc. in the emitter, base and collector respectively p. E(x’), n. B(x), p. C(x”) Steady-state minority carrier conc. in the emitter, base and collector respectively Fig: Minority carrier distribution in an npn bipolar transistor operating in the forward-active mode • In the forward active mode, the B-E junction is forward-biased and the B-C junction is reverse-biased • As there are two n-regions, there will be minority carrier holes in both emitter and collector

DMT 234 Semiconductor Physic & Device (a) Base region The excess minority carrier electron concentration in the base region is given as: Using the approximation that sinh(x) ~ x for x<<1, the excess electron concentration in the base is given by:

DMT 234 Semiconductor Physic & Device (b) Emitter region The excess minority carrier hole concentration is given by: This excess concentration will vary approximately linearly with distance if x E is small. Thus:

DMT 234 Semiconductor Physic & Device (c) Collector region The excess minority carrier hole concentration in the collector is given by: The result is exactly what we expect from the results of a reverse-biased pn junction.

DMT 234 Semiconductor Physic & Device 2. Cutoff mode Figure: Minority carrier distribution in an npn bipolar transistor operating in cutoff • In cutoff, both the B-E and B-C junction is reverse-biased; thus the minority carrier conc. are zero at each space charge edge • The emitter and collector regions are assumed to be “long” in this figure, while the base is narrow compared with the minority carrier diffusion length, X B << LB, essentially all minority carriers are swept out of the base region

DMT 234 Semiconductor Physic & Device 3. Saturation mode Figure: Minority carrier distribution in an npn bipolar transistor operating in saturation • In cutoff, both the B-E and B-C junction is forward-biased; thus excess minority carriers exist at the edge of each space charge region • Since a collector current still exists when the transistor is in saturation, a gradient will still exist in the minority carrier electron conc. In the base

DMT 234 Semiconductor Physic & Device 4. Inverse-active mode Figure: Minority carrier distribution in an npn bipolar transistor operating in inverse-active • In cutoff, both the B-E is reverse-biased and B-C junction is forward-biased; thus electrons from the collector are now injected into the base. • The gradient in the minority carrier electron conc. In the base is in the opposite directions compared with the forward active mode, so the collector and emitter currents will change direction

DMT 234 Semiconductor Physic & Device Current Gain Fig: Minority carrier distribution in an npn bipolar transistor operating in the forwardactive mode = Figure: Current density components in an npn bipolar transistor operating in the forward-active mode

DMT 234 Semiconductor Physic & Device • JRB , Jp. E and JR are B-E junction currents only; do not contribute to IC • Jpc 0 and JG are B-C junction currents only; do not contribute to the transistor action or current gain Notat ion Definition Jn. E Due to the diffusion of minority carrier electrons in the base at x=0 Jn. C Due to the diffusion of minority carrier electrons in the base at x=x. B JRB The difference between Jn. E and Jn. C , which is due to the recombination of excess minority carrier electrons with majority carrier holes in the base. JRB current is the flow of holes into the base to replace the holes lost by recombination Jp. E Due to the diffusion of minority carrier holes in the emitter at x’=0 JR Due to the recombination of carriers in the forward biased B-E junction Jpc 0 Due to the diffusion of minority carrier holes in the collector at x”=0 JG Due to the generation of carriers in the reverse-biased B-C junction

DMT 234 Semiconductor Physic & Device The dc common-base current gain is defined as: If we assume that the active cross-sectional area is the same for the collector and emitter, then 0 can be re-written in terms of current densities: or where

DMT 234 Semiconductor Physic & Device We now wish to determine each of the gain factors in terms of the electrical and geometrical parameters of the transistors: 1. Emitter Injection Efficiency Factor: If we assume that all parameters above except p. E 0 and n. B 0 are fixed, then in order for 1, we must have p. E 0 << n. B 0. We can write: where NE and NB are the impurity doping conc. In the emitter and base respectively

DMT 234 Semiconductor Physic & Device 2. Base Transport Factor: The base transport factor T will be close to one if x. B << LB.

DMT 234 Semiconductor Physic & Device 3. Recombination Factor: where

DMT 234 Semiconductor Physic & Device 4. Current Gain (a) Common base current gain: (b) Common emitter current gain:

DMT 234 Semiconductor Physic & Device Question : Plot the junction voltage conditions for the four operating modes for a pnp bipolar transistor in the following graph.

DMT 234 Semiconductor Physic & Device Question : Consider a silicon npn bipolar transistor with emitter and base regions uniformly doped at concentrations of 1018 cm-3 and 1016 cm-3, respectively. A forward bias B-E voltage of VBE = 0. 610 V is applied. The neutral emitter width is x. E =4 m. Calculate the excess minority carrier concentrations in the emitter at (a) x’ = 0 (b) x’ = x. E/2 [Answer: 3. 808 x 1012 cm-3, 1. 689 x 1012 cm-3]

DMT 234 Semiconductor Physic & Device Question : If the emitter doping concentrations is NE = 5 x 1018 cm-3, find the base doping concentrations such that the emitter injection efficiency is =0. 9950. Assume XE = 2 XB = 2 um. [Answer: NB = 1. 03 x 1016 cm-3]

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DMT 234 Semiconductor Physic & Device Next Topic : Fundamental of the metal oxide semiconductor field effect transistor.