Transistors BJT FET Notes FE Sem 1 BEE | Mumbai University

Module 6 introduces the transistor, the three-terminal semiconductor device on which all modern electronics is built. It explains the structure and operation of the bipolar junction transistor (BJT), its common emitter configuration and characteristics, and the structure and operation of the field effect transistor (FET). It ends with the three main uses of BJTs and FETs: amplification, switching and oscillators. The syllabus states that numericals are not expected, so concentrate on structure diagrams, working, characteristics and comparisons.

Summary

Structure of BJT: a bipolar junction transistor has three doped regions forming two PN junctions: emitter, base and collector. In an NPN transistor a thin, lightly doped P-type base is sandwiched between an N-type emitter and N-type collector; a PNP transistor has the opposite arrangement. The emitter is heavily doped (to inject many carriers), the base is very thin and lightly doped (so few carriers recombine there), and the collector is moderately doped and largest in size (to collect carriers and dissipate heat). The two junctions are the emitter-base junction (JE) and the collector-base junction (JC). In the symbol the arrow on the emitter shows conventional current direction: outward for NPN, inward for PNP.

Operation of BJT: in the active region, JE is forward biased and JC is reverse biased. In an NPN transistor, the forward bias makes the emitter inject a large number of electrons into the base. Because the base is thin and lightly doped, only a small fraction (typically less than 5 percent) recombine with holes to form the base current IB; the rest diffuse to the collector junction and are swept into the collector by its reverse bias, forming IC. Hence IE = IB + IC, IC = alpha IE (alpha about 0.95 to 0.995) and IC = beta IB (beta typically 50 to 300). A small base current thus controls a large collector current. Both electrons and holes take part, hence the name bipolar. The other regions are cut-off (both junctions reverse biased, transistor OFF) and saturation (both junctions forward biased, transistor fully ON).

Common emitter (CE) configuration: the emitter is common to input and output; input is applied between base and emitter and output taken between collector and emitter. Input characteristics (IB versus VBE at constant VCE) resemble a forward-biased diode curve, with conduction beginning at about 0.7 V for silicon. Output characteristics (IC versus VCE at constant IB) show a saturation region near the vertical axis (VCE below about 0.2 V), an active region where IC is nearly constant at about beta IB, and a cut-off region below the IB = 0 curve. CE has high current gain (beta), high voltage and power gain, moderate input resistance, moderate to high output resistance and a 180 degree phase shift between input and output voltage, which is why it is the most widely used configuration.

FET structure: a field effect transistor is a unipolar, voltage-controlled device in which current is carried by majority carriers only. In an N-channel JFET, a bar of N-type silicon forms the channel, with ohmic contacts at its ends called source (S, where carriers enter) and drain (D, where they leave). Heavily doped P-type regions diffused on both sides of the channel are connected together to form the gate (G). The gate-channel PN junctions are kept reverse biased. A P-channel JFET has a P-type channel and N-type gate. In the symbol the gate arrow points inward for N-channel and outward for P-channel. A MOSFET is the related type in which the gate is insulated from the channel by a thin silicon dioxide layer, giving even higher input impedance.

FET operation: with VGS = 0 and a positive VDS, electrons flow from source to drain through the channel. Increasing VDS increases ID linearly at first (ohmic region); the depletion layers widen near the drain end until the channel is pinched off at VDS = VP, after which ID stays nearly constant at IDSS (saturation or pinch-off region). When the gate is made negative (VGS reverse bias), the depletion layers widen and the channel narrows, reducing ID; at VGS = VGS(off) = -VP, the channel is completely closed and ID = 0. In saturation, the transfer characteristic follows Shockley's equation ID = IDSS(1 - VGS/VGS(off))^2. Because the gate junction is reverse biased, gate current is practically zero and the input resistance is very high (megaohms). The control parameter is the transconductance gm = delta ID/delta VGS.

BJT and FET as amplifiers: when biased in the active region (BJT) or the saturation/pinch-off region (FET), a small AC signal at the input (base or gate) produces a larger variation of output current, which through the collector or drain load resistor gives an amplified output voltage. A CE amplifier gives voltage gain of the order of a hundred with 180 degree phase reversal; the corresponding FET amplifier is the common source amplifier, which offers very high input impedance and lower noise but generally lower gain. Uses: audio amplifiers, pre-amplifiers in radios and microphones, instrumentation front-ends.

BJT and FET as switches: a BJT switch is driven between cut-off (IB = 0, IC about 0, VCE = VCC, switch OFF) and saturation (IB large enough that IC = VCC/RC, VCE about 0.2 V, switch ON). A MOSFET switch is driven between VGS below threshold (OFF, very high resistance) and VGS well above threshold (ON, low on-resistance). Uses: driving relays, LEDs, lamps and motors from microcontroller outputs, digital logic gates, SMPS and inverters. FET (MOSFET) switches need negligible gate drive current and are preferred in digital ICs and high-frequency power converters.

BJT and FET in oscillators: an oscillator is an amplifier with positive feedback that produces a sustained sinusoidal output without any input signal, converting DC power into AC. Oscillations are sustained when the Barkhausen criterion is met: the loop gain A beta = 1 (magnitude) and the total phase shift around the loop is 0 or 360 degrees. In an RC phase-shift oscillator, the CE amplifier provides 180 degrees and three RC sections provide another 180 degrees. LC oscillators such as Hartley and Colpitts use a tank circuit to set the frequency; crystal oscillators give very stable frequency. Both BJTs and FETs are used as the active amplifying device. Uses: signal generators, radio transmitters and receivers, clocks in digital systems.

Mind map

Current flow in an NPN transistor (active region)

Important definitions

Formulas

Key points to remember

• Numericals are not expected in this module; structure, operation, CE characteristics, FET working and applications are asked.
• Emitter: heavily doped; base: thin and lightly doped; collector: moderately doped and largest.
• For amplification, the BJT must be in the active region: emitter junction forward biased, collector junction reverse biased.
• BJT is current controlled and bipolar; FET is voltage controlled and unipolar.
• In CE configuration output voltage is 180 degrees out of phase with the input.
• In small-signal transistors IB is typically in microamperes while IC and IE are in milliamperes.
• In a JFET the gate-channel junction is always reverse biased, so gate current is nearly zero and input resistance is very high.
• Beyond pinch-off, JFET drain current stays nearly constant; this is the region used for amplification.
• For switching, the transistor operates only in cut-off (OFF) and saturation (ON).
• An oscillator needs positive feedback, a frequency-determining network and an amplifier with enough gain to satisfy A beta = 1.
• The arrow in a transistor symbol is always on the emitter (BJT) or gate (JFET) and points in the direction of conventional current (P to N).

Solved examples

Example 1. (Illustrative, numericals not asked in the MU paper.) In an NPN transistor the base current is 40 microampere and beta = 100. Find IC, IE and alpha.

IC = beta IB = 100 x 40 microampere = 4 mA.

IE = IB + IC = 0.04 + 4 = 4.04 mA.

alpha = IC/IE = 4/4.04 = 0.990 (check: beta/(1 + beta) = 100/101 = 0.990).

Example 2. (Illustrative.) A transistor has alpha = 0.98. Find beta. If the emitter current is 2 mA, find IC and IB.

beta = alpha/(1 - alpha) = 0.98/0.02 = 49.

IC = alpha IE = 0.98 x 2 = 1.96 mA.

IB = IE - IC = 2 - 1.96 = 0.04 mA = 40 microampere (check: IC/IB = 1.96/0.04 = 49).

Example 3. (Illustrative.) An NPN transistor switch drives a load of RC = 1 kilo-ohm from VCC = 5 V. With beta = 100 and VCE(sat) = 0.2 V, find the collector current when ON and the minimum base current needed for saturation.

IC(sat) = (VCC - VCE(sat))/RC = (5 - 0.2)/1000 = 4.8 mA.

Minimum IB = IC(sat)/beta = 4.8 mA/100 = 48 microampere.

In practice IB is made two to five times larger (for example about 0.2 mA) so that the transistor stays saturated despite variation in beta. When IB = 0 the transistor is cut off, IC is about 0 and VCE = 5 V.

Example 4. (Illustrative.) An N-channel JFET has IDSS = 10 mA and VGS(off) = -4 V. Find the drain current at VGS = 0, -2 V and -4 V.

Using ID = IDSS (1 - VGS/VGS(off))^2:

At VGS = 0: ID = 10 (1 - 0)^2 = 10 mA (= IDSS).

At VGS = -2 V: ID = 10 (1 - (-2)/(-4))^2 = 10 (0.5)^2 = 2.5 mA.

At VGS = -4 V: ID = 10 (1 - 1)^2 = 0 (channel pinched off).

This shows that a negative gate voltage controls the drain current.

Example 5. (Illustrative.) An amplifier with gain A = 50 is used in an oscillator. What feedback fraction is needed to sustain oscillations, and what phase shift must the feedback network provide if the amplifier is a single-stage CE amplifier?

Barkhausen criterion: A beta = 1, so beta = 1/A = 1/50 = 0.02 (in practice A beta is kept slightly above 1 at start-up so oscillations build up).

A single-stage CE amplifier gives 180 degrees phase shift, so the feedback network must provide another 180 degrees, making the total loop phase shift 360 degrees. In the RC phase-shift oscillator this is done by three RC sections of about 60 degrees each.

Important questions and answers

Q1. Explain the structure of an NPN and a PNP transistor with symbols. (4 marks)

A BJT has three regions, emitter, base and collector, forming two junctions: emitter-base (JE) and collector-base (JC).

NPN: a thin P-type base between N-type emitter and N-type collector. PNP: a thin N-type base between P-type emitter and P-type collector.

Doping and size: emitter heavily doped to supply carriers; base very thin (a few micrometres) and lightly doped so that most carriers pass through it; collector moderately doped and largest in area to collect carriers and dissipate heat.

Symbols: three terminals E, B, C; the arrow on the emitter points outward for NPN and inward for PNP, showing the direction of conventional emitter current.

Diagrams of the layer structure and symbols should be drawn.

Q2. Explain the operation of an NPN transistor. (6 marks)

For normal (active) operation, the emitter-base junction is forward biased by VBE and the collector-base junction is reverse biased by VCB.

The forward bias reduces the potential barrier at JE; the heavily doped N emitter injects a large number of electrons into the base, constituting the emitter current IE.

The base is very thin and lightly doped, so only a small fraction of these electrons (less than about 5 percent) recombine with holes in the base. The holes lost by recombination are replaced by the external base current IB, which is small (microamperes).

The remaining electrons diffuse across the base and reach JC, where the strong field of the reverse bias sweeps them into the collector, forming IC.

Thus IE = IB + IC, IC = alpha IE (alpha about 0.95 to 0.995), and IC = beta IB. A small change in base current causes a large change in collector current, which is the basis of amplification. A small leakage current ICBO also flows due to minority carriers in the reverse-biased collector junction. Diagram with biasing batteries and current directions should be drawn.

Q3. Draw and explain the input and output characteristics of a transistor in common emitter configuration. (6 marks)

Circuit: NPN transistor with emitter common; VBB and RB supply the base, VCC and RC the collector; meters measure IB, VBE, IC and VCE.

Input characteristics (IB versus VBE at constant VCE): similar to a forward-biased diode; IB is negligible until VBE reaches the cut-in voltage (about 0.5 to 0.7 V for silicon) and then rises rapidly. Increasing VCE shifts the curve slightly to the right because of base-width modulation (Early effect). Input resistance = delta VBE/delta IB, moderate (about 1 kilo-ohm).

Output characteristics (IC versus VCE for different constant IB): for each IB, IC rises steeply for very small VCE and then becomes nearly flat. Three regions: (1) saturation region, VCE below about 0.2 V, both junctions forward biased; (2) active region, the flat portion, where IC is about beta IB and increases only slightly with VCE; used for amplification; (3) cut-off region, below the IB = 0 curve, where only a small leakage current ICEO flows. Output resistance = delta VCE/delta IC, high (tens of kilo-ohms).

Q4. Why is the common emitter configuration most widely used? (3 marks)

It provides high current gain (beta, 50 to 300), high voltage gain and the highest power gain of the three configurations. Its input resistance is moderate and output resistance moderately high, so CE stages can be cascaded conveniently. It needs only one supply in practical biasing circuits. It provides 180 degree phase inversion, useful in amplifiers and oscillators. Hence it is used in most amplifier and switching circuits.

Q5. Explain the construction and working of an N-channel JFET. (6 marks)

Construction: a bar of lightly doped N-type silicon forms the channel. Ohmic contacts at its two ends are the source (S) and drain (D). Two heavily doped P-type regions are diffused on opposite sides of the bar and internally connected to form the gate (G). PN junctions are thus formed between gate and channel, with depletion layers extending mainly into the lightly doped channel.

Working: (1) VGS = 0, VDS increased from zero: electrons flow from source to drain and ID increases linearly (ohmic region). Because the drain end is more positive, the gate-channel junction is more reverse biased there and the depletion layers are wider near the drain. At VDS = VP the depletion layers almost touch (pinch-off) and ID becomes nearly constant at IDSS (saturation region). At very high VDS, breakdown occurs.

(2) Negative VGS: the gate-channel junctions are more reverse biased, the depletion layers widen and the channel narrows, so pinch-off occurs at lower VDS and the saturation current is less than IDSS. At VGS = VGS(off), the channel is completely closed and ID = 0.

Thus drain current is controlled by gate voltage; gate current is practically zero, giving very high input resistance. Drain and transfer characteristics should be drawn.

Q6. Compare BJT and FET. (5 marks)

Control: BJT is current controlled (IB controls IC); FET is voltage controlled (VGS controls ID).

Carriers: BJT is bipolar (electrons and holes); FET is unipolar (majority carriers only).

Input impedance: BJT low to moderate (kilo-ohms); FET very high (megaohms for JFET, higher for MOSFET).

Noise: BJT more noisy; FET less noisy.

Thermal stability: BJT less stable, risk of thermal runaway; FET more stable (negative temperature coefficient of drain current at high currents).

Gain: BJT has higher voltage gain; FET has lower voltage gain.

Size and fabrication: FET (especially MOSFET) is smaller and easier to fabricate in ICs.

Terminals: emitter, base, collector versus source, gate, drain.

Applications: BJT in audio and power amplifiers; FET in high input impedance amplifiers, digital ICs and switching.

Q7. Explain how a transistor can be used as a switch. (5 marks)

Circuit: NPN transistor in CE mode with load (for example a relay, LED or lamp) as RC between VCC and collector; input applied to the base through RB.

OFF state: when the input is low (0 V), IB = 0; both junctions are reverse biased and the transistor is in cut-off. IC is nearly zero, VCE = VCC; the transistor behaves like an open switch and the load is off.

ON state: when the input is high, a base current large enough to drive the transistor into saturation flows (IB greater than IC(sat)/beta). Both junctions are forward biased, VCE(sat) is about 0.2 V and IC = (VCC - VCE(sat))/RC; the transistor behaves like a closed switch and the load is on.

The transistor is never left in the active region in switching applications, which keeps power dissipation low. A MOSFET switch works similarly, driven OFF and ON by the gate voltage with negligible gate current. Applications: relay and motor drivers, LED drivers, logic gates (NOT gate), SMPS and inverters. A diode across a relay coil protects the transistor from inductive voltage spikes.

Q8. Explain the application of a BJT as an amplifier. (5 marks)

A CE amplifier uses an NPN transistor biased in the active region by a voltage-divider network (R1, R2), with collector resistor RC, emitter resistor RE (bypassed by capacitor CE) and coupling capacitors at input and output.

The input AC signal is superimposed on the DC bias between base and emitter. During the positive half cycle, VBE increases, IB increases, IC increases by beta times, the drop IC RC increases and VCE (the output) decreases. During the negative half cycle the opposite occurs. Thus a small input signal produces a large output voltage variation of opposite phase (180 degrees).

Voltage gain Av = output voltage/input voltage is typically of the order of 100 or more. Proper biasing ensures that the operating point stays in the active region so that the output is undistorted. The FET equivalent is the common source amplifier, which offers very high input impedance. Applications: audio amplifiers, radio receivers, pre-amplifiers for microphones and sensors.

Q9. What is an oscillator? Explain the Barkhausen criterion and the use of transistors in oscillators. (5 marks)

An oscillator is an electronic circuit that generates a continuous periodic waveform (usually sinusoidal) without any external input signal, converting DC energy into AC.

It consists of an amplifier (BJT or FET), a feedback network that feeds a fraction beta of the output back to the input in phase (positive feedback), and a frequency-determining network (RC, LC or crystal).

Barkhausen criterion: oscillations are sustained when (1) the loop gain magnitude A beta = 1, and (2) the total phase shift around the loop is 0 or 360 degrees. At start-up A beta is kept slightly greater than 1 so that noise builds up into oscillations, which then stabilise.

Transistor oscillators: in an RC phase-shift oscillator, a CE BJT (or common source FET) amplifier gives 180 degrees and three RC sections give 180 degrees. Hartley and Colpitts oscillators use LC tank circuits for radio frequencies, and crystal oscillators provide highly stable frequency for clocks and communication. Applications: signal generators, radio and TV transmitters and receivers, clocks of computers and microcontrollers.

Q10. State the relation between alpha and beta. (3 marks)

alpha = IC/IE and beta = IC/IB. Since IE = IB + IC, divide by IC: 1/alpha = IB/IC + 1 = 1/beta + 1. Hence alpha = beta/(1 + beta) and beta = alpha/(1 - alpha). Because alpha is close to 1 (for example 0.99), beta is large (99). A small change in alpha causes a large change in beta.

Exam tips

• Draw the layer structure and symbols of NPN, PNP and N-channel JFET with arrows in the correct direction; wrong arrows lose marks.
• Learn the doping and size of emitter, base and collector with the reason for each.
• For CE characteristics, draw both graphs with axes labelled and mark saturation, active and cut-off regions.
• Prepare a BJT versus FET comparison table of at least six points.
• For applications, give a circuit sketch and a short explanation for each of amplifier, switch and oscillator.
• Only the common emitter configuration is in the syllabus; do not waste time on CB and CC characteristics.

FAQs

Which BJT configuration is in the FE BEE syllabus?

Only the common emitter configuration. You should know its circuit, input and output characteristics, regions of operation and why it is preferred.

Why is the base of a BJT thin and lightly doped?

So that very few carriers injected from the emitter recombine in the base; most of them reach the collector. This makes the base current small and the current gain high.

Why does a FET have very high input impedance?

In a JFET the gate-channel junction is reverse biased, and in a MOSFET the gate is insulated by an oxide layer, so practically no gate current flows.

Are numericals asked on transistors in FE BEE?

No. The MU syllabus states numericals are not expected; questions are on structure, operation, CE characteristics, FET working and applications.

These notes follow the University of Mumbai first-year engineering syllabus (NEP 2020, Basic Electrical & Electronics Engineering) and were prepared with AI help and checked.