Bipolar Junction Transistors (BJTs)

• Transistor theory uses diode theory, operating with two, or more, diodes and extra electronics.
• Transistors amplify, control, and generate electrical signals, acting as components in microchips.
• A transistor used as a binary switch provides the fundamental building block for computer circuits.

Bipolar Junction Transistors (BJTs)

• BJTs use three doped semiconductor regions separated by two PN junctions.
• The regions are called the emitter, base, and collector.
• One type consists of two N regions separated by a P region, known as NPN.
• The other consists of two P regions separated by an N region, known as PNP.
• The PN junction joining the base and emitter is the base-emitter junction.
• The PN junction joining the base and collector is the base-collector junction.
• Wire leads connect to each of the three regions, labelled E, B, and C for emitter, base, and collector.
• The base region is lightly doped and thin, compared to the heavily doped emitter and the moderately doped collector.
• This allows current to flow from the emitter to the collector and vice versa.
• Bipolar use holes and electrons as carriers in the transistor structure.
• Mnemonics help remember circuit symbol configurations:
  • NPN emitter points outward (Never Points iN).
  • PNP emitter always points in (Points in Permanently).

Transistor Operation

• Amplification means increasing the amplitude of electrical signals.
• Circuits need sufficient power to amplify an input signal.
• Transistors are components in amplifier circuits, using a small base current to control a larger collector/emitter current.
• For a transistor to amplify properly, PN junctions must be correctly biased with external DC voltages.
• PNP operation mirrors NPN, but reverses electrons and holes, bias voltage polarities, and current directions.
• In both NPN and PNP:
  • The base-emitter (BE) junction is forward biased.
  • The base-collector (BC) junction is reverse biased.
• Each type has reversed current flow, and two DC voltage sources connected to the BJT.
• In NPN transistors, forward bias from base to emitter narrows the BE depletion region.
• Reverse bias from base to collector widens the BC depletion region.
• The N-type emitter region has conduction-band electrons that diffuse through the BE junction into the P-type base region.
• The base region is lightly doped, limiting holes.
• Most electrons diffuse into the BC depletion region.
• Electrons are pulled through the reverse-biased BC junction by the electric field.
• Electrons move through the collector region, out through the collector lead, and into the collector voltage source.
• Collector current typically exceeds the base current, making transistors appear to have current gain.

Transistor Current

• Electron flow (current) directions differ in NPN and PNP transistors; schematic symbol directions are opposites.
• The emitter current (IE) equals the sum of the collector current (IC) and the base current (IB).
• IB is comparatively small relative to IE or IC.
• Capital-letter subscripts denote DC voltage values.
• When connecting a transistor to DC bias voltages:
  • VBB forward biases the base-emitter junction.
  • VCC reverse biases the base-collector junction.
• Voltages derive from a DC power supply; VCC is often taken directly from the supply, while VBB (smaller) comes from a voltage divider.

Transistor DC Gain

• Beta (β) is the basic notation for the forward current gain.
• DC beta (βDC) is the ratio of DC collector current (IC) to DC base current (IB); measures DC current gain of a transistor.
• Typical βDC values range from below 20 to over 200, often designated as hFE on data sheets.
• The h parameter is primarily used for simulation.
• Gain (βDC) = IC / IB, which can be expressed as βDC = hFE.
• DC alpha (αDC) measures the ratio of DC collector current (IC) to DC emitter current (IE).
  • Always less than 1, as IC is slightly less than IE by the amount of IB.
  • αDC is expressed as IC / IE.
• When IE = 100 mA and IB = 1 mA, then IC = 99 mA, and αDC is 0.99.

Transistor Currents and Voltages

• Three transistor DC currents: IB (base), IE (emitter), IC (collector).
• Three DC voltages: VBE (base to emitter), VCB (collector to base), VCE (collector to emitter).

Transistor Characteristic Curve

• Collector characteristic curves show how collector current (IC) varies with collector-to-emitter voltage (VCE) for different base current (IB) values.
• Both VBB and VCC are variable sources of voltage during testing.
• The characteristic zones include resistive, active, and breakdown regions described by the points A, B, and C.
• Assume VBB produces a specific IB and VCC is zero: Ic is zero (point A on graph).
• In this condition, both the base-emitter and base-collector junctions are forward biased.
• The base is around 0.7 V, while the emitter and the collector are at 0 V. The base current goes to ground via the base-emitter junction, so IC is zero.

Saturation Region

• Point A to Point B*
• Transistor operates with both junctions forward biased.
• As VCC increases, VCE increases gradually, causing the collector current to increase.
• IC increases as VCC increases because VCE remains below 0.7 V; due to forward-biased base-collector junction.
• The transistor acts as a switch, either on or off.
• As a saturated switch, base current is typically excessive.

Active/Linear Region

• Point B to Point C*
• VCE exceeds 0.7 V, the base-collector junction becomes reverse biased.
• Once the base-collector junction is reverse biased, IC levels off.
• It remains relatively constant for a specified IB as VCE increases.
• IC increases slightly as VCE increases as base-collector depletion region widens.
• Fewer holes for recombination in the base region causes a slight increase in βDC, as portrayed by the portion of the curve between B and C.
• Value of IC in this portion relies only on the relationship IC = βDCIB.
• The active region serves as the transistor's amplification zone, as increasing IC only requires raising the base current IB.

Breakdown Region

• Point C* VCE reaches a high voltage, and the reverse-biased base-collector junction breaks down and collector current increases rapidly.

Additional Notes

• Transistors should not operate in the breakdown region.
• The family of collector characteristic curves is generated by graphing IC versus VCE for different IB values.
• With IB = 0, the transistor is in cut-off, with minor collector leakage current.

Additional Regions

• In cut-off regions with base lead open (IB = 0), there is minor collector leakage current, ICEO, that results from thermally produced carriers. Since ICEO is small, it can be neglected in circuit analysis, meaning VCE = VCC. Both the base-emitter and base-collector junctions are reverse biased.
• In the saturation region, it occurs when the base-emitter junction is forward biased, causing the collector current to raise (IC = βDCIB), and VCE decreases owing to higher voltage drop along the collector resistor (VCE = VCC - ICRC). Once VCE hits its saturation point (VCE(sat)), the base-collector junction becomes forward biased. In this state, a connection relating to IC = βDCIB becomes invalid. VC(sat) for a transistor typically falls below the elbow, measuring around a tenth of a volt, in silicon transistors.

DC Load Line

• Illustrates cut-off and saturation relative to collector characteristic curves.
• The line connects the cut-off and saturation points on the curve.
• At ideal cut-off: IC = 0, VCE = VCC.
• At saturation: IC = IC(sat), VCE = VCE(sat).
• The range between cut-off and saturation is the transistor's active region.

Temperature effect

• βDC or hFE serve as parameters, though βDC does not remain constant due to alterations in collector current and temperature.
• If rising IC maintains a steady junction temperature, then βDC goes up, eventually declining post-peak.
• If temperature fluctuates with a constant IC, βDC shifts in sync (temperature up, then βDC goes up, and vice versa).
• Data sheets list BDC (HFE) at set IC levels.
• Inconsistencies in construction often cause each transistor’s device variance, though fixed operating levels of both IC and temperature come into play. BDC at a stipulated IC measures typically its value at minimum BDC(min); and typical and maximum values can, too, get specified.

Transistor Data Sheet

• Data on the 2N3903 and 2N3904 NPN transistors notes that the maximum collector-emitter voltage (VCEO) is 40 V.
  • VCEO derives from the measurement from the collector (C) to the emitter (E) where the base remains open (0).
  • Clarified as VCE(max) in documentation, and its maximum collector current should measure at 200 mA for design needs.
• BDC (HFE) is designated to several values of IC, where levels differ with values of the IC.
• The collector-emitter saturation voltage, VCE(sat), measures at 0.2 V at IC(sat)= 10 mA, rising to higher levels at peak current usage.

Transistor Amplification

• The collector current equals the base current multiplied by the current gain (β).
• Collector current usually exceeds the base current.
• The collector current roughly equals the emitter current.
• AC voltage (Vin) combines with a DC bias voltage (VBB) in series with the base resistor (RB).
• The DC bias voltage (VCC) is connected to the collector across the collector resistor (RC).
• The AC input voltage spawns an AC base current, leading to a magnified AC collector current.
• The AC collector current generates a voltage across Rc, rendering amplified, yet inverted, results of the AC input voltage while operating along the active region.
• The forward-biased base-emitter junction displays a low resistance level towards the AC signal where, internally, it holds the element/emitter resistance at r(e).

Transistor as a Switch

• In its basic operation as a switching device:
  • Transistors in cut-off because no forward-bias connects it to the base-emitter junction.
  • It creates opens across the collector and emitter, where their equivalent measures come from switch levels.
  • When saturation (bias, emitter, base, plus, for both forward direction plus greater base current to initiate saturation of collector).
• The levels create shorts at the collector with its emitter from its equivalent values. A voltage decline measuring tenths forms with each short at designated voltages known as VCE(sat).

Example Calculation

• Used with an RB for switching a 5V DC circuit through transistors for driving a 12V DC 170-Ω circuit breaker.
• Start with a datasheet for a 2N2222A transistor.
• Then with your datasheet, it's time to measure (IC) that would pass to the load (relay).
• Take a look at your datasheet for a minimum hFE as it would prevent some problems if you do acquire at levels of great capacity.
• After collecting the information, calculating "IB" would come next, assuming the following data:
  • Ic = (12 V)/(170 Ω) = 70 mA
  • hFE = 110
  • IB = Ic / hFE = (70 mA) / 110 = 0.64 mA
• Calculating the VDROP across RB (5 V – 0.7 V measured from transistor) for obtaining, with 4.3 V drop, via RB with levels at 0.64 mA.
• RB equals the following formula: R``B = (4.3 V) / (0.64 × 10-3 A) = 6.718 kΩ ≈ 6.8 kΩ
• By using these levels alongside 6.8 kΩ ,0.7-V DC , the meter reads IB at 0.63 mA where hfe =110 so with; IC = IB × hFE = 0.63 × 10-3 × 110 = 69.3, mA (More than adequate to trigger the relay using at base).

Transistor Categories

• Manufacturers organize bipolar junction transistors into:
  • General purpose/small signal devices
  • Power Devices
  • RF (radio frequency/microwave) devices.

General-Purpose/Small-Signal Transistors

• Used primarily for low or medium power amplification and as a source as switches.
• Uses either plastic or metal designs.
• Certain types can house multiple transistors.

Power Transistors

• Uses larger level currents that registers typically at 1A or over.
• Its audio has a large application setting, for use as parts towards amplifiers in speakers.
• Most come with metallic plates or metallic designs which can connect along the device for using heatsinks.
• RF Transistors: Are designed to operate as high frequency levels to use via systems such as for the most part communications, with different configurations.

Transistor Testing

• Faults in a basic transistor circuit include:
  • Open bias resistors.
  • Resistive connections.
  • Shorted connections.
  • Opens or shorts in the transistor itself.

Notes

• A basic transistor circuit has VBB = 3 V and VCC = 9 V.
• If βDC = 200: the data sheet provides minimum and maximum hFE values for the 2N3904.
• Troubleshooting shows symptoms regarding incorrect voltage levels.
• "Floating point" indicates points/parts open on the circuit plus a voltage.
• Minor "Floating points" (µV to low mV range).
• Schematics provided reveal possible issues; faults may not be restricted to what gets given here.
• DMMs test transistors practically, with "diode" settings to check components, with about a (2.5 V - 3.5 V) that marks the mark the level setting which the levels can test the device in both regular, (reverse biased).
• The meter calculates junction states by delivering a voltage readout.

Transistors in Normal Bias.

• To conduct, the transistor is to be set with voltage, with around (0.5 V - 0.9 V) for forward direction for typicality.

Defective Transistors

• As junction fails/shorts with no internal connection, it reveals 2.6 V results as consistent output from DMM which signifies forward biases.
• Shorts produce measurements at 0 V during the tests.
• Damaged components might undergo shorts and display slight resistance. With, a tester might read levels beneath normal (1.1 V) opposed from 0 V, or 2.6, via proper ratings.