BJT

In this article, we will explore the transistor - one of the revolutionary electronic components that changed everything. It is the core element, indispensable in modern electronic circuits. 

What is a Transistor?

A transistor is an active semiconductor device, commonly used as an amplifier or an electronic switch.

The name “transistor” is a combination of two English words: “Transfer” and “Resistor”, and it was coined by scientist John R. Pierce in 1948. The name reflects its function: amplifying by transferring resistance.

In simple terms, a transistor uses a small signal applied at one terminal to control a larger signal at another terminal, or it is used to switch a signal on or off as it passes through.

Later:

• Bardeen left Bell Labs to become an academic (he went on to achieve further success by studying superconductivity at the University of Illinois).

• Brattain stayed for a while before retiring to become a teacher.

• Shockley founded his own semiconductor company, helping to inspire the rise of what we now call Silicon Valley. Two of his employees, Robert Noyce and Gordon Moore, later went on to establish Intel, today the world’s largest microchip manufacturer.

Eventually, Bardeen, Brattain, and Shockley reunited briefly a few years later when they jointly received the 1956 Nobel Prize in Physics for their discovery. Their story is often cited as a fascinating tale of intellectual brilliance intertwined with personal rivalry—and is well worth reading more about.

Structure and Operating Principle

Structure

A transistor consists of three semiconductor layers joined together, forming two P–N junctions in opposite directions. Therefore, a transistor can be thought of as two diodes connected back-to-back.

They are divided into two types:

• NPN: If the two diodes share a p-type semiconductor region, the transistor is of the NPN type, also called a negative transistor.

• PNP: If the two diodes share an n-type semiconductor region, the transistor is of the PNP type, also called a positive transistor.

The three semiconductor regions are connected to three terminals, known as the transistor’s electrodes:

• Base (B): The middle region of the transistor is the base. It is thin, lightly doped, and small in size. Thus, it contains very few charge carriers. The base forms two circuits: the input circuit with the emitter, and the output circuit with the collector. The input circuit has low impedance, while the output circuit has high impedance.

• Emitter (E): This region is wide and heavily doped, and therefore provides a large number of charge carriers. The emitter is connected to the base because it supplies charge carriers to it. The junction between the emitter and the base injects a large number of charge carriers into the base region.

• Collector (C): This region collects the majority of charge carriers supplied by the emitter. The collector is larger in size than the other regions in order to effectively gather carriers injected from the emitter.

Although the emitter and collector are both made from the same type of semiconductor (either N or P), they differ in size and doping concentration and therefore are not interchangeable.

Operating Principle of a Transistor

There are two steps required to make a transistor operate:

• Biasing the collector–emitter (C–E) junction

• Biasing the base–emitter (B–E) junction

NPN transistor:

• If we connect the emitter (E) and collector (C) of an NPN transistor to a DC power supply Ecc​, with the negative terminal connected to the emitter and the positive terminal connected to the collector, while leaving the base terminal open, then the majority carriers (electrons) in the emitter region cannot pass through the base semiconductor region. As a result, no current flows through the transistor.

• However, due to thermal motion, a very small number of electrons can cross the collector–base junction, generating a tiny leakage current known as the reverse collector current (also called leakage current).

Now we connect an additional DC source Ebb​ between the emitter (E) and the base (B), with the negative terminal connected to the emitter and the positive terminal connected to the base.

• At this point, the BE junction is forward-biased, so electrons from the emitter region can easily pass through the base region. Meanwhile, the BC junction is reverse-biased, so electrons from the collector region cannot flow into the base.

• Because the base is very thin and lightly doped, the number of holes in it is very small. However, a large number of electrons move from the emitter into the base. Only a few of them recombine with holes in the base to form the base current (IB), while most electrons are pulled across into the collector region (since the collector is at a higher voltage), forming the collector current (IC).

Simply put, the base current IB​ helps to open the BE junction and allows electrons from the emitter to move across the BC junction. Once they pass through, they form a current flowing from the emitter to the collector.

It is easy to see that:  IE = IC + IB

For a PNP transistor, the operation is similar to the NPN transistor, but with the bias directions reversed.

Symbol and Appearance

Transistor Symbol

As mentioned above, a transistor can be formed in two ways: PNP transistor and NPN transistor.

The arrow in the symbol indicates the direction of current flow from the emitter toward the base–emitter junction. The only difference between an NPN and a PNP transistor lies in the direction of this current.

How to Read Information on a Transistor
 

Although there are many types of transistors on the market today from different countries, the most common ones are those manufactured in Japan, the United States, and China. In addition, transistors are also produced in Russia, Europe, and other regions.

Japanese Transistors

They start with “2S”, followed by a letter that indicates the type and application of the transistor. Finally, a group of numbers specifies the product code:

• 2SA: PNP transistor for high-frequency operation.

• 2SB: PNP transistor for low-frequency operation.

• 2SC: NPN transistor for high-frequency operation.

• 2SD: NPN transistor for low-frequency operation.

Examples:
2SA1015, 2SA1013, 2SA168, 2SB688, 2SB55, …

For some newer transistors, the prefix “2S” is dropped. Instead, they start directly with the letters A, B, C, or D, replacing 2SA, 2SB, 2SC, or 2SD.

Examples:
A564, B733, C828, D1555.

• Transistors starting with A or B are PNP types.

• Transistors starting with C or D are NPN types.

In general:

• A and C types → low power, high frequency.

• B and D types → higher power, lower frequency.

American Transistors

American transistors typically begin with the prefix “2N”, followed by a series of numbers indicating the product code.

Examples:
2N3055, 2N4073, 2N73A, …

To determine whether the transistor is made from silicon (Si) or germanium (Ge), as well as other specifications, one must consult a datasheet or reference book.

Chinese Transistors

The general formula for Chinese transistor codes is: start with the number 3, followed by two letters.

Examples:
3CP25, 3AP20 …

• The first letter indicates the transistor type:

  • A: PNP transistor, made from germanium.

  • B: PNP transistor, made from silicon.

  • C: NPN transistor, made from germanium.

  • D: NPN transistor, made from silicon.

• The second letter indicates characteristics and application:

  • V: semiconductor

  • Z: rectifier

  • S: tunnel

  • U: photoelectric

  • X: low-power audio (< 1W)

  • P: high-power audio (> 1W)

  • G: low-power high-frequency (< 1W)

  • A: high-power high-frequency (> 1W)

• Finally, a group of numbers specifies the product code.

Examples:

• 3AG11 → PNP transistor, Ge, low-power audio, product number 11.

• 3AX31B → PNP transistor, Ge, low-power audio, product number 31, with improvements (suffix “B”).

Physical Appearance of Transistors

Some common shapes and packaging types of transistors:

Transistor Characteristics

When studying the characteristics of a transistor, three fundamental relationships are considered:

• The relationship between the input current IBI_BIB​ and the input voltage VBE​: the graph showing how IB​ varies with VBE​ is called the input characteristic of the transistor.

• The relationship between the output current IC​ and the input voltage VBE​: the graph representing this relationship is called the transfer characteristic of the transistor.

• The relationship between the output current IC​ and the output voltage VCB​: the graph showing this relationship is called the output characteristic of the transistor.

Input and Transfer Characteristics of a Transistor

Two adjustable DC sources, VCC​ and VBB​, are typically used to bias the junctions of the transistor, as shown in the schematic diagram below:

• The Vcc supply has its positive terminal connected to the collector (C) of the transistor through a resistor RCR_CRC​, and its negative terminal connected to the emitter (E). This supply is usually referred to as the power supply for the transistor.

• The VBB supply has its positive terminal connected to the base (B) through a resistor RBR_BRB​, and its negative terminal connected to the emitter (E). This supply is usually referred to as the biasing supply for the transistor.

• Keep the Vcc supply fixed so that the collector–emitter voltage VCE​ has a certain constant value, then vary the VBB supply so that the base–emitter voltage VBE​ changes.

• By observing the measurement indicators, we see that both the base current IBI_BIB​ and the collector current IC​ vary with the value of V.

• For each value of VBE\​, we can record the corresponding values of IBI_BIB​ and IEI_EIE​.

• For example, we may obtain the following values:

Based on this data table, we can draw the input characteristic curve and the transfer characteristic curve of the transistor.

Input Characteristic:

Transfer characteristic:

These transfer characteristics are plotted for VCE = 2V, and the characteristics remain unchanged for VCE values greater than 2V. 

Output Characteristic IC / VCE

We use the same experimental circuit:

• This time, keep the base–emitter voltage VBE at a fixed value, for example 0.2 V, and adjust the supply voltage VCC to vary the collector–emitter voltage VCE. We observe that as VCE changes, the collector current IC also changes.
  
  

• From the measurements, we can see that:

  • When VCE = 0, then IC = 0.

  • After that, IC increases rapidly and almost linearly with VCE (nearly vertical).

  • Then, IC increases much more slowly with VCE (almost horizontal).

• Now, adjust VBB so that VBE has another value, for example 0.3 V. Again, vary VCE from 0 V to 10 V. Record the meter readings and draw the second characteristic curve.
  
  

• By repeating this procedure for different fixed values of VBE while varying VCE, we obtain multiple output characteristic curves. The collection of these curves is called the family of output characteristics of the transistor.

Load Line and the Quiescent Operating Point of a Transistor

To more clearly explain the operating principle and the regions of operation of a transistor, the concepts of the load line and the quiescent operating point (Q-point) are introduced. These represent the dependence of the output current IC on the voltage VCE when the transistor is connected with a load resistor RC in the circuit, as shown in the schematic:

Applying Ohm’s law to the circuit, we have: 

    VCC = VRC + VCE

⇒ VRC = -VCE + VCC

Since VRC is the voltage drop across RC, we have: 

    VRC = IC · RC

⇒ IC · RC = -VCE + VCC

⇒ IC = (-1/RC) · VCE + (VCC / RC)

From this equation, we see that the output current IC depends on VCE in the form of a linear function: 

y = a·x + b

Therefore, the DC load line of the transistor is a straight line that intersects the horizontal axis (VCE axis) at the point VCC, and intersects the vertical axis (IC axis) at the point b. 

• The intersection point between the load line and the transistor’s output characteristic curves is called the quiescent operating point, or the Q-point of the transistor.

• The intersection of the load line with the output characteristic corresponding to IB = 0 is called the cutoff point Qcutoff. At this point, the BE junction of the transistor is not forward-biased, the transistor is inactive, and the voltage VCE ≈ VCC (the supply voltage).

• The intersection of the load line with the output characteristic corresponding to the saturation base current IB(sat) is called the saturation point Qsat. At this point, VCE is approximately 0 volts, and the output current IC reaches its maximum value.

• The intersection of the load line with any output characteristic for IB > IB(sat) also gives a saturation point.

• On the load line:

  • Below Qcutoff is the cutoff region of the transistor.

  • Above Qsat is the saturation region.

  • Between Qcutoff and Qsat is the active region, also called the amplification region of the transistor.
    Thus, in order for the transistor to operate in the amplification region, we must bias it so that the Q-point lies somewhere between Qcutoff and Qsat. Depending on the circuit’s purpose, the exact position of the Q-point is chosen accordingly.
    
    

• The concepts of input characteristic, transfer characteristic, output characteristic, load line, and quiescent operating point are of great importance in circuit design.

Technical Specifications of a Transistor

The technical specifications of a transistor are characteristic quantities that define its properties. A technician must clearly understand these specifications in order to properly select and use a transistor that meets the circuit requirements while also saving costs when purchasing components. The main specifications of a transistor include:

Maximum Allowable Current:
This is the largest current that can flow through the transistor without damaging it. It includes:

1. • IC max: maximum collector current.

   • IB max: maximum base current.

Breakdown Voltage:
This is the maximum reverse voltage that can be applied to the BE, BC, and CE junctions. Exceeding this value will damage the transistor. The parameters include:

1. • BVCEO: collector–emitter breakdown voltage when the base is open.

   • BVCBO: collector–base breakdown voltage when the emitter is open.

   • BVEBO: emitter–base breakdown voltage when the collector is open.
     
     

Maximum Allowable Power (PDM):
When operating, a transistor dissipates power given by PT = I × UCE. If PT exceeds the maximum allowable power, the transistor will be damaged (typically by junction breakdown at C–E or B–E).

Current Gain Factor (β):
The current gain of a transistor, denoted by the Greek letter beta (β), indicates how much the collector current IC increases in response to a change in the base current IB. In other words, when IB changes by a certain amount, IC changes by a much larger multiple of IB.

Cutoff Frequency (transition frequency):

This is the frequency at which the transistor’s current gain drops to 0.7 of its value measured at the lowest operating frequency. At frequencies higher than this point, the current gain decreases sharply. 

Types of Transistors

Bipolar Junction Transistor (BJT)
A bipolar junction transistor, often abbreviated as BJT, is a type of transistor with three terminals: B (Base), C (Collector), and E (Emitter).

BJTs are classified into two types: PNP transistors and NPN transistors.

Advantages:

• Low power consumption.

• Almost no startup delay.

• Since BJTs do not have a heated cathode, they do not contain toxic materials.

• Small and lightweight, with continuous improvements in design.

• Can operate at voltages as low as a single battery cell.

• High operating efficiency.

• Long lifespan, with relatively low sensitivity to external factors such as mechanical shocks.

Disadvantages:

• Performance decreases over time with use.

• BJTs have limitations when operating at high power and high frequency.

• They are easily damaged by sudden changes in power or temperature and are highly sensitive to radiation.

Applications:
In practice, BJTs are used in almost all modern electronic devices — from the tiny transistors in computer microprocessors to those integrated into ICs (Integrated Circuits).

Field-Effect Transistor (FET)
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the conductivity of a semiconductor material.

FETs are unipolar devices (using only one type of charge carrier), meaning they operate with a single type of carrier. For this reason, they require almost no input current (only a very small leakage current), which gives them a significant advantage over BJTs — namely, a very high input impedance.

A field-effect transistor (FET) is a semiconductor device with three terminals — drain, source, and gate — through which a current of charge carriers flows. The carriers are electrons in an N-channel FET and holes in a P-channel FET.

FETs are classified into two main types:

• JFET (Junction Field-Effect Transistor): a type of transistor controlled by a P–N junction.

• MOSFET (Metal-Oxide-Semiconductor FET): a transistor with an insulated gate formed by an oxide layer.

  •  MOSFETs are further divided into:

    • DE-MOSFET (Depletion-type MOSFET)

    • E-MOSFET (Enhancement-type MOSFET)

Advantages:

• Unipolar structure, since current conduction is dominated by only one type of majority carrier.

• Lower noise compared to bipolar transistors.

• Very high input impedance.

• Provides good cut-off because no gate current flows when ID = 0.

• High thermal stability.

• Operates at high frequencies.

Disadvantages:

• Compared with BJTs, the main disadvantage of FETs is their lower current gain.

• Slower switching speed than BJTs.

Applications:
In practice, FETs are widely used in medium-power circuits due to their load-handling capability.

Unijunction Transistor (UJT)

A UJT (Unijunction Transistor) is a three-terminal transistor with only one P–N junction. It acts as a controlled switch. Although it is not as widely used as BJTs, it still plays an important role in waveform generation and timing circuits.

Basic Structure:

• The UJT consists of a lightly doped N-type semiconductor bar with two ohmic contacts at its ends, forming two base terminals B1 and B2.

• A small aluminum rod diffused into the N-type bar forms a P–N junction and serves as the emitter (E).

• The P-region (emitter) is placed closer to B2 than to B1 (around 70% of the distance from B1 to B2) to optimize its electrical characteristics.

Characteristics of UJT:

• UJT is often considered a “nonlinear” device since it has only one P–N junction.

• Structurally, it is similar to a diode but with three terminals.

• UJT resembles an N-channel JFET in structure, but differs in two main points: the P-region surrounds the N-material, and the gate area is larger in a JFET.

• The N-region of the UJT is lightly doped, while the emitter is heavily doped.

• The N-type bar has high resistance: the resistance between emitter and base B2 is lower than between emitter and base B1.

• UJT normally operates with the emitter forward-biased.

• Due to its negative resistance characteristic, a UJT can function as an oscillator.

Applications of UJT:
Because of its low cost and unique properties, UJTs are widely used in:

• Oscillators

• Pulse generators

• Trigger circuits

• Phase control circuits

• Sawtooth waveform generators

• Timing circuits

• Voltage- or current-controlled power supplies

Transistor Configurations

A transistor can operate in two states:

• Amplification state:
  In this mode, we need to calculate the current gain and select the appropriate configuration. This mode is commonly used in audio circuits, amplifiers, etc.

• Saturation state:
  In this mode, we do not need to consider the transistor’s gain. It simply functions as an on/off switch.

Transistor Connection in Saturation Mode – Load Switching (ON/OFF):

This configuration is commonly seen in digital circuits and microcontroller circuits. The transistor functions as an electrically controlled switch (representing logic states 0 and 1).

To use this configuration, we bias VBE > 0.6 V (typically 3.3 V or 5 V). At this point, the transistor operates in saturation mode.

Transistor in Amplification Mode

1. Common Emitter Configuration (Voltage Amplifier Circuit)
This is the most widely used configuration in electronic circuits.

Characteristics of a common-emitter transistor amplifier:

• The common-emitter amplifier is usually biased so that UCE is about 60%–70% of VCC.

• The output signal amplitude is much larger than the input signal amplitude, making it a voltage amplifier.

• The output current is slightly larger than the input current, but the increase is not significant.

• The output signal is 180° out of phase with the input signal.

2. Common Collector Configuration (Current Amplifier Circuit)
This configuration is often used in buffer (damper) circuits, before splitting a signal into multiple branches. It strengthens the signal so that it can drive more loads. It is also widely used in voltage regulator circuits.

Characteristics of a common-collector transistor amplifier:

• The input is applied to the base (B), and the output is taken from the emitter (E).

• The output signal amplitude is nearly equal to the input amplitude.

• The output signal is in phase with the input signal.

• The output current is much stronger than the input current.

3. Common Base Configuration
This configuration is rarely used in practice.

Characteristics of a common-base transistor amplifier:

• Input is applied at the emitter (E), and output is taken at the collector (C). The base (B) is grounded through a capacitor.
  
  

• Provides voltage amplification, but no current amplification.

Thermal Stability of a Transistor

Effect of Temperature on Transistor Operation:

• When a transistor operates, current flow generates heat, causing the transistor to warm up.

• As the temperature changes, the leakage current Ico, the bias voltage VCB, and the current gain of the transistor also change.

• It has been shown that for both silicon and germanium transistors, if temperature increases, Ico doubles. As Ico increases, IC also increases, making the transistor even hotter (thermal runaway).

• For VBE, in both silicon and germanium transistors, VBE decreases by about 2.5 mV per °C rise in temperature (this property is often used in temperature sensors).

• The current gain changes almost linearly with temperature.
  
  

To prevent temperature from shifting the transistor’s Q-point, bias circuits are designed with stabilization methods, such as:

1.Bias with emitter resistor RE: 

• As temperature rises, IC increases → IE increases (since IE = IC + IB).

• This increases the voltage drop across RE (UE = IE·RE), reducing UBE = UB – UE.

• This causes IB to decrease, which reduces IC.

• Thus, RE automatically stabilizes IC against temperature variations.

2. Bias with RE and feedback resistor RB from collector to base: 

• When temperature rises, IC increases → voltage at collector UC = VCC – IC·RC decreases.

• This reduces the base voltage, decreasing IB.

• As IB decreases, IC decreases accordingly → the circuit achieves thermal stabilization.

3. Bias stabilization using a thermistor: 

Voltage-divider bias with an additional thermistor connected in parallel with resistor RB2.

An NTC thermistor is used, placed in contact with the transistor or its heat sink.

As the transistor’s temperature rises, the base voltage UB decreases → UBE decreases → IC decreases.

This provides automatic thermal stabilization for the transistor.

Advantages and disadvantages of transistors

Advantages

• The advantage of transistors is that they consume relatively little power (operate at low voltage, can even work with AA batteries). The startup delay is almost zero.

• Transistors do not contain toxic substances like some other components because they have no cathode heating element.

• Extremely small and lightweight, so they can be used in many different products and devices.

• Although small in size, transistors have high operating efficiency, a lifespan of more than 50 years, and are durable and reliable.

• Vacuum tubes, when amplifying, produce very little noise and distortion, resulting in very clean sound. This is also the reason why transistors are widely used in audio processing devices.

• Less prone to shock or breakage when dropped or impacted.

Disadvantages

• When operating at high power and high frequency, vacuum tubes perform better than semiconductor transistors.

• Vacuum tubes, when amplifying, produce very little noise and distortion, creating “clean” sound, so they are still favored by many audio enthusiasts.

• Transistors are sensitive to radiation and cosmic rays (special radiation-resistant chips must be used for spacecraft equipment).

• Since transistors are made of semiconductors, they are easily damaged by electrical shock or thermal shock.

• Transistors can still “age” and degrade in performance over time

Functions of Transistors and Applications in Practice

After understanding the basic concept of transistors as well as their structure and operating principle, you can probably guess their functions, right? In reality, transistors have very wide applications today. Some of the main applications include:

1.Voltage amplification

• Transistors are used in DC current amplifier circuits, AC signal amplifiers, or in differential amplifiers, special amplifier circuits, voltage regulators.

2. Used as a switch

• Transistors are often used in digital circuits such as electronic locks that can be in ON or OFF states.

• They are used in high-power applications such as power switching mode supplies or in low-power applications such as digital logic gates.

3. Main component in logic gates – op-amps

• Transistors are the main components in ICs, logic gates, or op-amps.

• Here, transistors are very small and connected together in many ways to form logic gates, serving for computation, data storage, etc.

How to test a transistor

To measure a transistor, you can use a Volt-Ohm meter (VOM) set to the resistance range (R) to check whether the transistor is still good or has failed. In addition, you can also determine whether the transistor is NPN or PNP type and identify its terminals.

Method to check if a transistor is still good or faulty
Set the VOM to the Rx100 range, measure the resistance between the pairs of terminals BE, BC, CE. If the measured values match the table below, then the transistor is still good.

Notes:

• The VOM meter usually has the – probe (black) connected to the positive terminal of the battery and the + probe (red) connected to the negative terminal of the battery inside the meter.

• If when measuring a pair of pins you get forward resistance = reverse resistance = 0Ω, then that pair of pins is shorted.

Possible transistor faults:

• Measuring forward from B to E or from B to C → if the needle does not move, it means the transistor is open BE or open BC.

• Measuring from B to E or from B to C → if the needle moves in both directions, it means the transistor has a short or leakage BE or BC.

• Measuring between C and E → if the needle moves, then CE is shorted.

Measurement shows that the transistor is still good and working normally:

First, by looking at the symbol we can tell that the transistor above is an NPN type, and its pins are in the order E–C–B (based on the transistor’s label).

• Step 1: Set the multimeter to the x1Ω range.

• Step 2 and Step 3: Measure in the forward direction BE and BC → if the needle moves, it’s correct.

• Step 4 and Step 5: Measure in the reverse direction BE and BC → if the needle does not move, it’s correct.

• Step 6: Measure between C and E → if the needle does not move, it’s correct.

Conclusion: The transistor is good.

Measurement shows that the transistor is shorted at BE:

• Step 1: Preparation is the same as above.

• Step 2: Measure forward between B and E → if the needle goes up = 0 Ω.

• Step 3: Measure reverse between B and E → if the needle also goes up = 0 Ω.

Conclusion: The transistor is shorted at BE.

Measurement showing transistor with open BE:

• Step 1: Preparation before measurement is the same as in the two previous tests.

• Step 2 & 3: Measure both directions between B and E → if the needle does not move.

Conclusion: The transistor has an open BE junction.

Measurement showing transistor shorted at CE

• Step 1: Prepare before performing the measurement.

• Step 2: Measure both directions between C and E → if the needle goes up = 0 Ω.

Conclusion: The transistor is shorted at CE.

Method to Identify the B, C, and E Terminals of a Transistor

When encountering an unknown transistor or one with its marking erased, we can use a multimeter to determine its terminals as follows:

1. Identifying the Base (B) terminal:

A transistor can be considered as two diodes BC and BE sharing a common node. We can determine the base using one of the following methods:

Method 1:
Set the multimeter (VOM) to the Rx100 or Rx1K range. Measure between any two pins of the transistor.

• If in both forward and reverse measurements the needle does not move, or moves only slightly, then those two pins are the Collector (C) and Emitter (E).

• The remaining pin is the Base (B).

Method 2:
Place one probe on a chosen pin of the transistor. Touch the other probe to the two remaining pins one by one:

• If the needle does not move or only moves slightly, then swap the probes and test again.

• This time, if the needle rises about halfway or almost to full scale when measuring with both other pins, then the pin currently connected to one probe is the Base (B).

👉 In short: the pin that gives a forward reading (needle rises) with both other pins is the Base.

To determine transistor type (NPN or PNP):

• If the black probe (-) is connected to the base, the transistor is NPN.

• If the red probe (+) is connected to the base, the transistor is PNP.

2. Identifying the Collector (C) and Emitter (E):

Measure the forward resistance between the Base (B) and the two remaining pins:

• The pin with higher resistance is the Collector (C).

• The pin with lower resistance is the Emitter (E).

Conclusion

Transistors are used in almost all electronic circuits. With today’s increasingly advanced technology, transistors are becoming smaller and smaller. Thanks to that, we now have ultra-compact yet powerful chips as we see today.