Lesson 8. Transistor
Recommended article: Circuit Theory Table of Contents
1. Transistor
2. Bipolar Junction Transistor
3. Field-Effect Transistor
b. Semiconductor-related Problems
1. Transistor
⑴ Overview
① Compound word of “trans” and “resistor”
② First transistor patent: J. E. Lilienfeld (1926)
③ Functions: Signal amplification, switching
④ Structure 1: Emitter, source - analogous to the water inlet in plumbing
⑤ Structure 2: Base, gate - analogous to a faucet in plumbing
⑥ Structure 3: Collector, drain - analogous to water outlet in plumbing
⑵ Type 1: Bipolar Junction Transistor (BJT) - Constitutes TTL memory
① Manufactured by combining n-type and p-type semiconductors
② Controls current with current
③ Bipolarity
⑶ Type 2: Field-Effect Transistor (FET) - Constitutes MOS memory
① Manufactured by combining conductor, insulator, and semiconductor
② Controls current with voltage
③ Unipolarity
2. Bipolar Junction Transistor (BJT)
⑴ Overview
① Definition: Manufactured by combining n-type and p-type semiconductors
② First theoretical establishment: Bardeen, Brattain, Shockley
③ First experimental elucidation: Gordon Teal
⑵ Structure
① There are NPN and PNP transistors
○ NPN transistor is generally considered as the reference
○ PNP transistor exactly same if bias is reversed in NPN transistor
② BJT transistor consists of Base, Emitter, Collector
○ Emitter: Part emitting charge carriers
○ Collector: Part receiving charge carriers
○ Emitter doping concentration is higher than Collector’s
○ Base is lightly doped
③ Charge flows from Emitter (E) → Base (B) → Collector (C)
Figure. 1. Symbols of NPN transistor (left) and PNP transistor (right) [Footnote:1]
○ B-E junction, B-C junction can be seen as diodes
○ NPN transistor symbol: Electrons move from Emitter → Base → Collector. Current is in opposite direction
○ PNP transistor symbol: Holes move from Emitter → Base → Collector. Current is in the same direction
⑶ Region 1: Active Region
① Conditions (for NPN transistor)
○ B-E junction is forward-biased
○ B-C junction is reverse-biased
○ Thus, VE < VB < VC
② Mechanism
Figure. 2. Operating principle of NPN transistor in active region [Footnote:2]
(Red dots represent electrons)
○ 1st step: Forward bias applied to E-B moves electrons from Emitter to Base, forming IE
○ 2nd step: Base is lightly doped, typically 5% or less of electrons forming IE combine with holes
○ 3rd step: The remaining 95% of electrons move from Base to Collector, forming IC
○ 4th step: Most current flows to Collector - Collector current greatly influenced by incoming charge from Base, electric field influence low
○ 5th step: Similar diffusion current occurs in BJT transistor - not drift current due to electric field
○ The following equations hold true
③ Formulation
Figure. 3. Analysis of B-E junction in transistor
○ B-E junction: Can be seen as a diode. Density of minority carriers: Electrons more than holes
○ B-C junction: Can be seen as a diode. Can be considered in equilibrium state
○ Base current: Using the formula for forward-biased diode
○ Collector current: Mostly diffusion current. Depends only on VBE (except for WB, the thickness of Base)
○ Reason for non-zero Base current
○ Existence of holes moving from Base to Emitter to create pB-E boundary
○ Combination of electrons with holes in the Base
○ Conclusion
④ Equivalent Circuit: B-E junction modeled as a diode with a threshold voltage. B-C junction replaced by a dependent current source
Figure. 4. Equivalent circuit of active region [Footnote:3]
○ Dependent current source: IC = βIB
○ Typically β is an integer, not α, so β is commonly used
○ Current can only flow from Collector to Emitter, not the reverse
○ Amplification factor β varies with physical size of Base and doping density, generally around 100
○ Insight: Despite large current wanting to flow through Collector, only a portion proportional to IB can enter Collector
○ Continuous Base current necessary to maintain transistor in active state
○ Problem-solving methodology: Calculate Base current → Collector current → Emitter current → VCE → VCB
⑷ Region 2: Saturation Region
① Conditions (for NPN transistor)
○ B-E junction is forward-biased
○ B-C junction is forward-biased
○ Thus, VE, VC < VB
○ If VBC < 400 mV, it’s called soft saturation; if VBC > 400 mV, it’s called deep saturation
② Similar to having forward diodes at B-E and C-E
○ Internally, charge carriers gather in the Base region and saturate → Increase in Base current does not significantly change Collector current
○ Threshold voltage at B-E is the same as that of a typical diode: around 0.7 V
○ Threshold voltage at C-E is denoted as VCE(sat) and is around 0.2 V
○ As Base terminal is narrow, C-B forward bias is same as C-E forward bias
③ Equivalent circuit
Figure. 5. Equivalent circuit of saturation region [Footnote:4]
○ IC exponentially increases with VCE increase (Shockley diode model)
○ Considering operating point, forward diode can be approximated as constant-voltage source and ideal diode (characteristic curve)
○ VCE(sat) is constant, similar to VBE - simpl
ification of characteristic curve
○ βforced defined as Collector current ÷ Base current
○ βforced increases proportionally with VCE - not perfectly proportional
○ When βforced increases to βdc, the current amplification ratio of the active region, transition to active region occurs
⑸ Region 3: Cutoff Region
① Conditions (for NPN transistor)
○ B-E junction is reverse-biased
○ B-C junction is reverse-biased
○ Thus, VB < VE, VC
② Equivalent circuit
Figure. 6. Equivalent circuit of cutoff region
⑹ Region 4: Reverse Active Region (Reverse Active Area, breakdown region)
① Condition (NPN transistor basis)
○ B-E junction is reverse biased.
○ B-C junction is forward biased.
○ In other words, VE > VB > VC will be true.
② Not commonly used region due to signal degradation.
⑺ Transistor’s Current-Voltage Characteristics Curve : Also known as Collector’s Current-Voltage Characteristics Curve
① Ideal transistor’s current-voltage characteristics curve : Collector’s current-voltage characteristics curve
Figure. 7. Transistor’s Current-Voltage Characteristics Curve [Note: 6]
Figure. 8. Simplified Transistor’s Current-Voltage Characteristics Curve [Note: 7]
Figure. 9. Collector Characteristic Curve Experiment and Operating Point [Note: 8]
○ Saturation Region : A ~ B. In other words, when on the line segment with a slope
○ VB = 0.7 V
○ Keep VBB fixed and increase VCC to gradually raise VCE from 0 to 0.7 V.
○ VC = VCE < VB = 0.7, so BC junction is forward biased.
○ As VCC increases, and thus VCE increases, IC also increases gradually (Shockley Diode Model): Exponentially.
○ Increasing IB does not significantly affect IC : Both BE and BC junctions are forward biased, so there’s no control function ( Switch ON )
○ In the equivalent circuit, VCE remains constant, similar to representing an actual diode as an ideal diode + voltage source. While there are practical differences in the graph, the calculated operating points are not greatly different.
○ In the equivalent circuit, IC is unrelated to IB, resulting in overlapping lines. In a real circuit, larger IB leads to smoother collector current flow.
○ Active Region : B ~ C. In other words, on a parallel line segment
○ VB = 0.7 V
○ Increase VCC to set VCE > 0.7 V
○ VC = VCE > VB = 0.7, so BC junction is reverse biased.
○ Even with increasing VCC, IC remains constant: IC = β × IB, where β is a constant.
○ Increasing IB leads to an increase in IC.
○ Saturation Region : After C. In other words, on a line segment with a slope again
○ If VCC keeps increasing to make the BC junction excessively reverse biased, breakdown phenomenon occurs, similar to a diode.
○ In the saturation region, the transistor is destroyed, so it should be operated at VCC < VCE(max).
○ Cutoff Region : IB = 0
○ When VBB = 0 and IB = 0, increasing VCC makes both the BC and BE junctions reverse biased.
○ There exists a slight leakage current: IC ≒ 0
○ Saying a BJT transistor is controlled by IB means that its characteristic curve is drawn with IB fixed.
② Early Effect
Figure. 10. Actual Transistor’s Current-Voltage Characteristics Curve [Note: 9]
○ Also known as Base Width Modulation Effect.
○ As VCE increases, the depletion region of the collector-base reverse-biased junction increases, and the effective base thickness WB decreases.
○ As a result, collector current increases.
○ Early Voltage : When extending various sloped characteristic curves, they meet at a single point, the voltage at that point’s magnitude.
③ Load Line and Operating Point
Figure. 11. Example of an Operating Point [Note: 10]
○ The intersection of the characteristic curves for base current-voltage and collector current-voltage, and the load line is called the operating point (Q point).
○ The base current-voltage characteristic curve is similar to a diode.
○ The above current-voltage characteristic curves overlook the fact that IC is slightly influenced by IB in the saturation region.
⑻ Application : Transistor Switch
① Definition : Biasing set to oscillate between cutoff and saturation regions.
② Knowing the saturation voltage of the collector helps determine the minimum base current required for saturation region operation (reference: ⑼-③).
⑼ Application : Phototransistor
① Instead of connecting a conductor to the transistor’s base, it’s designed to allow light to induce photocurrent flow.
⑽ Summary
① Problem-solving should proceed in the order of Active Region → Cutoff Region → Saturation Region.
② Approach Method 1: Determine via voltage biasing.
Figure. 12. Transistor Operating Regions (Transistor Region of Operation)
③ Approach Method 2: Distinguish saturation and active regions based on the ratio of collector current to base current.
○ βforced < βdc, thus the observed relationship is as follows.
○ Judging the operating region based on the above relationship is Approach Method 1 and a necessary and sufficient condition.
○ The above relationship is also used to control saturation and active regions by attaching a current source to the base.
○ The above relationship can be used to calculate the minimum value of I B required for operation in the saturation region.
3. Field Effect Transistor (FET)
⑴ Definition : Manufactured by combining conductors, insulators, and semiconductors.
⑵ Comparison with BJT Transistor
① Terminology
○ BJT Transistor’s emitter, base, and collector correspond to FET Transistor’s source, gate, and drain, respectively.
○ Just as BJT Transistors are based on NPN, FET Transistors are based on n-channel FETs.
○ In FETs, current flows from source to drain: n-channel FET basis, current flows drain → source, electrons flow source → drain.
② Control of BJT Transistor : Controlled by base current.
○ Conduction Current : Flow of particles with negative charge such as electrons, positive holes, ions in electrolytes.
③ Control of FET Transistor : Controlled by voltage between gate and source.
○ Conduction Current : Movement of free electrons caused by the action of an electric field inside a conductor.
○ Electrons have higher mobility than positive holes.
○ In doped semiconductors, only the majority carrier is of concern.
④ Advantages : Very fast switching speed.
⑤ Disadvantages : Low capacitance, so functional within a relatively small power range.
⑶ Type 1: J-FET (Junction FET)
Figure. 13. Diagram and Symbol of J-FET [Note: 11]
① N-channel J-FET
○ Applying a negative voltage to the gate causes positive holes in the P-type semiconductor to move to the edge, increasing drain current.
○ Applying a positive voltage to the gate causes positive holes in the P-type semiconductor to move to the center, decreasing drain current.
② P-channel J-FET : Similar to N-channel J-FET but with opposite applied voltage.
⑷ Type 2: MOS-FET (Metal Oxide Silicon FET) : More commonly used than J-FET
① The gate is insulated from the drain and source by SiO2.
○ Function : Provides insulation. Prevents the flow of charge to the gate.
○ Acts as a type of capacitor : See the mechanism below
② SiO2 has polysilicon or metal attached : Allows uniform distribution of gate voltage
③ Divided into enhancement type and depletion type
④ CMOS (complementary MOS)
○ Most digital systems are based on CMOS technology
○ C represents complementary : Because there are p-type and n-type
○ n-channel MOS is also called n-type MOS or nMOS
○ p-channel MOS is also called p-type MOS or pMOS
○ nMOS must be connected to GND, pMOS must be connected to a voltage source
⑸ Type 2-1. Enhancement Type MOS-FET (E MOS-FET)
Figure. 14. Enhancement Type MOS-FET
Figure. 15. Symbol of Enhancement Type MOS-FET
① Explained with n-channel E MOS-FET as a reference
② 1st. There is an NPN semiconductor between drain and source by default, preventing current flow
③ 2nd. Channel Formation : When a higher voltage is applied to the gate electrode than the source, electrons gather under the oxide layer, forming an inversion layer
○ Capacitor Operation : For capacitors, remember that if one side is (+), the other side is (-)
④ E MOS-FET Threshold Voltage (VT, VTH)
○ Minimum gate voltage required to form a channel in E MOS-FET
○ Body Effect : Factor causing VT to increase. For PMOS, the expression representing the body effect is as follows
⑤ Saturation Region
○ Condition 1. VGS - VT ＜ Drain Voltage (VDS)
○ Condition 2. Gate Voltage (VGS) - Threshold Voltage (VT) ＞ 0
○ These conditions are similar to a situation where the gate and source are forward-biased, and the gate and drain are reverse-biased
○ Current Characteristics : Current constant K is already determined during manufacturing
○ Drain voltage not only affects the motion of electrons but also the shape of the channel
Figure. 16. Effect of Drain Voltage Application on Channel Shape
○ Pinch-off phenomenon : Electrons in the channel move toward the drain until the channel is pinched off
Figure. 17. Pinch-off Phenomenon
○ (Note) When the channel is pinched off, it means no current flows
○ (Note) Adjusting the pinch-off condition controls the current flow independently of the drain-source voltage
○ Channel Length Modulation : Results in increased drain current due to a shorter effective channel length
○ BJT transistor’s active region corresponds to FET transistor’s saturation region
⑥ Unsaturated Region (Ohmic region)
○ Condition 1. Drain Voltage (VDS) ＜ VGS - VT
○ Condition 2. Gate Voltage (VGS) - Threshold Voltage (VT) ＞ 0
○ These conditions are similar to a situation where the gate and source are forward-biased, and the gate and drain are forward-biased
○ Current Characteristics : Current constant K is already determined during manufacturing
○ Derivation of Current Equation
○ Carrier Velocity Saturation : The current equation becomes like this when L is very small or E is very large
○ BJT transistor’s saturation region corresponds to FET transistor’s unsaturated region
⑦ Cutoff Region
○ Condition : Gate Voltage (VGS) - Threshold Voltage (VT) ＜ 0
○ This condition is similar to a situation where the gate and source are reverse-biased
○ Gate voltage cannot exceed E MOS-FET threshold voltage, so channel formation does not occur → No current flows
⑧ Characteristic Curves
Figure. 18. Characteristic Curves of E MOS-FET
○ Unsaturated Region : Points on the sloped line segment
○ Saturation Region : Points on the parallel almost straight line segment, though a slight slope due to early effect is observed
○ Cutoff Region : ID = 0
○ Drain Current ID corresponds to Collector Current IC of BJT transistor
○ Drain-Source Voltage VDS corresponds to Collector-Emitter Voltage VCE of BJT transistor
○ Transition Point : Boundary between unsaturated and saturated regions
○ The statement that FET transistors are controlled by VGS means that the characteristic curves are drawn with VGS fixed
⑨ Summary : When a problem is given, assume situations in the order of saturation region → unsaturated region → cutoff region
⑹ Type 2-2. Depletion Type MOS-FET (D MOS-FET)
Figure. 19. Depletion Type MOS-FET [Footnote: 16]
Figure. 20. Symbol of Depletion Type MOS-FET [Footnote: 17]
① Explained with n-channel D MOS-FET as a reference
② 1st. Structurally, the channel is already formed : Drain current flows even without gate voltage
○ n-type semiconductor is thinly deposited under silicon oxide layer
③ 2nd. When gate voltage is positive, electrons in the n-type semiconductor gather under the oxide layer
○ Capacitor Operation : For capacitors, remember that if one side is (-), the other side is (+)
○ As electrons gather under the oxide layer, the channel thickens → Current is enhanced
○ Increased current mode operation is when current increases
④ 3rd. When gate voltage is negative, electrons in the n-type semiconductor scatter towards the edges
○ Capacitor Operation : For capacitors, remember that if one side is (+), the other side is (-)
○ As electrons scatter from under the oxide layer to the edges, the channel thins → Current is weakened
○ Depletion mode operation is when current decreases
⑤ D MOS-FET Threshold Voltage : The voltage that makes drain current 0
○ Even when VGS = 0, drain current is not 0 : This current is called IDSS
⑥ Characteristic Curves
Figure. 21. Characteristic Curves of D MOS-FET (1)
Figure. 22. Characteristic Curves of D MOS-FET (2)
Input : 2018.02.15 14:33
Modification : 2022.09.11 21:48