A transistor is a semiconductor device that controls current or voltage. It can operate as an electronic switch, a signal amplifier, a current source, or part of an oscillator and many other circuits. This chapter compares the three most important transistor families: the bipolar junction transistor (BJT), the junction field-effect transistor (JFET), and the metal-oxide-semiconductor field-effect transistor (MOSFET).
The equations below describe simplified device models. Real behavior also depends on temperature, manufacturing variation, parasitic effects, and the surrounding circuit.
Learning Objectives
After completing this chapter, you should be able to:- identify the terminals and operating principle of a BJT, JFET, and MOSFET;
- distinguish current-controlled and voltage-controlled device behavior;
- recognize cutoff, active or linear, and saturation operating regions;
- explain the purpose of a DC operating point;
- use basic large-signal and small-signal models;
- describe transistor operation as a switch and as an amplifier;
- select a suitable transistor type for a basic application.
1. What a Transistor Does
A transistor uses a smaller electrical signal at one terminal to control a larger current between two other terminals. This control principle enables two fundamental functions:- Switching: the device is intentionally driven between a low-current state and a high-current state.
- Amplification: the device is biased at an operating point where a small input variation produces a larger or otherwise useful output variation.
2. Bipolar Junction Transistors (BJTs)
A BJT consists of three semiconductor regions arranged as either npn or pnp. Its terminals are:- Emitter (E): injects majority carriers;
- Base (B): a thin control region;
- Collector (C): collects carriers that cross the base.
Basic Active-Region Relations
For an npn transistor in forward-active operation:\[I_C\approx I_S\exp\!\left(\frac{V_{BE}}{V_T}\right)\]
A commonly used approximate current relation is:
\[I_C\approx\beta I_B\]
Here, β is the common-emitter current gain. Its value varies substantially among devices and with operating current and temperature, so robust bias circuits should not depend on an exact β value.
BJT Operating Regions
| Region | Junction Conditions | Typical Use |
|---|---|---|
| Cutoff | Emitter-base junction not sufficiently forward biased | Switch off |
| Forward active | BE forward biased, BC reverse biased | Amplification |
| Saturation | Both junctions forward biased | Switch on |
| Reverse active | Roles of emitter and collector reversed | Rarely used |
3. Junction Field-Effect Transistors (JFETs)
A JFET has a conductive channel between source and drain. A reverse-biased gate-channel pn junction changes the depletion width and therefore controls the channel current. Its terminals are:- Source (S): carrier entry terminal;
- Drain (D): carrier exit terminal;
- Gate (G): controls the channel through its electric field.
Shockley Transfer Relation
For an ideal n-channel JFET in its pinch-off operating region:\[I_D=I_{DSS}\left(1-\frac{V_{GS}}{V_P}\right)^2\]
IDSS is the drain current at VGS = 0, and VP is the negative gate-source cutoff or pinch-off voltage under the chosen sign convention. The relation is useful within its specified operating range and is not a universal model for every bias condition.
4. Metal-Oxide-Semiconductor FETs (MOSFETs)
A MOSFET controls a semiconductor channel with an electric field from a gate insulated by a thin dielectric. Because the gate is insulated, its DC input current is extremely small. The principal terminals are gate, drain, source, and body (or bulk). In many discrete devices the body is internally connected to the source.Enhancement-Mode nMOS
An enhancement-mode nMOS transistor is normally off. A sufficiently positive gate-source voltage attracts electrons to the semiconductor surface and creates an inversion channel. The gate voltage at which strong inversion is conventionally established is represented by the threshold voltage Vth. The overdrive voltage is:\[V_{ov}=V_{GS}-V_{th}\]
Ideal Long-Channel Operating Regions
- Cutoff: VGS < Vth; drain current is ideally zero.
- Triode or linear region: VGS > Vth and VDS < Vov; the device behaves approximately as a voltage-controlled resistance.
- Saturation region: VGS > Vth and VDS ≥ Vov; the channel pinches off near the drain and current is primarily controlled by VGS.
\[I_D\approx\frac{1}{2}\mu_n C_{ox}\frac{W}{L}V_{ov}^2\]
A first-order channel-length-modulation correction is often written as a factor (1 + λVDS). Modern short-channel devices require more detailed models, but the square-law model remains useful for understanding basic circuits.
5. Comparing BJT, JFET, and MOSFET
| Property | BJT | JFET | MOSFET |
|---|---|---|---|
| Control | Base-emitter voltage and base current | Reverse-biased gate voltage | Insulated-gate voltage |
| Input current | Finite base current | Very small leakage | Very small DC current; capacitive during switching |
| Carrier type | Bipolar | Majority carrier | Majority carrier |
| Common strengths | High transconductance at a given current | High input impedance and low noise in suitable circuits | Fast switching and excellent scalability |
| Typical applications | Analog gain, current mirrors, switching | Low-noise inputs, analog switches, current sources | Digital logic, power switching, analog ICs |
6. The DC Operating Point
For linear amplification, a transistor must be biased at a suitable quiescent or Q point. The Q point defines the DC currents and voltages before an input signal is applied. It should provide enough signal range while keeping the device inside the intended operating region.BJT Bias Stabilization
A simple base resistor is strongly affected by β and temperature. Adding an emitter resistor introduces negative feedback: if collector current rises, the emitter voltage rises, reducing the effective base-emitter voltage and opposing the original change. Voltage-divider bias with an emitter resistor is therefore commonly used.FET Bias
FET bias circuits establish VGS and ID. A source resistor can provide self-bias and negative feedback. Because gate current is very small, large gate-bias resistances are possible, although leakage, noise, and input capacitance still matter.7. Load Lines
A load line represents the voltage-current combinations permitted by the external circuit. For a common-emitter BJT circuit with collector resistor RC:\[V_{CE}=V_{CC}-I_CR_C\]
The intersection of the load line with a transistor characteristic determines the operating point. A similar method can be used for FET drain circuits. The load line makes the tradeoff between current, voltage swing, and power dissipation visible.
8. Transistors as Switches
BJT Switching
In a simple low-side switch, cutoff represents the off state and saturation represents the on state. The base must receive enough current to ensure saturation at the required collector current. Excessive base drive can increase stored charge and slow turn-off.MOSFET Switching
A power MOSFET is off when VGS is below the relevant threshold region and on when the gate is driven high enough to achieve the specified low RDS(on). Threshold voltage does not indicate full enhancement. Gate charge, driver resistance, drain voltage, and the Miller plateau determine switching time and loss. Approximate conduction loss in the on state is:\[P_{cond}\approx I_{D,rms}^2R_{DS(on)}\]
Switching loss is associated with transitions during which appreciable voltage and current exist simultaneously.
9. Transistors as Amplifiers
An amplifier uses a DC-biased transistor and superimposes a small AC signal on the Q point. The transistor converts the input variation into a controlled output current. A load then converts that current variation into an output voltage.Common BJT Configurations
- Common emitter: voltage and current gain, with phase inversion.
- Emitter follower (common collector): voltage gain near unity, high input resistance, and low output resistance.
- Common base: low input resistance and useful high-frequency behavior.
Common FET Configurations
- Common source: voltage gain with phase inversion.
- Source follower (common drain): voltage gain below unity and useful buffering.
- Common gate: low input resistance and useful high-frequency behavior.
10. Large-Signal and Small-Signal Models
A large-signal model describes the nonlinear DC behavior over a broad operating range. It is used for biasing and switching. A small-signal model linearizes the device around a chosen Q point and is used to calculate incremental gain and impedance.BJT Small-Signal Parameters
At room temperature in forward-active operation:\[g_m=\frac{I_C}{V_T}\]
\[r_\pi=\frac{\beta}{g_m}\]
The incremental collector current is approximately:
\[i_c=g_mv_{be}\]
MOSFET Small-Signal Parameters
For the ideal square-law saturation model:\[g_m=\frac{\partial I_D}{\partial V_{GS}}\approx\frac{2I_D}{V_{ov}}\]
Channel-length modulation produces a finite output resistance, often approximated by:
\[r_o\approx\frac{1}{\lambda I_D}\]
The incremental drain current is then approximately \(i_d=g_mv_{gs}\), with an additional output-conductance term when ro is included.
11. Frequency Response and Capacitance
Every transistor contains capacitances. BJT junction capacitances and MOSFET gate, drain, and source capacitances affect gain and switching speed. A capacitance connected between input and output can be multiplied by the Miller effect, substantially increasing the effective input capacitance of an inverting amplifier. At low frequency, coupling and bypass capacitors may determine the lower cutoff frequency. At high frequency, internal device capacitances and carrier transit times reduce gain.12. Thermal Behavior and Safe Operation
Transistor characteristics change with temperature. BJT base-emitter voltage decreases with increasing temperature at a fixed current, while leakage currents generally increase. MOSFET on-resistance usually increases as the device heats. Bias stabilization and adequate thermal design are therefore essential. Power dissipation is approximately:\[P\approx VI\]
Junction temperature can be estimated using thermal resistance:
\[T_J\approx T_A+P\,\theta_{JA}\]
Voltage, current, power, junction temperature, and safe-operating-area limits from the device datasheet must all be respected.
13. Practical Selection Checklist
- Determine whether the application is switching, amplification, buffering, or current control.
- Check maximum voltage, current, power, and temperature ratings.
- For MOSFET switching, verify RDS(on) at the available gate-drive voltage, not merely the threshold voltage.
- For BJT switching, provide adequate base drive and consider storage time.
- Check input and output capacitances, gain, noise, and frequency requirements.
- Design for parameter variation rather than a single typical datasheet value.
- Verify thermal performance and transient safe operating area.