Cómo funciona un FET - Electrónica 1 - Clase 12

Transistors de Efecto de Campo: Introducción y Tipos

Introducción a los Transistores de Efecto de Campo

• En esta clase se abordarán los transistores de efecto de campo, con el objetivo de conocer su funcionamiento y características.

• La idea inicial para construir un dispositivo sólido a partir de semiconductores data desde 1945, pero el primer transistor funcional fue creado en 1948.

Desarrollo Histórico

• En 1960, los laboratorios Bell desarrollaron un nuevo diseño basado en teorías originales sobre transistores de efecto de campo.

• Los transistores FET (Field Effect Transistor) son controlados por un campo eléctrico en lugar de la inyección directa de electrones, lo que les diferencia de los bipolares.

Tipos y Clasificación

Clasificación General

• Se distinguen dos tipos principales: transistores con puerta aislada (MOSFET) y con puerta unión (JFET).

• Los JFET tienen una puerta y canal hechos de silicio dopado inversamente; mientras que los MOSFET utilizan metal como puerta.

Subtipos Específicos

• Dentro del MOSFET hay subgrupos como enriquecimiento y deflexión, que se dividen en canal N y P.

• Los JFET también se dividen en canal N y P, diferenciándose por la simbología del diodo entre la puerta y el canal.

Funcionamiento Interno

Estructura del JFET

• Un JFET consiste en una barra tipo N o P rodeada por material inversamente polarizado; esto afecta cómo fluye la corriente.

• La polarización inversa crea una región sin portadores móviles llamada región de carga espacial.

Comportamiento Eléctrico

• Cuando no hay tensión entre gate y source, no fluye corriente. Sin embargo, al aplicar un pequeño potencial positivo entre drain-source, comienza a fluir corriente.

• A medida que aumenta la tensión entre drain-source, el canal actúa como resistencia; inicialmente es lineal hasta alcanzar condiciones específicas.

Condiciones Críticas

Saturación del Canal

• Al aumentar aún más la tensión entre drain-source después del punto lineal, el canal puede saturarse; esto significa que incrementos adicionales no aumentan significativamente la corriente.

Transistor Operation and Characteristics

Current Flow and Resistance Behavior

• Electrons flow from the source to the drain, establishing a current through the channel, which behaves like a resistance until saturation is reached. Beyond this point, current remains constant despite increased voltage.

• When a negative potential is applied between the gate and source, it increases the space charge region, narrowing the channel and increasing its resistance while decreasing current. The slope of the curve decreases with more negative voltage.

Saturation Region Dynamics

• In saturation, if a sufficiently large reverse potential is applied between gate and source, it can completely deplete carriers in the channel, resulting in zero current. This condition is known as pinch-off voltage.

• The pinch-off voltage (indicated as -4V here) signifies that no free carriers exist in the channel; thus, current equals zero.

Transfer Characteristics of Transistors

• The transfer characteristic curve relates channel current to gate-source voltage during saturation. It highlights two key values:

• Current when gate-source voltage is zero (ideally should remain null).

• Pinch-off voltage where current ceases entirely.

• The equation defining this relationship shows a quadratic nature but only holds physical significance within certain regions of operation.

Conductance Parameters

• Analyzing transistor operation at specific points allows for linearization using tangents; this slope represents transconductance (gm), derived from output characteristics.

• gm serves as a constant parameter for specific operating points but real-world curves show slight deviations indicating non-ideal behavior due to output resistance.

Fixed Biasing Techniques

• A fixed bias configuration involves applying negative bias to stabilize transistor operation. The load line can be drawn based on equations relating source voltage drop across resistors to gate-source voltages.

• If Vgs equals zero under fixed bias conditions, maximum drain current occurs at Vdd/Rd; adjustments lead to various operational points along the load line.

Alternative Circuit Configurations

• Another circuit design includes placing a resistor in series with ground connected directly to the gate. Here, since gate current remains negligible, any drop across this resistor also approaches zero.

• As drain current flows through the transistor causing positive polarity drops across resistors, it results in negative polarization at the gate relative to source.

Equations for Polarization Analysis

• Key equations relate resistor voltages back to currents flowing through them; these help establish load lines by determining intersection points with characteristic curves for operational analysis.

Analysis of Transistor Operation and Circuit Design

Understanding the Circuit Configuration

• The operation point Q is defined by two intersections: one with the characteristic curve and another that lacks physical significance.

• The gate voltage relative to ground is not zero; it depends on resistors R1 and R2 . Analyzing this requires traversing the circuit mesh.

• By applying Kirchhoff's law, we can express the gate-source voltage ( V_gs ) in terms of other voltages in the circuit, leading to a clearer understanding of operational points.

Current Analysis at Operational Points

• The current through resistor R_s can be expressed as I_d = fracV_12R_s , establishing a relationship between input voltage and output current.

• Two significant points arise from analyzing the equation: one where current is negative (crossing into physical relevance), and another where it intersects with the characteristic curve.

Feedback Mechanisms in Amplification

• To analyze feedback effects, a capacitor ( C_s ) blocks resistance feedback, allowing for an equivalent circuit analysis.

• The transconductance ( g_m ) value is crucial for determining how input signals affect output currents within specific operating conditions.

Gain Calculation in Transistor Circuits

• The gain calculation involves assessing how output voltage relates to input voltage, yielding a formula based on parallel resistances within the circuit.

• Input resistance can be identified as parallel combinations of resistors R1 , R2 , while output resistance combines drain resistor ( R_d ) with biasing resistances.

Differences Between N-channel and P-channel Transistors

• For P-channel transistors, characteristics mirror those of N-channel but with reversed polarities; negative gate-source voltages become positive.

• In P-channel configurations, control over current flow shifts from being dependent on current (as in bipolar junction transistors) to being controlled by voltage levels across gates.