JFET - WikiHQ

The junction field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers.

Unlike bipolar junction transistors, JFETs are exclusively voltage-controlled in that they do not need a biasing current. Electric charge flows through a semiconducting channel between source and drain terminals. By applying a reverse bias voltage to a gate terminal, the channel is pinched, so that the electric current is impeded or switched off completely. A JFET is usually conducting when there is zero voltage between its gate and source terminals. If a potential difference of the proper polarity is applied between its gate and source terminals, the JFET will be more resistive to current flow, which means less current would flow in the channel between the source and drain terminals.

JFETs are sometimes referred to as depletion-mode devices, as they rely on the principle of a depletion region, which is devoid of majority charge carriers. The depletion region has to be closed to enable current to flow.

JFETs can have an n-type or p-type channel. In the n-type, if the voltage applied to the gate is negative with respect to the source, the current will be reduced (similarly in the p-type, if the voltage applied to the gate is positive with respect to the source). Because a JFET in a common source or common drain configuration has a large input impedance (sometimes on the order of 1010 ohms), little current is drawn from circuits used as input to the gate.

History

A succession of FET-like devices was patented by Julius Lilienfeld in the 1920s and 1930s. However, materials science and fabrication technology would require decades of advances before FETs could actually be manufactured.

JFET was first patented by Heinrich Welker in 1945. During the 1940s, researchers John Bardeen, Walter Houser Brattain, and William Shockley were trying to build a FET, but failed in their repeated attempts. They discovered the point-contact transistor in the course of trying to diagnose the reasons for their failures. Following Shockley's theoretical treatment on JFET in 1952, a working practical JFET was made in 1953 by George C. Dacey and Ian M. Ross.

High-speed, high-voltage switching with JFETs became technically feasible following the commercial introduction of Silicon carbide (SiC) wide-bandgap devices in 2008. Due to early difficulties in manufacturing — in particular, inconsistencies and low yield — SiC JFETs remained a niche product at first, with correspondingly high costs. By 2018, these manufacturing issues had been mostly resolved. By then, SiC JFETs were also commonly used in conjunction with conventional low-voltage Silicon MOSFETs. In this combination, SiC JFET + Si MOSFET devices have the advantages of wide band-gap devices as well as the easy gate drive of MOSFETs.

When the depletion layer spans the width of the conduction channel, pinch-off is achieved and drain-to-source conduction stops. Pinch-off occurs at a particular reverse bias (VGS) of the gate–source junction. The pinch-off voltage (Vp) (also known as threshold voltage or cut-off voltage) varies considerably, even among devices of the same type. For example, VGS(off) for the Temic J202 device varies from to , while the VGS(off) for the J308 varies between and . (Confusingly, the term pinch-off voltage is also used to refer to the VDS value that separates the linear and saturation regions.

In every case the arrow head shows the polarity of the P–N junction formed between the channel and the gate. As with an ordinary diode, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in".

Comparison with other transistors

At room temperature, JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. The JFET has higher gain (transconductance) than the MOSFET, as well as lower flicker noise, and is therefore used in some low-noise, high input-impedance op-amps. Additionally the JFET is less susceptible to damage from static charge buildup.

Mathematical model

Linear ohmic region

The current in N-JFET due to a small voltage VDS (that is, in the linear or ohmic or triode region

: <math>I_\text{D} = \frac{bW}{L} q N_d \mu_n V_\text{DS},</math>

where

: ID = drain–source current,

: b = channel thickness for a given gate voltage,

: W = channel width,

: L = channel length,

: q = electron charge = 1.6 C,

: μn = electron mobility,

: Nd = n-type doping (donor) concentration,

: VP = pinch-off voltage.

Then the drain current in the linear region can be approximated as

: <math>I_\text{D} = \frac{bW}{L} q N_d \mu_n V_\text{DS} = \frac{aW}{L} q N_d \mu_n \left(1 - \sqrt{\frac{V_\text{GS{V_\text{P}\right) V_\text{DS}.</math>

In terms of <math>I_\text{DSS}</math>, the drain current can be expressed as

: <math>I_\text{D} = \frac{2 I_\text{DSS{V_\text{P}^2} \left(V_\text{GS} - V_\text{P} - \frac{V_\text{DS{2}\right) V_\text{DS}.</math>

Constant-current region

The drain current in the saturation or active is often approximated in terms of gate bias as

: <math>b = a \left(1 - \sqrt{\frac{V_\text{GS{V_\text{P}\right),</math>

where

: VP is the pinch-off voltage the gate–source voltage at which the channel thickness goes to zero,

: a is the channel thickness at zero gate–source voltage.

Transconductance

The transconductance for the junction FET is given by

: <math>g_\text{m} = \frac{2 I_\text{DSS{|V_\text{P}|} \left(1 - \frac{V_\text{GS{V_\text{P\right),</math>

where <math>V_\text{P}</math> is the pinchoff voltage, and IDSS is the maximum drain current. This is also called <math>g_\text{fs}</math> or <math>y_\text{fs}</math> (for transadmittance).

References

External links

Physics 111 Laboratory -- JFET Circuits I
Interactive Explanation of n-channel JFET