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The Wonderful Journey of Semiconductors: How was JFET invented?
In modern electronic devices, field-effect transistors (FETs) play an indispensable role. The junction field effect transistor (JFET) is one of the basic building blocks in these devices. As a three-terminal semiconductor device, JFET has the function of electronically controlling switches and resistors, and can even be used to build amplifiers. Unlike bipolar junction transistors (BJTs), JFETs are fully voltage controlled, meaning they require no bias current, a feature that makes JFETs a great advantage in many applications.
A JFET usually conducts when the voltage between its gate and source is zero. If a bias voltage of the appropriate polarity is applied, it will reduce the flow of current.
How JFET works
The basic operating principle of JFET can be compared to a garden hose. The amount of water flow can be controlled by shrinking the diameter of the water pipe. When a voltage is applied between the gate and source of a JFET, a depletion region is formed that no longer conducts electricity due to a lack of mobile carriers. As the depletion region expands, the cross-section of the conductive channel decreases, thereby limiting the flow of current. When the depletion layer is thick enough to completely span the conductive channel, the JFET enters what is called a "squeeze-off" state.
JFET can be regarded as a depletion-mode component, relying on the principle of the depletion region to control the flow of current.
Historical evolution of JFET
The development of JFET can be traced back to the early 20th century. Julius Lilienfeld applied for a series of FET-like patents in the 1920s and 1930s. A true JFET was first patented in 1945 by Heinrich Welker. By the 1940s, Nobel Prize winners John Bardeen, Walter Houser Brattain, and William Shockley were also developing FETs, but the technology was not yet mature at that time, and one after another fail. Finally, the JFET derived from Shockley's theory in 1952 was successfully built in 1953 by George C. Dacey and Ian M. Ross.
In 1950, Japanese engineers Jun-ichi Nishizawa and Y. Watanabe patented a similar device known as the static-induced transistor (SIT).
Structure introduction
The basic structure of a JFET is composed of a long section of doped semiconductor material, which may be a p-type or n-type semiconductor. Each end forms an ohmic junction, a source (S) and a drain (D). A pn junction is formed on either side of or around this semiconductor channel and its voltage is biased through the ohmic gate contact (G).
JFET Ratio
Compared with other field-effect transistors, the gate current of JFET at room temperature (that is, the reverse leakage current from the gate to the channel junction) is comparable to that of MOSFET, but is much lower than the base current of bipolar junction transistors. JFET has a higher transconductance than MOSFET and has low flicker noise, so it is used in some low-noise and high input impedance operational amplifiers.
Because the JFET has extremely high input impedance in the circuit, it only consumes a trace amount of current for the circuit used as input.
The current situation and future of JFET
With the evolution of technology, especially the introduction of commercial silicon carbide (SiC) wide-bandgap devices in 2008, JFET has become feasible in high-speed, high-voltage switching applications. Although there were difficulties in the production of SiC JFETs in the early days, these problems have been basically solved and are widely used in scenarios where they are used with traditional low-voltage silicon MOSFETs.
With the development of electronic technology, JFET technology will also face more applications and challenges. Can we expect JFET to play a greater role and potential in future electronic devices?
