P-n junction isolation
Encyclopedia
p-n junction isolation is a method used to electrically isolate electronic components, such as transistors, on an integrated circuit
(IC) by surrounding the components with reverse biased p-n junctions.
, and connecting this surrounding material to a voltage which reverse-biases the p-n junction
that forms, it is possible to create a region which forms an electrically isolated "well" around the component.
is p-type material. Also assume a ring of n-type material is placed around a transistor, and placed beneath the transistor. If the p-type material within the n-type ring is now connected to the negative terminal of the power supply and the n-type ring is connected to the positive terminal, the 'holes
' in the p-type region are pulled away from the p-n junction, causing the width of the nonconducting depletion region
to increase. Similarly, because the n-type region is connected to the positive terminal, the electrons will also be pulled away from the junction.
This effectively increases the potential barrier and greatly increases the electrical resistance
against the flow of charge carriers. For this reason there will be no (or minimal) electric current across the junction.
At the middle of the junction of the p-n material, a depletion region
is created to stand-off the reverse voltage. The width of the depletion region
grows larger with higher voltage. The electric field grows as the reverse voltage increases. When the electric field increases beyond a critical level, the junction breaks down and current begins to flow by avalanche breakdown
. Therefore care must be taken that circuit voltages do not exceed the breakdown voltage or electrical isolation ceases.
", September 1977 Volume 23, Number 3, pp. 63-9, Robert Noyce
wrote:
Lehovec's received for isolation. He is reported to have said "I never got a dime out of [the patent]."
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
(IC) by surrounding the components with reverse biased p-n junctions.
Introduction
By surrounding a transistor, resistor, capacitor or other component on an IC with semiconductor material which is doped using an opposite species of the substrate dopantDopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
, and connecting this surrounding material to a voltage which reverse-biases the p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
that forms, it is possible to create a region which forms an electrically isolated "well" around the component.
Operation
Assume that the semiconductor waferWafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
is p-type material. Also assume a ring of n-type material is placed around a transistor, and placed beneath the transistor. If the p-type material within the n-type ring is now connected to the negative terminal of the power supply and the n-type ring is connected to the positive terminal, the 'holes
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
' in the p-type region are pulled away from the p-n junction, causing the width of the nonconducting depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
to increase. Similarly, because the n-type region is connected to the positive terminal, the electrons will also be pulled away from the junction.
This effectively increases the potential barrier and greatly increases the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
against the flow of charge carriers. For this reason there will be no (or minimal) electric current across the junction.
At the middle of the junction of the p-n material, a depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
is created to stand-off the reverse voltage. The width of the depletion region
Depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...
grows larger with higher voltage. The electric field grows as the reverse voltage increases. When the electric field increases beyond a critical level, the junction breaks down and current begins to flow by avalanche breakdown
Avalanche diode
In electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...
. Therefore care must be taken that circuit voltages do not exceed the breakdown voltage or electrical isolation ceases.
History
In an article entitled "Microelectronics", published in "Scientific AmericanScientific American
Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...
", September 1977 Volume 23, Number 3, pp. 63-9, Robert Noyce
Robert Noyce
Robert Norton Noyce , nicknamed "the Mayor of Silicon Valley", co-founded Fairchild Semiconductor in 1957 and Intel in 1968...
wrote:
"The integrated circuit, as we conceived and developed it at Fairchild Semiconductor in 1959, accomplishes the separation and interconnection of transistors and other circuit elements electrically rather than physically. The separation is accomplished by introducing pn diodes, or rectifiers, which allow current to flow in only one direction. The technique was patented by Kurt LehovecKurt LehovecKurt Lehovec is one of the pioneers of the integrated circuit, 1959. He innovated the concept of p-n junction isolation used in every circuit element with a guard ring: a reverse-biased p-n junction surrounding the planar periphery of that element. This patent was assigned to Sprague Electric...
at the Sprague Electric Company". http://www.virtuallystrange.net/ufo/updates/1997/sep/m01-033.shtml
Lehovec's received for isolation. He is reported to have said "I never got a dime out of [the patent]."