The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor)#

This is the big one, invented with Mohamed Atalla in 1959 at Bell Labs.

MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor): A type of transistor used for amplifying or switching electronic signals. It’s a fundamental building block in modern electronics, essentially acting like an electrically controlled switch.

Before the MOSFET, the prevalent transistor was the bipolar junction transistor (BJT). While important, BJTs had limitations, especially regarding power consumption and ease of manufacturing for complex circuits. The MOSFET offered advantages that made it ideal for scaling down and integrating many transistors onto a single chip.

Kahng and Atalla didn’t just invent the concept; they built working versions. They fabricated both the PMOS (P-channel MOSFET) and NMOS (N-channel MOSFET) devices.

PMOS (P-channel MOSFET): A type of MOSFET where the channel is made of p-type semiconductor material. It conducts current when a negative voltage is applied to the gate.

NMOS (N-channel MOSFET): A type of MOSFET where the channel is made of n-type semiconductor material. It conducts current when a positive voltage is applied to the gate.

These early devices were made using a 20 micrometre (μm) process, which sounds large by today’s standards (modern processes are measured in nanometres, like 5nm or 3nm), but it was state-of-the-art at the time.

Why is the MOSFET so important? It is the most widely used type of transistor in existence. Look around – your smartphone, computer, smart TV, microwave, car electronics, everything with a digital brain likely has billions of MOSFETs inside. It’s the basic switching element that makes digital logic circuits possible and the foundation for integrated circuits (ICs), also known as microchips.

The Concept of the MOS Integrated Circuit#

Following their success with the individual MOSFET device, Kahng and Atalla also proposed the idea of combining multiple MOSFETs and other components onto a single chip using their MOS technology. This concept was a crucial step towards realizing the powerful microprocessors and memory chips we have today.

Integrated Circuit (IC): A tiny electronic circuit printed on a piece of semiconductor material (like silicon). It contains many transistors, resistors, and capacitors connected together to perform complex functions.

Pioneering Work on Schottky Diodes#

Kahng and Atalla’s work extended beyond just the basic MOSFET. In the early 1960s, they did pioneering research on devices that utilized what would later be called a Schottky barrier.

Schottky Barrier: A potential energy barrier formed at a metal-semiconductor junction. This barrier allows current to flow easily in one direction but resists flow in the other, which is the fundamental behavior of a diode.

Schottky Diode (Schottky-Barrier Diode): A diode formed by a metal-semiconductor junction, characterized by a low forward voltage drop and very fast switching speed compared to conventional PN junction diodes.

While the concept of a metal-semiconductor junction acting as a rectifier (allowing current one way) had been known, Kahng and Atalla were among the first to practically realize efficient Schottky diodes. They published their findings in 1962, describing a device they called the “hot electron” triode structure. Their work was significant because it showed how to control these junctions effectively for device applications. Schottky diodes found immediate use, particularly in mixer applications in radio frequency (RF) circuits, where their fast switching speed is a big advantage. They also conducted further research specifically on high-frequency Schottky diodes.

Early Nanolayer-Base Transistors#

In 1962, Kahng and Atalla also demonstrated an early version of a metal nanolayer-base transistor. This was quite an advanced concept for the time, exploring different transistor structures.

Nanolayer-Base Transistor: A type of transistor structure where the base (the control layer) is made of a very thin metallic film, typically just nanometres thick, sandwiched between two semiconductor layers acting as the emitter and collector.

The idea behind this structure was to achieve high operating frequencies. Because the metal base was so thin (nanometric thickness), the time it took for charge carriers to cross the base (transit time) was very short, leading to faster operation compared to traditional bipolar transistors.

Their experimental setup involved depositing incredibly thin films of gold (Au), about 10 nanometres (nm) thick, onto a single crystal semiconductor substrate made of n-type germanium (n-Ge). This metal film served as the base layer, and the n-Ge acted as the collector. The emitter was made from a piece of n-type silicon (n-Si) shaped to have a sharp point or a blunt corner, which was then pressed against the thin gold layer – this was the “point contact”. It was a sophisticated piece of materials engineering and device physics for the era.

The Floating-Gate MOSFET (FGMOS)#

Another major invention by Dawon Kahng, this time with his colleague Simon Min Sze, was the floating-gate MOSFET (FGMOS), first reported in 1967.

Floating-Gate MOSFET (FGMOS): A MOSFET structure that includes a second gate, called the “floating gate,” electrically isolated from the control gate and the rest of the transistor by insulating layers. Charge can be stored on this floating gate, trapping information.

The key feature here is that the floating gate is completely surrounded by an insulator. This means charge placed on the floating gate is trapped there for a very long time, even when the power is off. This property makes the FGMOS ideal for memory applications, particularly non-volatile memory.

Non-Volatile Memory (NVM): A type of computer memory that can retain stored information even when not powered. Examples include ROM, EPROM, EEPROM, and flash memory.

Kahng and Sze realized this potential. They invented the floating-gate memory cell and proposed that the floating gate’s ability to store charge could be used to store data in a reprogrammable read-only memory (ROM). This idea was foundational.

Impact on Memory Technologies#

The floating-gate memory cell proposed by Kahng and Sze became the direct basis for several hugely important non-volatile memory technologies:

• EPROM (Erasable Programmable ROM): Memory that could be programmed electronically but erased by exposing the chip to strong ultraviolet light.
• EEPROM (Electrically Erasable Programmable ROM): An improvement on EPROM that can be erased electrically, byte by byte.
• Flash Memory: An advanced form of EEPROM that allows large blocks of data to be erased and written at once. This is the memory technology used in USB drives, solid-state drives (SSDs), memory cards for cameras, and storage in smartphones and tablets.

Without Kahng’s invention of the FGMOS and his insight into its use as a memory cell, the flash memory that is so common and essential today might not exist, or at least would have developed much differently.

Other Research#

Beyond these major inventions, Dawon Kahng also conducted research in other areas, including ferro-electric semiconductors (materials where electric polarization can be reversed by an electric field) and luminous materials, making contributions to the field of electroluminescence (the emission of light from a material when an electric current is passed through it).