By analyzing the MOSFET, it is known that the switch can be realized by controlling the change of the PN junction.

Field effect transistor	Type	Name	Principle
FET	JFET	Junction	PN junction
	MOSFET	Metal oxide semiconductor
	MESFET	Metal semiconductor	Schottky junction
	MODFET	Modulation doping
	HEMT	High electron mobility

 

In fact, a Schottky junction can be realized through another structure.

 

Definitions of Junctions

Starting from the concept of junction, junction is the contact area of the two materials A and B.

PN junction is a space charge area formed at the interface. Using different doping processes, the P-type semiconductor and the N-type semiconductor are fabricated on the same semiconductor substrate (usually silicon carbide substrate or germanium) through diffusion. The electrons of the N-type semiconductor diffuse into the P-type semiconductor filled with holes; the P-type semiconductor is transformed into a semi-insulator in this area, and the N-type semiconductor is transformed into a semi-insulator. There is an electric field from the N-type semiconductor to the P-type semiconductor.

Schottky junction is the contact area between metal and N-type semiconductor. The electrons of the N-type semiconductor diffuse into the free electron gas of the metal with a low energy level. The metal in this region is transformed into an N-type semiconductor, and the N-type semiconductor is transformed into a P-type semiconductor; there is an electric field from the N-type semiconductor to the metal.

That is, by changing the gate of the MOSFET from metal + oxide + semiconductor to metal + N-type semiconductor, a MESFET (Metal-Semiconductor FET) is made.

MESFET is divided into two types:

1. Normally open/enhanced type: If the N-type semiconductor is very thick, the gate is turned on from the source to the drain when no voltage is applied, just as a switch is open at the beginning.

2. Normally closed/depleted type: If the N-type semiconductor is very thin, the gate is blocked from the source to the drain when no voltage is applied. In other words, a switch is closed at the beginning.

 

Reasons for Choosing Semi-insulating Substrate

Thus, why choose a semi-insulating substrate? Because if it is a conductive substrate, the structure will always be on, unable to achieve switching control; and the semi-insulating substrate can reduce parasitic capacitance. Generally speaking, the N-type semiconductor is gallium arsenide, and the gallium arsenide semi-insulating substrate will be selected; the N-type semiconductor is gallium nitride, and the choice of substrate will be SiC semi-insulating substrate, GaN semi-insulating substrate, etc. Semi-insulating silicon carbide is useful in this regard.

So why is the N-type semiconductor emphasized in the structure? Because the electron mobility is greater than the hole mobility, a faster migration can be achieved. This is also the reason for choosing Schottky junction.

To further improve the migration rate, a semi-insulating-semi-insulating heterojunction should be fabricated in the structure.

Modulation of AlxGa1-xAs/GaAs heterojunction or AlxGa1-xN/GaN heterojunction formed by doping metal aluminum Al: the contact area between semi-insulator and semi-insulator.

Since the energy of the Al-doped region is low, the electrons in the undoped region are attracted to the contact surface, which is the two-dimensional electron gas.

The migration rate of these electrons has been improved. This structure is called MODFET (Modulation-Doped FET) or HEMT (High Electron Mobility Transistor).

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