As is known, the quest in the semiconductor industry is for integrated circuit chips of ever larger throughput. One factor that can limit this throughput is overheating from the heat dissipated in the circuit. Overheating can destroy the relatively fragile tiny semiconductive circuit elements. To limit such heat dissipation, it has become common practice to employ complementary MISFETS circuitry whereby the quiescent current can be kept low, as is desirable to keep heat dissipation low. Such complementary circuits include both P-type and N-type MISFETS and for an integrated circuit it is necessary to have both types on the same chip. Moreover, it is also known to be preferable to have available field-effect transistors of the enhancement-mode type as compared to depletion-mode type since it is usually advantageous that the transistors be essentially off when in their quiescent state to reduce the amount of current dissipated by the integrated circuit.
Another expedient to avoid overheating is to refrigerate the integrated circuit. Operation at a low temperature has the added advantage of reducing the thermal generation of charge carriers, holes and electrons, making the semiconductor circuit element less noisy and thereby permitting operation with lower current levels, which results in lower power dissipation.
Another factor that is important to high throughput for an integrated circuit is keeping short the delay introduced by the time it takes a transistor to switch between conducting and non-conducting states. Generally this time is determined by how long it takes the active charge carriers, electrons in N-type MISFETS and holes in P-type MISFETS, to transverse the channel region of the transistor. This time is dependent not only on the length of the channel but also on the mobility of the active charge carrier. Accordingly, in complementary circuits, the mobilities of both the holes and electrons are important to achieve short delays and so high speed.
A shortcoming of gallium arsenide, the most popular component semiconductor, for use in high throughput complementary circuitry is the low mobility of its holes since this limits the speed with which the P-type MISFETS can be switched between states. This in turn limits the speed that can be achieved in a complementary circuit using gallium arsenide circuit elements.
It is known that this shortcoming is essentially absent in gallium antimonide in which both holes and electrons have relatively high mobilities, especially when the gallium antimonide is operated below room temperature, for example, at about liquid nitrogen temperature (77K). As mentioned above, operation below room temperature is desirable also to reduce thermal noise and thereby desirably to permit operation with lower levels for the signals.
However, a problem hitherto with the use of gallium antimonide as the active semiconductor in a MISFET has been the difficulty of establishing a good insulated layer for the gate connection to its channel region. As is well known, in an MISFET, it is important to establish an insulated layer for the gate connection to its channel region that is relatively free of interfacial states, since these states undesirably act to shield the channel against penetration of the electric field being supplied for control by the gate voltage during operation. A critical factor in the success of the silicon MISFET particularly that of the enhancement mode type, is the ability of its thermally-grown native, or genetic, oxide to provide an insulating layer for the gate connection that is relatively free of interfacial states. For operation with low threshold levels which is necessary to support low signal levels, for example, no more than a few hundredth of a volt, as is desirable is high density integrated circuits, the concentration of interfacial states should be less than 10.sup.11 per cubic centimeter, and preferably less than 10.sup.10 per cubic centimeter. However, with gallium antimonide, a native oxide layer does not possess similar properties and thus cannot be used as an insulating layer, so that an alternative approach is needed.
It is known that the intrinsic resistivity of a semiconductive material is related to its band gap, the minimum energy separation between its highest occupied state and its lowest empty state in its energy diagram. Accordingly, a semiconductor with a relatively wide band gap can act essentially as an insulator with respect to a semiconductor with a relatively narrow band gap.
Accordingly, there previously have been proposals, for use in a MISFET as the gate insulating layer, a layer of semiconductor of a wider band gap than that of the semiconductor forming the channel. In practice, this proved to be difficult in the case of a gallium antimonide channel without increasing significantly the interfacial states since the layer of semiconductor typically had to include foreign atoms to facilitate lattice matching and to function effectively in its intended role, and such doping added interfacial states.