This invention relates to field effect transistors and more particularly to metal-semiconductor field effect transistors or MESFETS. This invention is more specifically concerned with modulation doped field effect transistors (MODFETs) or high-electron mobility transistors (HEMTs) which can operate at microwave and millimeter wave frequencies.
HEMTs have demonstrated excellent gain and low noise at extremely high frequencies, but high output conductance and significant gate leakage current have limited the performance of HEMTs.
Generally speaking, a HEMT transistor has a substrate on which a buffer layer is grown of a semiconductor material such as a III-V compound having a lattice constant that closely matches the substrate. A layer of low band-gap semiconductor material is grown on the buffer layer, and a layer of high-band gap material is grown on the low-band gap layer. Source, gate, and drain electrodes are metallized onto an upper surface.
A cap layer of Si-doped low-band gap material is formed on the upper layer of high band-gap material, and the electrodes are metallized onto this cap layer. A recess is etched through this for the metallization of the gate electrode onto the high band gap material. For the surface doped device, there is a reasonably high output conductance, e.g. 50 mS/mm. The highly doped cap layer has a high conductance, so that the entire cap layer on the drain side of the recess is at substantially the same potential as the drain. While this has the effect of shortening the gate channel-to-drain current path, it also places a sharp potential drop in the short distance from the gate to the edge of the recess (i.e., 100 to 300 angstroms). For a given gate potential V.sub.gs, as the drain potential V.sub.ds is increased, the channel potential gradient in the short zone near the gate becomes quite large because the potential difference between the gate and drain is concentrated in the short distance from the drain-side edge of the gate to the edge of the recess. This imparts a high electric field near the drain side edge of the gate.
A consequence of increasing the drain to gate potential V.sub.dg is to generate a high electric field domain near the drainside gate edge. Because the buffer layer below the low band gap material is close to source potential, there is some leakage from the two dimensional electron gas sheet charge into the buffer. This reduces the charge control capability of the gate electrode, and the output conductance g.sub.o increases (where g.sub.o =I.sub.ds /V.sub.ds).
A high-performance quarter-micron gate length surface-doped InAlAs/InGaAs/InP high electron mobility transistor can have a DC output conductance g.sub.o of 50 mS/mm or more, and its maximum or cutoff frequency f.sub.max is limited to perhaps 100 GHz, with a unity current gain cutoff frequency of perhaps 70 GHz.
The sharp potential drop at the gate edge is also responsible for significant gate leakage current. This is important especially in power applications where the gate input voltage is limited by gate leakage.