Excess drain current in heterojunction FETs due to substrate space-charge-limited current

A comparison of gated and ungated heterostructure FETs shows that high output conductance, poor pinchoff, and kinks in the I-V characteristics of heterojunction FETs (HFETs) are related to space-charge-limited conduction (SCLC) associated with deep traps in the substrate. The dominant trap level was determined from I-V measurements of an ungated structure as a function of temperature and the trap density was determined from SCLC characteristics. Reduction in excess drain current can be achieved by using a high trap concentration substrate and by increasing the electron barrier between the channel and the substrate.<>