The electrical and optical properties of a four-terminal top back gate FET

A novel four-terminal device, the top back gate FET (TBGFET) grown by MBE is fabricated. A n⁺Sn-doped GaAs buried-contact layer is grown on Cr-doped SI first, and an undoped Al0.4- Ga0.6As insulating layer and Sn-doped GaAs channel are grown subsequently. Nonselective mesa etch is used for device isolation and selective etch is adopted to open the back-gate contact window. TBGFET Works as a MESFET and MISFET with a common channel, but can be separately controlled by the top gate and the back gate. We can adjust the transconductance and current level at room temperature as we wish by properly applying negative bias to the back gate or top gate. The top-gate threshold voltag V and back-gate threshold voltage Vbtfrom -0.5 to -10 V are obtained under different insulator thickness and trap density. If a proper criterion is satisfied, either top-gate threshold voltage Vtor back-gate threshold voltage Vbtcan be independently chosen without influencing each other. The decoupling of Vtand Vbtare well explained by the Lampert\´s law of the trap-fill-limited carrier injection in semi-insulator. The device interaction with light is also investigated. It is foUnd that the back-gate threshold voltage shift as well as the conduction current enhancement is caused principally by photon-induced detrapping in the semi-insulated AlGaAs layer. Photon generated e-h pairs in the device channel play a minor role either in current enhancement or in back-gate threshold voltage shift. Both and are increased with the light intensity and tend to saturate at some fixed values. Based on the above properties, a number of applications of TBGFET in digital and microwave circuit are proposed.