Fig. 4: The ionic BJT based on hydrogel diodes.

a The schematic of the ionic switch circuit and a photograph showing the Al3+-based hydrogel ionic NPN BJT. b The current response of IB and IC with VB as the input. VB was a square wave with an amplitude of 5 V and a frequency of 0.05 Hz. VC was set as 0 V. c IC versus VC with different IB as inputs, showing a typical output characteristic curve for NPN BJT. d IC versus IB under different voltages of VC. e IC versus VC with different VB as inputs