The invention discloses a 4H-SiC 
PIN diode based on gradient 
doping of a P region and an I region. The 4H-SiC 
PIN diode mainly solves the problem that low on-resistance and low junction 
capacitance cannot be achieved in the prior art. The 4H-SiC 
PIN diode comprises an n-type substrate (1), an n + layer (2), an i layer (3) and a p + layer (4) from bottom to top, a 
cathode (5) is arranged on the lower portion of the substrate, an 
anode (6) is arranged on the upper portion of the p + layer, a 
passivation layer (7) is arranged on the upper portion of the 
anode, the i layer (3) is of a multi-layer structure composed of n 
layers, and all the 
layers are made of 4H-SiC 
semiconductor materials with linearly decreasing 
doping concentration; the p + layer (4) is of a multi-layer structure composed of m 
layers, and each layer is made of a 4H-SiC 
semiconductor material with linearly increased 
doping concentration. According to the invention, low on-resistance and low 
zero bias capacitance can be realized at the same time, the response speed of the PIN 
diode can be accelerated, the 
cut-off frequency is improved, and the PIN 
diode can be used as a 
microwave diode to be applied to a 
microwave amplitude limiting circuit.