Basic principle of semiconductor pressure sensor
Semiconductor pressure sensors can be divided into two main categories. One category consists of various piezoresistive diodes or transistors based on the principle that the I-υ characteristics of a semiconductor PN junction (or Schottky junction) change under stress. These pressure-sensitive devices have unstable characteristics and have not seen significant development. The other category comprises sensors based on the semiconductor piezoresistive effect, which is the main type of semiconductor pressure sensor. Initially, most were made by attaching semiconductor resistance strain gauges to elastic elements to create various stress and strain measuring instruments. In the 1960s, with the development of semiconductor integrated circuit chip technology, semiconductor pressure sensors using diffused resistors as piezoresistive elements emerged. These pressure sensors have a simple and reliable structure, with no moving parts; the pressure-sensitive element and elastic element are integrated, eliminating outdated mechanical components and stress relaxation, thus improving sensor performance.
The piezoresistive effect in semiconductors refers to a property where the resistivity (denoted by ρ) changes with the applied force. The relative change in resistivity per unit stress is called the piezoresistive coefficient, denoted by π. Mathematically, it is expressed as ρ/ρ = πσ.
In the formula, σ represents stress. The change in resistance (πR/R) that a semiconductor resistor needs to withstand stress is determined by the change in resistance. Therefore, the above piezoresistive relationship can also be written as πR/R = πσ.
Under the influence of external forces, certain stresses (σ) and strains (ε) are generated in the semiconductor crystallization process. The intrinsic relationship between them is determined by the Young's modulus (Y) of the raw material, i.e., Y = σ/ε.
If the piezoresistive effect is expressed in terms of the strain that a semiconductor can withstand, then R/R = Gε
G is called the sensitivity factor of a pressure sensor, which represents the relative change in resistance caused by a unit strain.
The piezoresistive index, or sensitivity factor, is a fundamental physical parameter of the piezoresistive effect in semiconductors. The relationship between them, like that between stress and strain, is determined by the Young's modulus of the raw material, i.e., G = πY.
Because semiconductor crystals exhibit anisotropic elasticity, Young's modulus and piezoresistive coefficient change with crystal orientation. The magnitude of the piezoresistive effect in semiconductors is also closely related to the semiconductor's resistance; the lower the resistance, the smaller the sensitivity factor. The piezoresistive effect of a diffusion resistor is determined by the crystallization tendency of the diffusion resistor and the impurity concentration. The impurity concentration specifically refers to the surface impurity concentration of the diffusion layer.
Semiconductor pressure sensor construction
Common semiconductor pressure sensors use N-type single-crystal silicon wafers as substrates. First, the silicon wafer is shaped into a malleable, force-bearing component with a specific geometric shape. Four P-type diffused resistors are fabricated along different crystal orientations at the force-bearing locations on the single-crystal silicon wafer. These four resistors are then used to form a four-arm Wheatstone bridge. Under the action of external force, the change in resistance is converted into an electrical signal output. This Wheatstone bridge with pressure effect is the heart of the pressure sensor and is generally called a piezoresistive bridge circuit (Figure 1). The characteristics of a piezoresistive bridge circuit are: ① The resistance values of the four arms of the bridge circuit are the same (all R0); ② The piezoresistive effect values of adjacent arms of the bridge circuit are the same but with opposite signs; ③ The resistance temperature coefficients of the four arms of the bridge circuit are the same, and they are always at the same temperature. In the diagram, R0 represents the resistance under no stress at indoor temperature; ΔRT represents the change in resistance caused by the temperature coefficient of resistance (α) when the temperature changes; ΔRδ represents the change in resistance caused by strain (ε); the output voltage of the bridge is u = I0ΔRδ = I0RGδ (constant current source bridge).
In the formula, I0 is the constant current source current, and E is the constant voltage source voltage. The output voltage of the piezoresistive bridge circuit is directly positively correlated with the strain force (ε) and is independent of RT caused by the temperature coefficient of the resistor, which greatly reduces the temperature drift of the sensor. The most widely used semiconductor pressure sensor is a fluid pressure sensor. Its key structure is a diaphragm cell made entirely of photovoltaic materials (Figure 2). The diaphragm is cup-shaped, and the part inside the cup bears the external force. The pressure bridge circuit is made on the bottom of the cup. A circular base is made of the same silicon single crystal material, and then the diaphragm is bonded to the base. This pressure sensor has the advantages of high sensitivity, small size, and solid-state design, and has been widely used in aerospace, instrumentation, and medical equipment.
