What is Carrier Mobility?

What is Carrier Mobility?

Carrier mobility is often used to refer to the overall movement of electrons and holes in semiconductors. Mobility refers to the average drift velocity of carriers (electrons and holes) under the action of a unit electric field. That is a measure of the speed of the movement of carriers under the action of an electric field, the faster the movement, the greater the mobility; the slower the motion, the lower the mobility. In the same semiconductor material, the types of carriers are different, and the mobility is different. Generally, the mobility of electrons is higher than that of holes. For example, at room temperature, the mobility of electrons in the lightly doped silicon material is 1350cm2/(VS), while the mobility of holes is only 480cm2/(VS). PAM-XIAMEN can test carrier mobility on semiconductor wafers offered.

1. How does Mobility Affect the Device?

On one hand, the carrier concentration together determines the size of the conductivity (reciprocal of resistivity) of the semiconductor material. The higher the mobility, the lower the resistivity, the lower the power consumption and the greater the current carrying capacity when the same current is passed. Since the mobility of electrons is generally higher than that of holes, power MOSFETs usually always use an n-channel structure in which electrons are used as carriers, rather than a p-channel structure in which holes are used as carriers.

On the other hand, it will affect the operating frequency of the device. The most important limitation of the frequency response characteristics of bipolar transistors is the time for minority carriers to transit the base region. The greater the mobility, the shorter the required transit time, and the cut-off frequency of the transistor is proportional to the carrier mobility of the base material. Therefore, increasing the carrier mobility can reduce power consumption and improve the current carrying capacity of the device. At the same time, the switching speed of the transistor is improved. Generally speaking, the mobility of P-type semiconductors is 1/3 to 1/2 of that of N-type semiconductors.

2. How to Measure Carrier Mobility of Semiconductor Wafers?

We can use following methods for measuring carrier mobility:

  • Transit time (TOP) method: suitable for the measurement of the carrier mobility of materials with better photogenerated carrier function and can measure the low mobility of organic materials;
  • Hall Effect method: suitable for the measurement of the carrier mobility of larger inorganic semiconductors;
  • Radiation-Induced Conductivity (SIC) method: ideal for conductive materials where the conduction mechanism is space charge limited.
  • Current-voltage characteristic method: mainly applicable to the measurement of the carrier mobility of the inversion layer of the MOSFET operating at room temperature;
  • Surface wave transmission method;
  • Voltage decay method;
  • The polarity reversal method of the applied electric field;

In addition, drift experiments, analysis of ion diffusion, and analysis of transient response of pyroelectric current polarization and charge can also be used to measure carrier mobility.

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