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The DEPFET Principle

The DEPFET concept was first introduced by Kemmer and Lutz in 1987. The principal idea is to integrate the first amplifying transistor directly into high ohmic silicon substrate (fig. 1). This way, stray capacitances due to connections between sensor and amplifier can be avoided and a small input capacitance can be realized. A cross section of a DEPFET pixel structure is shown in fig.2. 

 

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Fig. 1: Principle idea of DEPFET pixel detectors. 

 

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Fig. 2: Cross section of DEPFET pixel. The source is the symmetry axis of the pixel structure.

 

By sidewards depletion and an additional n-implantation, a potential minimum for electrons is created right underneath the transistor channel, which is called the "internal gate". The holes generated by an impinging particle drift to the rearcontact, while the electrons are collected in the internal gate of the transistor. This results in a modulation of the transistor current, which is proportional to the collected charge. Since the internal gate fills up with electrons from signals as well as from leakage current, it has to be reset periodically by applying a positive voltage to an external clear contact (fig. 3).

 

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Fig. 3: Reset of the internal gate by applying a positive voltage pulse to the clear contact.

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