Constructional Features of GTO
Constructional Features of GTO:
- Like a thyristor, a GTO is also a four layer three junction p-n-p-n device. In order to obtain high emitter efficiency at the cathode end, the n cathode layer is highly doped. Consequently, the break down voltage of the function J3 is low (typically 20-40V).
- The p type gate region has conflicting doping requirement. To maintain good emitter efficiency the doping level of this layer should be low, on the other hand, from the point of view of good turn off properties, resistively of this layer should be as low as possible requiring the doping level of this region to be high. Therefore, the doping level of this layer is highly graded.
- Additionally, in order to optimize current turn off capability, the gate cathode junction must be highly interdigitated. A 3000 Amp GTO may be composed of upto 3000 individual cathode segments which are a accessed via a common contact. The most popular design features multiple segments arranged in concentric rings around the device center.
- The maximum forward blocking voltage of the device is determined by the doping level and the thickness of the n type base region next. In order to block several kv of forward voltage the doping level of this layer is kept relatively low while its thickness is made considerably higher (a few hundred microns).
- Beyond the maximum allowable forward voltage either the electric field at the main junction (J2) exceeds a critical value (avalanche break down) or the n base fully depletes, allowing its electric field to touch the anode emitter (punch through).
- The junction between the n base and p anode (J1) is called the “anode junction”. For good turn on properties the efficiency of this anode junction should be as high as possible requiring a heavily doped p anode region. However, turn off capability of such a GTO will be poor with very low maximum turn off current and high losses. There are two basic approaches to solve this problem.
- In the first method, heavily doped n layers are introduced into the p anode layer. They make contact with the same anode metallic contact. Therefore, electrons traveling through the base can directly reach the anode metal contact without causing hole injection from the p anode. This is the classic “anode shorted GTO structure” .
- Due to presence of these “anode shorts” the reverse voltage blocking capacity of GTO reduces to the reverse breakdown voltage of junction J3 (20-40 volts maximum). In addition a large number of “anode shorts” reduces the efficiency of the anode junction and degrades the turn on performance of the device. Therefore, the densities of the “anode shorts” are to be chosen by a careful compromise between the turn on and turn off performance.
- In the other method, a moderately doped n type buffer layer is juxtaposed between the n- type base and the anode. As in the case of a power diode and BJT this relatively high density buffer layer changes the shape of the electric field pattern in the n- base region from triangular to trapezoidal and in the process, helps to reduce its width drastically.
- However, this buffer layer in a conventional “anode shorted” GTO structure would have increased the efficiency of the anode shorts. Therefore, in the new structure the anode shorts are altogether dispensed with and a thin p type layer is introduced as the anode. The design of this layer is such that electrons have a high probability of crossing this layer without stimulating hole injection. This is called the “Transparent emitter structure”.