Power Discrete Epi

Traditionally, power discrete devices and power IC's are built on an epitaxial wafer because it provides the device fab with a great controllability of series resistance (or conductivity) in the base of diodes, rectifiers, thyristors, or bipolar junction transistors (BJT), or in the drift zone of MOSFETs and IGBTs. Typically, thin base or drift is preferred, which is achievable and cost-effective using epi wafers. Additionally, an epi wafer's inherently uniform resistivity distribution, both from wafer to wafer and within the wafer, provides additional benefit. Lower resistivity of the epi layer is the preferred solution for lower Rds(on), but it may cause breakdown voltage to be below the acceptable level. As a result, the resistivity and thickness specifications for power discrete epi wafers are very tight. Rds(on) is the series resistance of the device during its "on" state that is a very critical parameter and affects the device's power consumption. SunEdison Semiconductor provides epi wafers specifically for power MOSFET, bipolar and other discrete applications. Combinations of N-type and P-type epi layers and heavily doped substrates are offered for these applications.

 

Epi Reactor Diameter Type Epi Thickness
Target, UM
Epi Resistivity
Target, Ohm-cm
Uniformity, +/-%
Grade Epi
Resistivity
Epi
Thickness
Batch 150mm
200mm
P/P++
N/N+
N/N++
N/N+/N++
N/P/P+
P/N/N+
3-70 0.2-30 Standard 8-10 5-8
Enhanced 6 4
Single Wafer 150mm
200mm
P/P++
N/N++
N/N+/N++
3-30 0.1-30 Standard 5 5
Enhanced 3 2


Certain power discrete devices such as IGBT and special BJT products are built in a double-layer epi and, in a special case, a triple-layer epi to simplify device process design or reduce fabrication cost. For example, IGBT epi wafer has two layers of epi deposited onto heavily doped substrate of the opposite type (e.g., N/N+/P++.) A special BJT epi wafer has double layers of epi deposited onto heavily doped substrate of the same type (e.g., N/N+/N++.) Another special BJT epi wafer has two layers of epi of the opposite type (e.g., N/P/P++). Multiple layer epi wafers are mostly used for high voltage/power devices and tend to be thick. However, the resistivity uniformity of multiple-layer power discrete epi wafers is comparable to that of single layer epi with the same thickness. )

 

Sub Dopant Resistivity Target, Ohm-cm Diameter
Boron <0.001-0.02 150mm, 200mm, 300mm
Arsenic 0.0017-0.007 150mm, 200mm
Phosphorus <0.001-0.003 150mm, 200mm
Antimony 0.005-0.03 150mm, 200mm, 300mm

Red phosphorus doped substrate for epi: The resistance of epi substrate is also an important component of the series resistance of power discrete devices and its contribution to the final device Rds(on) becomes increasingly significant in low voltage (<= 30V) power devices, such as cell phones and other hand-held electronics where low power consumption is most critical to long battery life. The beneficial effect of low resistivity substrate is particularly important, as it does not limit breakdown voltage. Achieving low Rds(on) by using very low resistivity substrate can eliminate the need to develop the next technology platform, and thus reduce cost and time to market. For this matter, n-type epi deposited onto n-type substrate with very low resistivity (e.g., < 0.0015 ohm-cm) suits the need, which is achievable through the recent development of elemental red phosphorus doped crystal growth technology.