Cut Rectangular Transistor Bars
# Cut Rectangular Transistor Bars: Radial Non-Uniformity, Not Just Axial
Every segregation and drift calculation this process has used so far — zone refining, the melt-concentration drift between pellet drops — describes how doping varies along the pull axis, from one end of the ingot to the other. Cutting individual rectangular bars introduces a different axis of variation entirely: doping can also vary radially, across the round cross-section, at a single fixed position along the pull. Imperfect convective mixing and the crystal's own rotation during growth don't guarantee perfectly uniform dopant incorporation at every point around the circumference — a bar cut near the ingot's center and a bar cut nearer the edge, even from the identical axial position, can carry measurably different base widths or junction depths, simply because of where within the round cross-section each bar happened to come from.
## 1. Radial Variation Is a Different Mechanism From Everything Covered So Far
Axial segregation, the subject of steps one, thirteen, and seventeen, arises because the melt's bulk composition changes as more of it solidifies over time — a one-dimensional problem along the growth direction. Radial variation arises instead from imperfect mixing at the growth interface itself: convective flow patterns and the crystal's rotation relative to the melt create small, systematic differences in local growth conditions across the interface's own width, which can imprint as rotational striae or a smooth radial gradient in incorporated dopant concentration. This is a genuinely separate error source from anything the process has tracked before, and it only becomes relevant once a round ingot section is divided into multiple individual bars rather than treated as a single axial structure.
## 2. Real Diagram: Where in the Cross-Section Each Bar Comes From Matters
## 3. Why This Matters Only Now, Not Earlier in the Process
Radial dopant variation existed in the crystal since the moment it grew — nothing about this cutting step creates it — but it only becomes operationally relevant at this step, because this is the first point in the process where the single continuous ingot is divided into multiple separate devices that will be evaluated and specified individually. Every earlier step treated the ingot as one axial structure with one set of concentrations; from this step forward, bar-to-bar gain variation, traceable back to exactly where within the round cross-section each bar was cut, becomes a real yield and binning consideration that the parametric test near the end of this process will eventually have to sort through.
## Cut Rectangular Transistor Bars's Place in the Process Lineage
Cutting rectangular transistor bars is step twenty-three of the 1951 grown-junction transistor's full manufacturing sequence — immediately after the junction-bearing section has been isolated, and before the individual bars are inspected. It is the step that first exposes radial dopant non-uniformity as a practical concern, converting a single continuous crystal structure into multiple individual devices whose electrical properties can differ based on where in the round cross-section each one was cut from.