The Structure of GaN HEMT Devices.器件的结构

GaN HEMT器件的结构

The Structure of GaN HEMT Devices.

HEMT器件是三端电压控制器件，它有三个电极，分别是栅极、源极和漏极。栅极通常是肖特基接触电极，源极和漏极是欧姆接触电极。通过调节外加栅极电压(相对于源极)，可以调控沟道中的二维电子气(2DEG)密度，从而实现栅极电压和漏极电压对漏极电流(输出电流)的控制。其工作原理可以用沿着垂直于异质结结面方向的一维线性电荷模型来定量描述。所谓电荷控制模型是指在无漏极电压时，栅极电压对2DEG密度的控制作用。典型AlGaN/GaN HEMT的结构示意图如图1所示。由于栅极、源极和漏极均制作在器件的上表面，2DEG所形成的电流在沟道中水平流动，故称为横向结构器件。该器件结构可通过异质外延生长技术(如MOCVD技术)或外延结构转移技术制备在不同的衬底材料上，如SiC衬底、蓝宝石衬底、Si衬底、GaN衬底、金刚石衬底或其它合适衬底材料上，图1中略去了具体的衬底材料。如果不考虑三个电极，在图1中，在纵向(垂直于异质结结面方向)方向上从衬底材料向上，依次是高阻GaN缓冲层材料、非故意掺杂AlGaN势垒层材料、N型掺杂AlGaN势垒层材料。

HEMT devices are three-terminal voltage-controlled devices with three electrodes: the gate, source, and drain. The gate is typically a Schottky contact electrode, while the source and drain are Ohmic contact electrodes. By adjusting the applied gate voltage relative to the source, you can control the density of the two-dimensional electron gas (2DEG) in the channel, thereby controlling the drain current (output current) based on the gate voltage and drain voltage. The operating principle can be quantitatively described using a one-dimensional linear charge model along the direction perpendicular to the heterojunction interface. The so-called charge control model refers to the control of the 2DEG density by the gate voltage when there is no drain voltage.

A typical AlGaN/GaN HEMT structure is schematically illustrated in Figure 1. Since the gate, source, and drain are all fabricated on the device's upper surface, the current formed by 2DEG flows horizontally in the channel, making it a lateral structure device. This device structure can be prepared on different substrate materials using heteroepitaxial growth techniques such as MOCVD technology or epitaxial structure transfer technology. The specific substrate material is not mentioned in Figure 1. If we exclude the three electrodes, in Figure 1, from the substrate material upwards in the vertical direction (perpendicular to the heterojunction interface), you will find the following layers: a high-resistance GaN buffer layer material, unintentionally doped AlGaN barrier layer material, and N-type doped AlGaN barrier layer material.

2DEG位于Undoped-AlGaN/GaN异质结界面GaN—侧的沟道中。通常，为减少AlGaN势垒层中的合金无序对沟道中2DEG的散射作用，在Undoped-AlGaN/和GaN层之间插入一薄层(厚度约为1nm左右)的AlN材料；为了保护AlGaN层免受氧化，在n-AlGaN表面还会生长一层3nm左右的GaN帽层材料。在图1中，为简单起见，略去了AlN插入层和GaN帽层，这样处理并不影响一般地对AlGaN/GaN HEMT器件的工作原理进行讨论。

The 2DEG is located in the channel on the GaN side at the interface of the undoped AlGaN/GaN heterojunction. Typically, to reduce the scattering effect of alloy disorder in the AlGaN barrier layer on the 2DEG in the channel, a thin layer of AlN material (approximately 1nm thick) is inserted between the undoped AlGaN and GaN layers. Additionally, to protect the AlGaN layer from oxidation, a thin GaN cap layer (approximately 3nm thick) is grown on the surface of n-AlGaN. In Figure 1, for simplicity, the AlN insertion layer and GaN cap layer have been omitted, and this simplification does not affect the general discussion of the operation principles of AlGaN/GaN HEMT devices.